Read the instrumented suite, and find the test that proves nothing

Four coverage waves have been steered by JaCoCo, which measures
testDebugUnitTest only and cannot see app/src/androidTest at all. So
nothing had ever asked what the 60 device tests pin, only that they were
green. This is that read: a triage, not a test push, in the shape of
docs/coverage-read-findings.md.

Six findings (E1-E6) are things a test would not fix, and five of those
six are prose rather than code — the suite itself is in good condition.
Eight tickets carry the rest (#223-#230), each naming the mutation that
has to go red rather than a coverage delta.

The one that matters is #223. HardwareFallbackTest is the only automated
check of the hardware->software fallback against a real codec failure,
and it has never attempted the hardware path. Measured on run
34004304566: the API 33, 34, 35 and 37 legs each log

  Routing sample_h264_444.mp4 -> ... via FFMPEG (NO_HARDWARE_ENCODER)

because emulators expose no hardware encoder, so the router sends the job
straight to FFmpeg and runMedia3OrFallBack's catch is never entered. Its
two assertions — succeeded, output non-empty — are true anyway. It
finishes in 448 ms, which is not long enough to fail a hardware export
and then software-encode a three-second clip. Deleting that catch reddens
nothing anywhere.

Two things generalise. A test can assert and still not reach, which
neither a coverage number nor a "does it assert something" review can
see; the filter that works is whether the test's premise holds on the
machine running it. And the codebase already knew — ForcedFailureTest
pins DeviceCodecs.PERMISSIVE against this exact hazard and says why, as
does ConversionWorkerTest. Their assertions are about the path, so
without the pin they fail loudly; HardwareFallbackTest's are about the
output, so it passes quietly. That asymmetry is why nobody noticed.

E5 records the structural reason this document is separate: F7 in the
coverage findings calls probeWithExtractor's catch uncovered when
RemuxTest drives it on a device every leg. A JaCoCo-derived document
cannot see androidTest, so it will keep re-deriving that.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/scripts/e2e-report-shape-test.sh

@@ -0,0 +1,193 @@
+#!/usr/bin/env bash
+#
+# Exercises e2e-report-shape.sh's baseline counter against fixture source, with no emulator and
+# no CI run. Run it directly:
+#
+#   .github/scripts/e2e-report-shape-test.sh
+#
+# WHY THIS CAN EXIST AT ALL: the counter is a pure function of the working tree. It greps
+# `app/src/androidTest` for `@FailsOnEmulatorApi37` and compares the total against the number
+# committed in FailsOnEmulatorApi37.kt. Nothing about that needs a device, which is the whole
+# reason #120 could be measured rather than argued about.
+#
+# WHY A THROWAWAY REPO ROOT rather than a knob on the script. The report finds its own root from
+# `BASH_SOURCE`, so a copy of it placed at `<root>/.github/scripts/` reads `<root>/app/src/...`.
+# Building that root is three mkdirs and costs the shipped script nothing:
+#
+#   - the REAL script is what runs, byte for byte, so reverting the matcher reddens this test
+#     rather than a testing-only code path beside it;
+#   - no environment variable exists that could point the LIVE count somewhere else, which is
+#     the failure mode #83 built the baseline check to prevent in the first place;
+#   - XML_DIR resolves inside the throwaway root, so a stale app/build/outputs left by a real
+#     run on a developer machine cannot leak into the numbers here.
+#
+# WHAT IT GUARDS (#120). The old matcher looked for the string anywhere on any line, so a KDoc
+# saying `Deliberately not @FailsOnEmulatorApi37` counted as a marked test and every PR got a
+# deviation notice that was wrong. The obvious repair -- count only lines that are nothing but
+# the annotation -- silently stops counting `@FailsOnEmulatorApi37 @Test`, which is legal Kotlin,
+# and undercounting is the direction that hides a genuine new marker. The fixture carries every
+# shape at once -- three that count and three that must not, enumerated in its own header -- so
+# both mistakes fail here instead of on a PR: against testdata/marker-shapes the old matcher says
+# 5, own-line-only says 2, and the shipped one 3.
+#
+# NOT WIRED INTO CI, deliberately and as a known gap. Adding a step to the Static analysis job
+# would add a new way for a gating job to go red, and #120 was explicit that nothing about it may
+# change any job's status or the pass/fail rules. shellcheck still covers this file, since that
+# step reads `git ls-files '*.sh'` rather than a fixed list.
+set -uo pipefail
+
+SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+REPORT="$SCRIPT_DIR/e2e-report-shape.sh"
+FIXTURE_DIR="$SCRIPT_DIR/testdata/marker-shapes"
+FIXTURE="$FIXTURE_DIR/MarkerShapes.kt"
+
+TMP="$(mktemp -d)"
+# Single quotes: the path is expanded when the trap fires, not when it is set.
+trap 'rm -rf -- "$TMP"' EXIT
+
+failures=0
+
+pass() { printf 'ok    %s\n' "$1"; }
+
+fail() {
+  failures=$((failures + 1))
+  printf 'FAIL  %s\n' "$1"
+  shift
+  printf '        %s\n' "$@"
+}
+
+# assert_contains <name> <haystack> <needle>
+# `case` rather than grep: the strings being matched carry backticks and an em dash, and this
+# way neither the shell nor a regex engine gets an opinion about them.
+assert_contains() {
+  case "$2" in
+    *"$3"*) pass "$1" ;;
+    *) fail "$1" "wanted to find: $3" "in:" "$2" ;;
+  esac
+}
+
+assert_absent() {
+  case "$2" in
+    *"$3"*) fail "$1" "did NOT want to find: $3" "in:" "$2" ;;
+    *) pass "$1" ;;
+  esac
+}
+
+# make_root <marked-tree-dir> <baseline-number>
+# Assembles a throwaway repo root around the given tree and prints its path.
+make_root() {
+  local tree="$1" baseline="$2" root testdir
+  root="$(mktemp -d "$TMP/root.XXXXXX")"
+  testdir="$root/app/src/androidTest/java/org/libremediaconverter"
+  mkdir -p "$root/.github/scripts" "$testdir"
+  cp -- "$REPORT" "$root/.github/scripts/"
+  cp -- "$tree"/*.kt "$testdir/"
+
+  # The synthetic stand-in for the committed baseline. Its KDoc names the marker the way the real
+  # file does -- in brackets, never with an `@` -- because the real file lives inside the tree
+  # being counted, so an `@` spelling here would add a phantom to every number below.
+  cat > "$testdir/FailsOnEmulatorApi37.kt" <<EOF
+package org.libremediaconverter
+
+/** Stand-in for the real marker file. Only [FAILS_ON_EMULATOR_API37_BASELINE] is read. */
+const val FAILS_ON_EMULATOR_API37_BASELINE = $baseline
+EOF
+
+  # A clean, untruncated run of exactly <baseline> tests, all failing -- which is what the
+  # advisory leg looks like when nothing has drifted. It leaves the marked count as the only
+  # field that can deviate, so every assertion below is about the thing under test.
+  cat > "$root/gradle.log" <<EOF
+> Task :app:connectedDebugAndroidTest
+Starting $baseline tests on test(AVD) - 16
+There was $baseline failure(s).
+EOF
+
+  printf '%s\n' "$root"
+}
+
+# run_report <root>  -- stdout of the real script; its summary lands in <root>/summary.md.
+run_report() {
+  E2E_WEDGED_AFTER='' GITHUB_STEP_SUMMARY="$1/summary.md" \
+    bash "$1/.github/scripts/e2e-report-shape.sh" 37 "$1/gradle.log" \
+    "$1/app/src/androidTest/java/org/libremediaconverter/FailsOnEmulatorApi37.kt"
+}
+
+# ---------------------------------------------------------------------------
+# The fixture still carries every shape.
+#
+# Three of the checks below are covered twice over -- deleting a real annotation moves the count
+# and fails a case further down. The two decoys are not: drop the KDoc mention and the count
+# stays 3, so the precision this whole ticket is about would stop being tested and nothing would
+# say so. That asymmetry is why the shapes are asserted by name rather than only by their effect
+# on the total.
+# ---------------------------------------------------------------------------
+fixture_text="$(cat -- "$FIXTURE")"
+assert_contains "fixture: the import"           "$fixture_text" 'import org.libremediaconverter.FailsOnEmulatorApi37'
+assert_contains "fixture: annotation own line"  "$fixture_text" '
+    @FailsOnEmulatorApi37
+    @Test'
+assert_contains "fixture: annotation with @Test on one line" "$fixture_text" '@FailsOnEmulatorApi37 @Test'
+assert_contains "fixture: annotation nested and indented" "$fixture_text" '
+        @FailsOnEmulatorApi37'
+assert_contains "fixture: KDoc mention (this is #120)" "$fixture_text" "* Deliberately not \`@FailsOnEmulatorApi37\`"
+assert_contains "fixture: commented-out annotation" "$fixture_text" '// @FailsOnEmulatorApi37'
+
+# ---------------------------------------------------------------------------
+# 1. The fixture's three real annotations against a baseline of 3: no deviation.
+#
+# This one case fails under both wrong matchers -- the old one counts 5, own-line-only counts 2 --
+# which is why it is first.
+# ---------------------------------------------------------------------------
+root="$(make_root "$FIXTURE_DIR" 3)"
+out="$(run_report "$root")"
+assert_contains "3 real markers, baseline 3: reports a match" "$out" '  baseline: matches (3 expected, 3 failed)'
+assert_absent   "3 real markers, baseline 3: says nothing about the tree" "$out" 'the tree carries'
+assert_contains "3 real markers, baseline 3: summary agrees" \
+  "$(cat -- "$root/summary.md")" '**Matches the committed baseline of 3**'
+
+# ---------------------------------------------------------------------------
+# 2. A fourth REAL annotation. The count has to move and the deviation has to fire.
+#
+# The important half of #120: precision was the bug, but a matcher that stopped noticing a new
+# marker would have been a worse one, silently.
+# ---------------------------------------------------------------------------
+plus_one="$(mktemp -d "$TMP/plusone.XXXXXX")"
+cp -- "$FIXTURE" "$plus_one/"
+cat > "$plus_one/FourthMarker.kt" <<'EOF'
+package org.libremediaconverter.fixture
+
+class FourthMarker {
+    @FailsOnEmulatorApi37
+    @Test
+    fun addedToday() = Unit
+}
+EOF
+root="$(make_root "$plus_one" 3)"
+out="$(run_report "$root")"
+assert_contains "a 4th real marker: the deviation fires, and counts 4" "$out" \
+  "  baseline DEVIATION: the tree carries 4 tests marked \`@FailsOnEmulatorApi37\` but the baseline says 3 — update FAILS_ON_EMULATOR_API37_BASELINE"
+assert_contains "a 4th real marker: the summary carries it too" "$(cat -- "$root/summary.md")" \
+  "- the tree carries 4 tests marked \`@FailsOnEmulatorApi37\` but the baseline says 3"
+
+# ---------------------------------------------------------------------------
+# 3. Delete the same-line annotation and the count must drop to 2.
+#
+# This is the trap, pinned down. `@FailsOnEmulatorApi37 @Test` on one line is what separates the
+# shipped matcher from `^[[:space:]]*@NAME[[:space:]]*$`, and without this case the fixture entry
+# guarding it could be deleted as decoration -- case 1 would then pass under the wrong matcher.
+# Here the same-line entry is worth exactly one, and it is asserted to be.
+# ---------------------------------------------------------------------------
+minus_same_line="$(mktemp -d "$TMP/minus.XXXXXX")"
+sed -e '/@FailsOnEmulatorApi37 @Test/d' -- "$FIXTURE" > "$minus_same_line/MarkerShapes.kt"
+root="$(make_root "$minus_same_line" 3)"
+out="$(run_report "$root")"
+assert_contains "same-line annotation removed: counts 2, so it was worth 1" "$out" \
+  "  baseline DEVIATION: the tree carries 2 tests marked \`@FailsOnEmulatorApi37\` but the baseline says 3 — update FAILS_ON_EMULATOR_API37_BASELINE"
+
+echo
+if [ "$failures" -eq 0 ]; then
+  echo "e2e-report-shape-test.sh: all checks passed"
+  exit 0
+fi
+echo "e2e-report-shape-test.sh: $failures check(s) failed"
+exit 1

.github/scripts/e2e-report-shape.sh

@@ -30,16 +30,31 @@
 #
 # Usage:
 #   e2e-report-shape.sh <label> <gradle-log> [<baseline-file>]
+#   E2E_WEDGED_AFTER=<seconds>   the wrapper timeout killed gradle after that many seconds
 #
 # With a third argument the run is compared against the baseline in that file (advisory mode)
 # and a `::notice::` is emitted per deviation. NEVER `::error::`: the advisory job is
 # `continue-on-error: true` and stays that way, and an error annotation would be a new way for
 # a diagnostic to change a conclusion.
+#
+# WHY THE WEDGE ARRIVES AS AN ENV VAR (#118) rather than being read out of the log like every
+# other field: there is nothing in the log to read. A wedge is gradle never returning, so gradle
+# never printed a verdict, never printed a truncation line, and never aborted instrumentation --
+# the log of a wedged leg is the log of a run that simply stops. Measured on job 98035980326:
+# `expected: 59`, `received: 59`, `completed cleanly: yes`, six seconds before the wedge warning,
+# for a leg that the timeout had killed 22 minutes in. Only e2e-run.sh knows, because only it
+# saw `timeout` exit 124, so it says so. Guessing it from a log that ends abruptly would call
+# every cancelled run a wedge.
+#
+# It is read as a STRING and only ever interpolated into one. `[ -n ... ]`, never `-gt`: it
+# crosses a process boundary from a shell that deliberately sets it EMPTY on every non-wedge
+# path, and an arithmetic test on an empty string is the header's rule four paragraphs up.
 set -uo pipefail
 
 LABEL="${1:-unknown}"
 LOG="${2:-}"
 BASELINE_FILE="${3:-}"
+WEDGED_AFTER="${E2E_WEDGED_AFTER:-}"
 
 SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
 REPO_ROOT="$(cd -- "$SCRIPT_DIR/../.." && pwd)"
@@ -115,6 +130,13 @@ elif [ -n "$abort_received" ]; then
 elif [ -n "$expected" ] && [ -z "$abort_line" ]; then
   received="$expected"
   received_src="the run was not truncated, so every expected test reported"
+  # ... unless it was killed, in which case "not truncated" is only "gradle never got as far as
+  # saying so". This is the branch the wedged leg in #118 took -- with no XML written yet, the
+  # number is what the runner was TOLD to run, and the source line said the opposite in the same
+  # table that called the leg clean. The number is deliberately left alone: it is still the best
+  # available answer, and only the claim about where it came from was wrong.
+  [ -n "$WEDGED_AFTER" ] \
+    && received_src="no test XML was written and gradle never printed a truncation line — but the leg was killed mid-run, so this is what it was told to run, not what reported"
 fi
 
 failed="unknown"
@@ -152,6 +174,12 @@ elif [ "$no_run" = "nothing" ]; then
   # "cleanly" would be a lie about a run that left no evidence it happened.
   completed="unknown"
   completed_src="no runner output to read"
+elif [ -n "$WEDGED_AFTER" ]; then
+  # The wedge is checked LAST of the four, so it only ever overrides the `yes`. The two "no"s
+  # above are already right and name the abort, which the wedge row does not; `unknown` is
+  # already right too. A wedge on top of an abort is both facts, and both get printed.
+  completed="**no**"
+  completed_src="the wrapper timeout killed gradle after ${WEDGED_AFTER}s — instrumentation itself was never aborted, which is why nothing in the log says so"
 else
   completed="yes"
   completed_src="no truncation line and no \`INSTRUMENTATION_ABORTED\`"
@@ -180,11 +208,34 @@ if [ -n "$BASELINE_FILE" ]; then
   advisory="yes"
   [ -f "$BASELINE_FILE" ] \
     && baseline="$(sed -nE 's/^const val FAILS_ON_EMULATOR_API37_BASELINE = ([0-9]+).*/\1/p' "$BASELINE_FILE" | head -1)"
-  # The #81 check, verbatim: what the tree actually carries. Reported next to the baseline so a
-  # stale baseline shows up here rather than only once the emulator disagrees with it.
+  # What the tree actually carries. Reported next to the baseline so a stale baseline shows up
+  # here rather than only once the emulator disagrees with it.
+  #
+  # ANCHORED AT LINE START, AND WHITESPACE-OR-END-OF-LINE AFTER THE NAME (#120). The #81 check
+  # this replaces matched the string anywhere on any line, so #113's KDoc reading `Deliberately
+  # not @FailsOnEmulatorApi37` counted as a fourth marked test and the report announced a
+  # deviation on every PR. That is worse than a wrong number: #83 built this so a new failure
+  # could not be invisible, and a notice that is wrong every time teaches everyone to skim past
+  # deviation notices.
+  #
+  # THE OBVIOUS REPAIR IS A TRAP, and the reason for the second half of the pattern.
+  # `^[[:space:]]*@NAME[[:space:]]*$` -- "the annotation on a line of its own" -- also stops
+  # counting `@FailsOnEmulatorApi37 @Test`, which is legal Kotlin, and UNDERcounting is the
+  # dangerous direction: it hides a genuine new marker, which is the one thing this exists to
+  # catch. Measured against `testdata/marker-shapes`, a fixture carrying every shape at once:
+  # the old matcher says 5, own-line-only says 2, this one says 3. On the real tree, 4 / 3 / 3.
+  # e2e-report-shape-test.sh runs that fixture through this whole script.
+  #
+  # `^[[:space:]]*@` cannot match an `import` line, so the old `grep -v import` goes with it
+  # rather than staying to imply a filter is still doing work.
+  #
+  # This is a regex over source text and not a parser. An annotation inside a multi-line string,
+  # or inside a `/* */` block that opened mid-line, would still be counted. Neither exists here;
+  # if one ever does, this check wants a different tool rather than a longer regex.
   if [ -d "$REPO_ROOT/app/src/androidTest" ]; then
-    marked="$(grep -rn "@FailsOnEmulatorApi37" "$REPO_ROOT/app/src/androidTest" --include='*.kt' \
-      | grep -v import | grep -c FailsOn || true)"
+    marked="$(grep -rhcE '^[[:space:]]*@FailsOnEmulatorApi37([[:space:]]|$)' \
+      "$REPO_ROOT/app/src/androidTest" --include='*.kt' \
+      | awk '{ total += $1 } END { print total + 0 }' || true)"
   fi
 fi
 
@@ -217,6 +268,11 @@ echo "----- RUN SHAPE (api${LABEL}) -----"
 echo "  expected:          $expected"
 echo "  received:          $received"
 echo "  failed:            $failed"
+# Above `completed cleanly`, because it is the line that says what happened to the leg and the
+# other one only qualifies it. A reader who stops after three rows still sees it.
+if [ -n "$WEDGED_AFTER" ]; then
+  echo "  wedged:            yes -- gradle was killed after ${WEDGED_AFTER}s and never returned"
+fi
 echo "  completed cleanly: ${completed//\*/}"
 if [ -n "$abort_received" ]; then
   echo "  received before the abort: $abort_received"
@@ -249,6 +305,9 @@ if [ -n "${GITHUB_STEP_SUMMARY:-}" ]; then
     echo "| expected | $expected | $expected_src |"
     echo "| received | $received | $received_src |"
     echo "| failed | $failed | $failed_src |"
+    if [ -n "$WEDGED_AFTER" ]; then
+      echo "| wedged | **yes** | \`timeout\` fired after ${WEDGED_AFTER}s and killed gradle (exit 124), which is what e2e-run.sh then captured the wedge diagnostics for |"
+    fi
     echo "| completed cleanly | $completed | $completed_src |"
     if [ -n "$abort_received" ]; then
       echo "| received before the abort | $abort_received | the same line — the XML above counts the truncated test as a failure, this number does not |"

.github/scripts/e2e-run.sh

@@ -239,20 +239,34 @@ timeout -k 30s "$WEDGE_TIMEOUT" \
   ${E2E_EXTRA_GRADLE_ARGS:-} 2>&1 | tee "$GRADLE_LOG" || status=$?
 echo "::endgroup::"
 
+# Whether the wrapper timeout fired, decided ONCE. 124 is `timeout` saying it killed the
+# command, and two places downstream need that fact: capture_wedge below, and the report, which
+# otherwise calls a killed leg `completed cleanly: yes` (#118). Deriving it twice is how those
+# two would drift apart -- the report would keep printing after someone changed what a wedge
+# means here. It stays a string: empty on every other path, so those legs pass an empty
+# E2E_WEDGED_AFTER and the report behaves exactly as before.
+wedged=""
+[ "$status" -eq 124 ] && wedged="$WEDGE_TIMEOUT"
+
 # The run-shape report: expected/received/failed and whether the run finished, every time,
 # green or red. It never changes `status` -- it is a diagnostic, and the header's rule about
 # diagnostics applies to it as much as to every probe below.
 #
+# E2E_WEDGED_AFTER is the wedge, told to the report rather than left for it to infer. It cannot
+# be inferred: a wedge is gradle never returning, so gradle printed no verdict at all, and the
+# log the report reads looks like a run that simply stopped. Only this script knows the
+# difference, because only this script saw the exit status.
+#
 # The baseline argument, and only it, turns on the comparison, and only the advisory API 37 job
 # passes E2E_ADVISORY=1. Comparing on the gating legs would announce a deviation on all five of
 # them every run, since they run the whole suite rather than the marked three. They still get
 # the report: a truncated run reporting fewer results than it ran is what #108 looks like, and
 # `completed cleanly` is the field that shows it.
 if [ "${E2E_ADVISORY:-}" = "1" ]; then
-  bash "$SCRIPT_DIR/e2e-report-shape.sh" "$LABEL" "$GRADLE_LOG" \
+  E2E_WEDGED_AFTER="$wedged" bash "$SCRIPT_DIR/e2e-report-shape.sh" "$LABEL" "$GRADLE_LOG" \
     "$REPO_ROOT/app/src/androidTest/java/org/libremediaconverter/FailsOnEmulatorApi37.kt" || true
 else
-  bash "$SCRIPT_DIR/e2e-report-shape.sh" "$LABEL" "$GRADLE_LOG" || true
+  E2E_WEDGED_AFTER="$wedged" bash "$SCRIPT_DIR/e2e-report-shape.sh" "$LABEL" "$GRADLE_LOG" || true
 fi
 
 if [ "$status" -eq 0 ]; then
@@ -260,7 +274,7 @@ if [ "$status" -eq 0 ]; then
   exit 0
 fi
 
-if [ "$status" -eq 124 ]; then
+if [ -n "$wedged" ]; then
   capture_wedge "api${LABEL}"
 else
   echo "::error::E2E api${LABEL} failed (exit $status)"

.github/scripts/testdata/marker-shapes/MarkerShapes.kt

@@ -0,0 +1,54 @@
+// NOT A TEST, AND NEVER COMPILED. This is fixture data for e2e-report-shape-test.sh, which
+// copies it into a throwaway repo root and runs the real report script against that. It lives
+// under .github/ deliberately: Gradle only compiles app/src/**, ktlint and detekt are applied to
+// :app only, and the report's own count reads app/src/androidTest -- so nothing here can reach
+// the build, the linters, or the number the advisory job compares against. Verified by running
+// the report against the real repo root with this file committed: still 3.
+//
+// It carries every shape the counter has to tell apart, in one file, because the bug in #120 was
+// exactly that two of them look alike to a substring match. Three count and three must not:
+//
+//   COUNTS       the annotation on its own line
+//   COUNTS       the annotation sharing a line with @Test -- legal Kotlin, and the case the
+//                obvious "own line only" repair silently drops
+//   COUNTS       the annotation indented inside a nested class
+//   must NOT     a KDoc mentioning it -- this is #120 itself, copied from Media3EngineTest
+//   must NOT     a commented-out annotation
+//   must NOT     the import
+//
+// Three count. That is what the synthetic baseline in the test is set to, so the fixture and the
+// baseline agree exactly the way the real tree and FAILS_ON_EMULATOR_API37_BASELINE do.
+//
+// The `@Test` here is spelled the way a real test spells it so the fixture reads like source
+// rather than like a regex exercise. Nothing runs it.
+
+package org.libremediaconverter.fixture
+
+import org.junit.Test
+import org.libremediaconverter.FailsOnEmulatorApi37
+
+class MarkerShapes {
+    @FailsOnEmulatorApi37
+    @Test
+    fun ownLine() = Unit
+
+    @FailsOnEmulatorApi37 @Test
+    fun sameLineAsTest() = Unit
+
+    /**
+     * Deliberately not `@FailsOnEmulatorApi37`: nothing here decodes or encodes, so no emulator
+     * codec is involved and the API 37 image has no quarrel with it.
+     */
+    @Test
+    fun mentionedInKdoc() = Unit
+
+    // @FailsOnEmulatorApi37 -- taken off on 2026-01-01, kept as a note rather than deleted
+    @Test
+    fun commentedOut() = Unit
+
+    class Nested {
+        @FailsOnEmulatorApi37
+        @Test
+        fun indentedDeeper() = Unit
+    }
+}

.gitignore

@@ -13,6 +13,7 @@
 .externalNativeBuild
 .cxx
 local.properties
+.kotlin
 
 # The FFmpeg AAR is committed under bin/ so test runs do not depend on a rebuild.
 # Build outputs from tools/ffmpeg are not.

CLAUDE.md

@@ -98,6 +98,13 @@ days. Read it as the current answer, and see the git history if you need the old
   is written anyway and says nothing about it — `.github/scripts/e2e-report-shape.sh` is where that
   is measured and explained.
 
+  Every leg prints that table, advisory or not, and **on the wedge path it also carries a `wedged:`
+  row** (#118). `completed cleanly` only ever meant "instrumentation was not aborted", which stays
+  true of a leg the `WEDGE_TIMEOUT` killed 22 minutes in — so without that row the table read
+  `received: 59, completed cleanly: yes` for a leg that had just died. The wedge is passed to the
+  report as `E2E_WEDGED_AFTER` by `e2e-run.sh`, which is the only thing that can know it: a wedge
+  is gradle never returning, so the log it left says nothing about it.
+
 Still true, and the reason the advisory job is not simply deleted: **API 37 needs a manual check on
 the Pixel 10 Pro XL before each release.** Those three tests are the one thing CI cannot answer
 for.
@@ -123,10 +130,11 @@ install for code that can never run — and on API 37 the full APK does not fit
 - The `model` package is excluded from `ReturnCount` and `CyclomaticComplexMethod` only. It is the
   decision layer, where one branch is one documented user-visible outcome and the metric counts
   answers rather than complexity. Every other rule still applies there.
-- **Coverage is reported, not gated** — **69.2% of lines (1519/2194), 53.2% of branches**,
-  measured 2026-08-24 with `./gradlew :app:jacocoTestReport`.
+- **Coverage is reported, not gated** — **94.2% of lines (2234/2372), 87.5% of branches
+  (1171/1338)**, measured 2026-09-05 with `./gradlew :app:jacocoTestReport`, against 628 JVM tests
+  in 96 classes.
 
-  **Every figure this file carried before that date was an artifact, roughly half the real one.**
+  **Every figure this file carried before 2026-08-24 was an artifact, roughly half the real one.**
   Robolectric loads classes through its own sandbox classloader with no source location, JaCoCo
   skips no-location classes by default, and nothing told it otherwise — so **not one Robolectric
   test counted**, and Robolectric is what exercises the framework edge here. The
@@ -140,9 +148,193 @@ install for code that can never run — and on API 37 the full APK does not fit
   disproportionately Robolectric, so each one added denominator and no numerator — the measurement
   was punishing exactly the tests that were hardest to write.
 
-  Two things still hold. A floor needs a baseline that has settled, and this one has now moved by
-  39 points in a single build change, so it has not. And **re-measure before quoting** — that
-  instruction is the only reason this was caught.
+  Two things still hold. A floor needs a baseline that has settled, and this one has not. It moved
+  39 points in a single build change on 2026-08-24; then another 16 as the #52 test push and the
+  fixes it turned up landed — 69.2% -> 84.9% line, 53.2% -> 63.8% branch — while the denominator
+  grew 2194 -> 2321, because that work added production code of its own; then again on 2026-08-27
+  as #132 and #133's ten children landed — 84.9% -> 87.1% line, 63.8% -> **69.1%** branch, 454 ->
+  502 tests. **Branch moved four times as far as line that last time**, and that is the shape to
+  expect from this kind of work rather than a surprise: those children targeted decision code —
+  enum fallbacks, refusal arms, cursor shapes, a `when` over container rules — where one test
+  chooses a branch the suite had never taken. Line coverage barely notices; branch coverage is the
+  whole point.
+
+  Then #153's five children on 2026-08-29 — 87.1% -> 88.9% line, 69.1% -> **75.4%** branch, 502 ->
+  546 tests.
+
+  **That last branch figure moved for two reasons, and only one of them is new tests.** The
+  numerator rose 974 -> 1011; the denominator *fell* 1410 -> 1340. Both are the seam work. Pulling
+  a `when` out of a lambda inside a `collect` deletes the coroutine state machine's synthesized
+  branches around it, and what is left is a plain function whose branches a test can choose:
+  `ConversionViewModel$observe$1$1` went from carrying the whole mapping to 6 branches, while the
+  extracted `ConversionViewModelKt` covers 41 of 42 and `JoinViewModelKt` 38 of 39. So a seam is
+  worth more than the tests it enables — it also stops the measurement counting scaffolding.
+
+  Be careful quoting a branch move on its own for that reason. A percentage that rises because the
+  denominator shrank is not the same claim as one that rises because more branches are tested, and
+  this entry has a history of explaining its own numbers wrongly.
+
+  Then wave 3 (#167-#178) on 2026-09-02 — 88.9% -> 92.8% line, 75.4% -> **81.3%** branch, 546 ->
+  584 tests, in ten PRs from #179 to #188.
+
+  **Its shape is different from the two before it, and the difference is the thing to carry
+  forward.** Waves 1 and 2 were finding uncovered code. By wave 3 there was not much of that left,
+  so the gaps were sorted into two kinds before any test was written:
+
+  - **coverage gaps** — the line never executes. Filtered to sites where JaCoCo reports `mi > 0`, a
+    concrete instruction no test runs, which is what separates a real gap from a partial branch on
+    a compound condition. That filter cut the candidate list roughly in half and was right to.
+    **Wave 4 found it wrong in both directions, though — use the two filters below instead.**
+  - **assertion gaps** — JaCoCo is green and nothing checks the answer. `MainActivity`'s rail and
+    bottom bar were both *executed* by `AppRootRestorationTest` and **transposing them passed the
+    entire suite**; so did swapping the two progress-notification strings, and swapping `Content`'s
+    two destinations. No coverage number would ever have found any of the three.
+
+  **Wave 4 (2026-09-02) corrected that first filter, and the correction is the reusable part.**
+  `mi > 0` fails in both directions. It *over-reports* on Compose: `JoinScreen.kt:222` reads
+  `mi=10` and also `ci=38`, and `JoinStateAffordancesTest` already clicks that Save button and
+  asserts `save:joined.mp4` — the missed instructions are the synthesized `$changed`/`$dirty`
+  recomposition-skip path, the same codegen this file already warns about for *branch* counts,
+  showing up in the instruction count too. And it *under-reports* on warm methods with cold arms:
+  `ConversionViewModel.cancel()` misses no line, yet `activeWorkId?.let(...)` had only ever been
+  entered on the null side in 584 tests. Use two filters together instead:
+
+  - **`ci == 0`** — the line never executed. This is JaCoCo's own missed-line definition, so it
+    totals exactly the reported missed-line count and needs no judgement.
+  - **`ci > 0 && mb > 0` at method level** — a covered method with an arm nothing takes. This is
+    the only one that finds the `cancel()` shape.
+
+  Of wave 4's 251 missed branches, just **18** sat on lines that do execute, so the branch gap and
+  the line gap are largely the same gap; the second filter is about which of them are reachable.
+
+  So **every ticket named the mutation that had to go red, and that was its acceptance criterion
+  rather than a coverage delta**. It caught **two vacuous tests written in the same session**,
+  before either shipped:
+
+  - a `firstContainerHolding` test asserting a refusal still offered *something*. True, and
+    useless: the source container is a candidate in its own right, so the list stays non-empty
+    whatever the fallback does. What it actually buys is the codec the user asked for.
+  - a staged-delete test scanning for a `"join-"` prefix `StagingNames.forJob` does not produce —
+    it names files `<jobId>.<ext>`, so the assertion was true of everything.
+
+  It also corrected a *third* test that was not vacuous: `probeForConcat`'s KDoc claimed to drive
+  the `catch` arm, and rethrowing from that catch left it green. That is how the arm turned out to
+  be unreachable — see the next paragraph. A passing test with a wrong explanation is its own
+  failure mode.
+
+  **A green mutation is only evidence when the mutation is a real change**, which is the mirror
+  trap: one `classify` mutation stayed green because reordering two arms was semantically
+  equivalent for every reachable input. A bad mutation and a weak test look identical in the output.
+
+  Three things came back **not as the ticket described them**, which is a result rather than a
+  shortfall:
+
+  - `ContainerCapabilities:282`'s `exclude` filter **cannot drop anything**. `repair` always
+    changes a codec on the shared container — a codec it left alone is one `validate` would not
+    have refused — and the one non-default `exclude` carries `COPY` while every candidate carries
+    `NONE`. F4-shaped.
+  - `probeForConcat`'s catch arm is **unreachable on this runtime**. Robolectric's `MediaExtractor`
+    never throws from `setDataSource`, measured across an unregistered `content://` authority, a
+    missing `file://`, a file of garbage bytes and an `http://` URL — all four returned with
+    `trackCount = 0`. It stays device-only.
+  - `JobSnapshots:31`'s missed arm was **not** the `!isFile` one the ticket named — that is already
+    covered by the `reclaimed` fixture. It was `path == null`: a job carrying no output path at
+    all. Read the report, not the ticket, when the two disagree.
+
+  One item was **included against** the F4 rule rather than exempted by it, and the distinction is
+  worth having written down since both live in the same function: `ContainerCapabilities:94`
+  (`accepts(container, VideoCodec.NONE, mode)`) is dead in production today — every caller guards
+  `NONE` first — and was tested anyway, because its audio twin at `:101` has had a test since #136
+  and the asymmetry was the argument. The `COPY -> error(...)` arms beside it stay exempt, because
+  a second line of defence that can be provoked is not one.
+
+  Denominators moved here too, in both directions and for two different reasons: 1340 -> 1342
+  branches from `MediaProbe.merge`, 2348 -> 2352 lines from the `ConcatJoiner` interface. Neither
+  is new untested code.
+
+  And **re-measure before quoting**: this entry was once written quoting 81.4%, measured four hours
+  earlier, and was already three points stale by the time it was ready to merge.
+
+  **Wave 4's read (2026-09-02) moved no number at all, and that is its result.** It was a triage
+  rather than a test push: twelve tickets (**#192-#203**), four deferred candidates (**#204**), and
+  five findings (**F6-F10** in `docs/coverage-read-findings.md`). What it establishes is the shape
+  of what is left, which is different again from wave 3's:
+
+  - Of 169 never-executed lines, **81 are native or device edges and stay that way** —
+    `FFmpegEngine` 33, `Media3Engine` 24, `ConcatEngine` 14, `MainActivity.onCreate` 10 — their
+    zeroes being the `testDebugUnitTest`-only measurement boundary that #84, #85, #86 and #88 each
+    recorded before. A further **34 are device-bound only until a seam moves them**:
+    `AndroidDeviceCodecs` 20 (#194) and the 14 of `MediaProbe`'s 26 that are `readMediaInformation`
+    (#195). Do not read that second group as exempt — the two tickets exist because it is not.
+  - Most of the rest is **already closed with a reason on record**, or compiler-generated: default-arg
+    bridges, DI factory lambdas, synthetic `NoWhenBranchMatchedException` arms, coroutine completion.
+  - Six of the ten findings in that document are now "no action" or "not a test gap". By this point
+    the report's remaining red is mostly arms nothing can reach, members nothing calls, and arms a
+    test *can* reach but cannot pin — and a coverage number tells none of them apart.
+
+  **The biggest single gap it found was not a missed line.** `ConversionViewModel.cancel()` and
+  `JoinViewModel.cancel()` report every line covered; only the null arm of
+  `activeWorkId?.let(workManager::cancelWorkById)` had ever been entered, so nothing in 584 tests
+  connected the Cancel button to WorkManager (#192). That is what the second filter above is for.
+
+  It also re-opened a mechanism, not a close: #86 and #133 ruled `AndroidDeviceCodecs.probe()` out
+  **through `ShadowMediaCodecList`**, on the grounds that the builder cannot set `isAlias` or
+  `canonicalName`. A pure seam does not have that constraint, and #133 did not evaluate one. Read
+  #194 before re-arguing either way — and note the reason it is worth cutting is not coverage but
+  that the `runCatching` fallback logs "assuming permissive" while returning empty sets, which makes
+  `canEncode` and `canDecode` answer *no* for everything.
+
+  **Wave 4's tests then landed on 2026-09-05**, as #206-#217 for the twelve tickets plus #218
+  (#159) and #219 (#122): 92.8% -> **94.2%** line, 81.3% -> **87.5%** branch, 584 -> 628 tests in 87
+  -> 96 classes. Missed lines 169 -> 138, missed branches 251 -> 167.
+
+  **Its branch move is a different animal from the 2026-08-29 seam work's, and the difference is the
+  point.** That one gained 6.3 branch points with the numerator up 37 (974 -> 1011) while the
+  denominator *fell* 70 (1410 -> 1340) — much of the rise was scaffolding leaving the measurement
+  rather than arms being covered. Here the numerator is up **80** (1091 -> 1171) and
+  the denominator moved **-4** (1342 -> 1338). So this one is almost entirely tests choosing arms
+  nothing had chosen, which is what the entry above warns to check before quoting a branch figure.
+  The line denominator rose the other way, 2352 -> 2372, and that is new production code rather than
+  untested code: the seams the wave cut — `capabilitiesFrom`, `ffprobeInfoFrom`, `sessionOutcome`,
+  and `sweepScope`/`startupSweep`.
+
+  **The two-filter method above is what found the work**, and its second filter earned its place:
+  the largest single gap of the wave (#192, the Cancel button never shown to reach WorkManager) sits
+  on lines that were already green and no line-level filter could see it.
+
+  One result worth carrying forward about *evidence* rather than coverage. #218 fixed a flake whose
+  reproduction is statistical, and running the whole suite six times per arm caught nothing either
+  way — at the observed rate a clean six-run arm is roughly a coin flip, so the comparison was
+  underpowered and proved nothing. What settled it was a deterministic mutation, and then the merge
+  train confirmed it by accident: the race reproduced on #217's Unit tests leg, which sits below
+  #218 and carries the unfixed scope. **Prefer a mutation that must go red to a repetition count**
+  when a fix is for something intermittent.
+
+  **Every number above is `testDebugUnitTest` only, and on 2026-09-05 the instrumented suite got its
+  first read for that reason** — `docs/e2e-read-findings.md`, entries **E1-E6**, tickets
+  **#223-#230**. Four waves had been steered by a figure that **cannot see `app/src/androidTest` at
+  all**, so nothing had ever asked what those 60 device tests pin, only that they were green.
+
+  **It found one test that passes while testing nothing, and it is the one that matters most.**
+  `HardwareFallbackTest` is the only automated check of the hardware→software fallback against a
+  *real* codec failure, and on run `34004304566` the API 33, 34, 35 and 37 legs each log
+  `Routing sample_h264_444.mp4 -> ... via FFMPEG (NO_HARDWARE_ENCODER)` (API 36's logcat artifact on
+  that run is truncated, so it is unread rather than different): emulators expose no
+  hardware encoder, so the job never reaches Media3 and the `catch` it exists to prove is never
+  entered. Its two assertions — succeeded, output non-empty — are true anyway, and it finishes in
+  448 ms. **Deleting that `catch` reddens nothing on any leg** (#223).
+
+  Two things generalise from it. **A test can assert and still not reach**, which no coverage
+  number and no "does it assert something" review would catch — the filter that works is *does this
+  test's premise hold on the machine that runs it?*. And the codebase **already knew**: the sibling
+  `ForcedFailureTest` pins `DeviceCodecs.PERMISSIVE` against exactly this hazard and writes out why,
+  as does `ConversionWorkerTest`. The difference is that their assertions are about the *path*, so
+  without the pin they would fail loudly; `HardwareFallbackTest`'s are about the *output*, so it
+  passes quietly. **Prefer asserting the path over asserting the artefact** where the two differ.
+
+  The read was a triage, not a test push, and five of its six findings are prose rather than code —
+  the suite itself is in good shape. What had drifted is its self-description.
+
 - **Testable code is not done until it is tested.** If a piece is unit testable, it gets unit
   tests before it counts as done. If it is e2e testable, it gets e2e tests. Both clauses apply —
   a change that is both needs both.
@@ -164,6 +356,32 @@ install for code that can never run — and on API 37 the full APK does not fit
 
 - `kotlin.code.style=official`. Gradle stays Kotlin DSL.
 
+- **A stacked PR does not merge with `gh pr merge`, and `MERGED` is not proof it reached `main`.**
+  Two separate traps, both measured on 2026-08-27 while landing #144-#151.
+
+  `gh pr merge` uses the GraphQL mutation, which refuses a stacked PR outright: *"This pull request
+  is part of a stack and must be merged using the asynchronous merge REST API."* So does
+  `PUT .../pulls/{n}/merge`. The one that works is
+  `gh api -X PUT repos/OWNER/REPO/pulls/N/merge-async -f merge_method=merge`, which returns a uuid
+  to poll at `.../merge-async/{uuid}` until `status` is `merged` or `failed`.
+
+  **The second trap is worse, because nothing looks wrong.** GitHub retargets a stacked PR's base
+  to `main` when the PR below it merges, but *asynchronously*. Merge a stack faster than that
+  settles — five PRs about thirty seconds apart, in the case that found this — and each one merges
+  into its own base branch, which has itself already been merged and left behind. Every call
+  returns `status: merged` and every one is true. `gh pr list --state open` comes back empty, every
+  PR shows `MERGED`, and **none of the content is on `main`**.
+
+  What caught it was a coverage re-measure reading two points lower than the same tree had measured
+  an hour earlier; a fresh `git pull` changed nothing, which is what turned it into a question.
+  `git merge-base --is-ancestor <merge-sha> origin/main` answers it in one line. Do that after
+  merging a stack, or merge one at a time and re-read `baseRefName` between. #160 is what the
+  recovery cost.
+
+  The auto-retarget belongs to the stacking feature specifically. A PR opened with a plain
+  `gh pr create --base some-branch` does **not** retarget when that branch merges — it is left
+  pointing at a dead base and has to be moved by hand.
+
 - **File one-off issues with `tools/github/file-issue.sh`, not `gh issue create`.** `gh issue
   create` does not touch the project board, so the issue exists, carries its labels, and is
   invisible in the Kanban — indistinguishable from never having been filed. Measured 2026-08-24:
@@ -236,3 +454,37 @@ Because versions float, a build can change without a commit. `./gradlew :app:dep
   `@OptIn`. Android lint's `UnsafeOptInUsageError` catches a missed one.
 - **Release builds ship both ABIs.** `-PabiFilters` is a test-run override only; `build.yml`
   verifies the released APK carries every ABI and that all native libraries are 16 KB aligned.
+- **A JUnit `Timeout` — rule or `@Test(timeout=)` — cannot be used in the JVM suite.** Both run the
+  test body on a separate thread, and every Compose test here goes through Robolectric's paused
+  main looper: `UnsupportedOperationException: main looper can only be controlled from main
+  thread`, from `ShadowPausedLooper` under `RobolectricIdlingStrategy.runUntilIdle`. The identical
+  tests pass with the timeout removed, so it is the mechanism, not the test. What bounds a hung
+  run instead is `timeout` on the `Test` tasks plus the jstack watchdog beside it in
+  `app/build.gradle.kts`, neither of which moves a thread. `HangBoundTest` guards both numbers,
+  and **a timed-out run writes no XML for the class that hung** — the dump is its only
+  attribution, so do not delete the watchdog as stray config. It has since been exercised in anger:
+  on 2026-09-05 it caught #125's Room/WorkManager deadlock on CI, failing in 10m57s with the hung
+  test named, where that ticket had predicted a 60-minute cap and no cause. #125 is closed as
+  bounded on the strength of it — the inversion itself is internal to the two libraries and still
+  live at `work-runtime` 2.11.2 / `room` 2.7.0.
+- **The JVM suite does not run `LibreMediaConverterApp`.** `app/src/test/resources/robolectric.properties`
+  names `TestLibreMediaConverterApp` for every test, and it differs from the real class in exactly
+  one thing: `sweepScope` is `Dispatchers.Unconfined`, so the startup staging sweep finishes before
+  `onCreate()` returns instead of running on `Dispatchers.IO`.
+
+  **That line is load-bearing — do not delete it as stray config.** Robolectric builds an
+  `Application` per test class that asks for one, and each `onCreate` launched a sweep over the
+  shared `<cacheDir>/conversions/` that nothing joined. So a test asserting about a staged file was
+  racing every sweep the classes before it had left in flight (#159). It was CI-only until wave 4
+  added ten Robolectric classes, at which point `OutputPublisherStagingTest` failed on roughly one
+  local run in six. Per-test opt-in was measured and rejected: **27 of the 58 Robolectric classes
+  touch that directory**. The `SupervisorJob` is kept in the test scope so a throwing sweep is
+  swallowed there exactly as in production — the dispatcher is the only intended difference.
+
+  **It cost one assertion, knowingly.** `AppStartSweepTest` used to open by asserting that the
+  manifest's `android:name` is what Robolectric instantiated, so the sweep is code that actually
+  runs. An `application=` override *replaces* the manifest rather than being checked against it, and
+  `applicationInfo.className` reports the override too — measured — so that claim is not merely
+  unasserted on the JVM now, it is unobservable, and a rewritten version would assert the override
+  against itself. **The manifest link is device-only.** What remains is the `as LibreMediaConverterApp`
+  cast in that class's `setUp`, which catches only the test app ceasing to extend the real one.

app/build.gradle.kts

@@ -1,5 +1,7 @@
 import org.gradle.api.tasks.PathSensitivity
 import org.gradle.testing.jacoco.tasks.JacocoReport
+import java.io.File
+import java.time.Duration
 
 plugins {
     // Applied by id: these two come from the root buildscript classpath, which is what
@@ -217,6 +219,115 @@ tasks.withType<Test>().configureEach {
         .withPropertyName("releaseWorkflow")
         .withPathSensitivity(PathSensitivity.RELATIVE)
 
+    // Same reasoning, same trap: HangBoundTest reads the two numbers below out of this file, and
+    // they are the one part of the change that does not compile. Without this the task stays
+    // UP-TO-DATE when the build script changes, so the guard would go stale on exactly the edit
+    // it exists to catch.
+    inputs.file(project.file("build.gradle.kts"))
+        .withPropertyName("moduleBuildScript")
+        .withPathSensitivity(PathSensitivity.RELATIVE)
+
+    // --- Bounding a hung run (#125) -----------------------------------------------------------
+    //
+    // This suite had no timeout of any kind, so a hang ran until something outside it gave up.
+    // #125 is a real Java-level deadlock -- a lock-order inversion between Room's
+    // TransactionExecutor and WorkManager's SerialExecutorImpl, reached through the WorkInfo flow
+    // -- and one local run sat in it for 47 minutes. On CI it would burn the Unit tests job's
+    // 30-minute cap and report as a job timeout with no cause at all.
+    //
+    // WHY NOT A JUnit `Timeout` RULE, which is the obvious answer: it runs the test body on a
+    // separate thread, and this suite is thread-affine. Measured here, `@Rule Timeout` and
+    // `@Test(timeout = ...)` against a `createComposeRule()` Robolectric test both give:
+    //
+    //     java.lang.UnsupportedOperationException: main looper can only be controlled from main
+    //       at org.robolectric.shadows.ShadowPausedLooper.executeOnLooper
+    //       at androidx.compose.ui.test.RobolectricIdlingStrategy.runUntilIdle
+    //
+    // The same two tests with the timeout removed pass, so that is the mechanism and not the
+    // probe. Nothing that moves a test off its own thread can be used here.
+    //
+    // `Task.timeout` moves nothing -- it stops the forked test JVM from outside. Its weakness is
+    // that it kills without a thread dump, and the jstack is the only reason #125 could be named
+    // at all; the watchdog below is what answers that, and it only dumps.
+    //
+    // THE NUMBER, against the slowest observed *pass* rather than the typical one. Eight CI runs
+    // sampled 2026-08-26, whole `./gradlew :app:testDebugUnitTest` invocation with compilation in
+    // it and this task a subset: 62, 76, 77, 79, 81, 84, 86 and 90 seconds. Locally the task
+    // itself is ~11 s over 454 tests. Ten minutes is ~6.7x the slowest of those and a third of
+    // the job's 30-minute cap, so a fired timeout still has room to be reported and uploaded. It
+    // is deliberately nowhere near the observed duration: a timeout that fires on a healthy slow
+    // runner turns a real signal into noise and teaches people to re-run reflexively.
+    timeout.set(Duration.ofMinutes(10))
+
+    // The dump, two minutes before the kill. jstack is what turned #125 from "CI timed out" into
+    // a named lock-order inversion, and `Task.timeout` on its own would have thrown it away.
+    //
+    // It is deliberately incapable of failing a build: it reads a live process and writes a file.
+    // Nothing here kills, interrupts or signals anything, so the worst a misfire can do is leave a
+    // stack trace nobody needed. It has one, and it is the ordinary CI shape rather than an exotic
+    // case: the worker is found by scanning this daemon's descendants for GradleWorkerMain, which
+    // cannot tell one invocation's worker from the next, and the Unit tests job runs
+    // testDebugUnitTest and jacocoTestReport back to back against the same daemon. If this task's
+    // own worker lived and died inside a single poll, the watchdog can adopt the following one.
+    //
+    // Everything it needs is read here, at configuration time, and captured by value. Reaching
+    // back through the task or the project from inside the action would not survive the
+    // configuration cache, which `gradle.properties` turns on for every build.
+    val threadDump = layout.buildDirectory.file("reports/hang/$name-threads.txt").get().asFile
+    val taskPath = path
+    val dumpAfterNanos = Duration.ofMinutes(8).toNanos()
+    val captureWindowNanos = Duration.ofMinutes(1).toNanos()
+    val pollMillis = 1_000L
+    doFirst {
+        val watchdog = Thread {
+            val startedAt = System.nanoTime()
+            var worker: ProcessHandle? = null
+            while (true) {
+                Thread.sleep(pollMillis)
+                val elapsed = System.nanoTime() - startedAt
+                val watched = worker
+                if (watched == null) {
+                    // Gradle forks the worker moments after this task starts. If none has shown
+                    // up by the end of the capture window there is nothing to watch, and going on
+                    // polling would only risk adopting some other build's.
+                    if (elapsed > captureWindowNanos) return@Thread
+                    worker = ProcessHandle.current().descendants()
+                        .filter { it.info().commandLine().orElse("").contains("GradleWorkerMain") }
+                        .findFirst().orElse(null)
+                } else if (!watched.isAlive) {
+                    return@Thread // the run finished; this is the healthy exit
+                } else if (elapsed >= dumpAfterNanos) {
+                    val jstack = File(File(System.getProperty("java.home"), "bin"), "jstack")
+                    threadDump.parentFile.mkdirs()
+                    if (jstack.canExecute()) {
+                        ProcessBuilder(jstack.absolutePath, "-l", watched.pid().toString())
+                            .redirectErrorStream(true)
+                            .redirectOutput(threadDump)
+                            .start()
+                            .waitFor()
+                    } else {
+                        threadDump.writeText("no jstack at ${jstack.absolutePath}\n")
+                    }
+                    // To stdout as well as to the file, and that is the half that matters on CI:
+                    // the Unit tests job uploads app/build/reports/tests/ and nothing else, so a
+                    // dump that only ever existed under reports/hang/ would be unreachable from a
+                    // red run -- which is the "timed out with no cause" this exists to end. The
+                    // step log always survives, and needs no workflow edit to say so.
+                    println(
+                        "$taskPath is still running after ${Duration.ofNanos(elapsed).toMinutes()} " +
+                            "minutes and is about to be timed out. Thread dump of pid " +
+                            "${watched.pid()}, also written to $threadDump -- look for 'Found one " +
+                            "Java-level deadlock' (that is #125).\n" + threadDump.readText(),
+                    )
+                    return@Thread
+                }
+            }
+        }
+        watchdog.isDaemon = true
+        watchdog.name = "hang-watchdog"
+        watchdog.start()
+    }
+
     extensions.configure<JacocoTaskExtension> {
         isIncludeNoLocationClasses = true
         excludes = listOf("jdk.internal.*")

app/src/androidTest/java/org/libremediaconverter/saf/SafPickerRoundTripTest.kt

@@ -10,6 +10,9 @@ import androidx.compose.ui.test.performClick
 import androidx.media3.common.util.UnstableApi
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import androidx.test.platform.app.InstrumentationRegistry
+import androidx.test.runner.lifecycle.ActivityLifecycleCallback
+import androidx.test.runner.lifecycle.ActivityLifecycleMonitorRegistry
+import androidx.test.runner.lifecycle.Stage
 import androidx.test.uiautomator.By
 import androidx.test.uiautomator.BySelector
 import androidx.test.uiautomator.Configurator
@@ -24,6 +27,7 @@ import org.junit.runner.RunWith
 import org.libremediaconverter.FailsOnEmulatorApi37
 import org.libremediaconverter.MainActivity
 import org.libremediaconverter.ui.TestTags
+import java.util.concurrent.atomic.AtomicInteger
 
 /**
  * Choosing a file, through the real system picker, and still having it after a rotation.
@@ -251,6 +255,21 @@ class SafPickerRoundTripTest {
     /** Set by the one test that rotates, read by [restoreOrientation]. See its KDoc. */
     private var rotated = false
 
+    /** Counts [MainActivity] creations from the moment [watchForRecreation] is called. */
+    private val recreations = AtomicInteger()
+
+    /**
+     * Counts a rotation's recreation without asking the Activity anything.
+     *
+     * Deliberately not `composeRule.activity`, which resolves through `scenario.onActivity` and so
+     * blocks on the main thread. Polling *that* across a recreation is a plausible reading of the
+     * 20-minute wedges in #122, which would make the obvious barrier the bug it is meant to fix.
+     * The runner's lifecycle monitor is a callback: reading the counter touches no looper.
+     */
+    private val recreationWatcher = ActivityLifecycleCallback { activity, stage ->
+        if (activity is MainActivity && stage == Stage.CREATED) recreations.incrementAndGet()
+    }
+
     /**
      * Leave the device the way it was found — and only if this test moved it.
      *
@@ -270,6 +289,7 @@ class SafPickerRoundTripTest {
      */
     @After
     fun restoreOrientation() {
+        ActivityLifecycleMonitorRegistry.getInstance().removeLifecycleCallback(recreationWatcher)
         if (!rotated) return
         device.setOrientationNatural()
         device.unfreezeRotation()
@@ -303,9 +323,11 @@ class SafPickerRoundTripTest {
         // The identity hash rather than the Activity itself, so nothing here keeps a destroyed
         // Activity reachable across the recreation it is being used to detect.
         val before = System.identityHashCode(composeRule.activity)
+        watchForRecreation()
 
         device.setOrientationLandscape()
         rotated = true
+        awaitRecreation()
         composeRule.waitForIdle()
 
         // Two guards before the assertion that matters, because both of the ways this test could
@@ -675,6 +697,36 @@ class SafPickerRoundTripTest {
      * `Condition still not satisfied after 30000 ms` — which names neither the node nor the test.
      * With the description it says which affordance never arrived, which is the whole finding.
      */
+    /** Starts counting [MainActivity] creations, so [awaitRecreation] can wait for the next one. */
+    private fun watchForRecreation() {
+        recreations.set(0)
+        ActivityLifecycleMonitorRegistry.getInstance().addLifecycleCallback(recreationWatcher)
+    }
+
+    /**
+     * Waits for the rotation to actually rebuild [MainActivity], which `waitForIdle` does not.
+     *
+     * **This is #122.** `waitForIdle()` waits for the compose hierarchy to settle. Immediately
+     * after a rotation the window manager has accepted but not yet delivered as a configuration
+     * change, the *old* Activity's composition is already idle — so it returns, `composeRule
+     * .activity` still resolves to the old instance, and the guard below reads an unchanged
+     * identity hash. That is the clean `AssertionError` seen on the API 33 gating leg of #217, and
+     * the wedges on #122 are the same race taken the other way: land while the composition is
+     * being torn down and there is nothing coherent for `waitForIdle` to settle on.
+     *
+     * A bounded wait is worth having even if that second half is wrong. It turns a 20-minute
+     * `WEDGE_TIMEOUT` — which costs the leg and names no test — into a fast failure that says which
+     * test and what it was waiting for.
+     */
+    private fun awaitRecreation() {
+        composeRule.waitUntil(
+            "the rotation did not recreate MainActivity within $RECREATION_TIMEOUT_MS ms",
+            RECREATION_TIMEOUT_MS,
+        ) {
+            recreations.get() > 0
+        }
+    }
+
     private fun awaitNode(tag: String) {
         composeRule.waitUntil("a node tagged $tag exists", APP_TIMEOUT_MS) {
             composeRule.onAllNodesWithTag(tag).fetchSemanticsNodes().isNotEmpty()
@@ -703,6 +755,15 @@ class SafPickerRoundTripTest {
          */
         const val REOPENED_TIMEOUT_MS = 10_000L
 
+        /**
+         * How long a rotation is given to destroy and rebuild the Activity.
+         *
+         * Generous against the API 33 and 34 emulators #122 was measured on, where the rotation is
+         * slow enough for the gap this bound exists to cover to be observable at all — and still
+         * two orders of magnitude inside the 1200 s `WEDGE_TIMEOUT` it replaces.
+         */
+        const val RECREATION_TIMEOUT_MS = 15_000L
+
         /**
          * How long the app is given to take the window focus back after a back press.
          *

app/src/main/java/org/libremediaconverter/LibreMediaConverterApp.kt

@@ -3,6 +3,7 @@ package org.libremediaconverter
 import android.app.Application
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
 import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.launch
 import org.libremediaconverter.convert.OutputPublisher
@@ -17,14 +18,36 @@ import org.libremediaconverter.convert.OutputPublisher
  * ever becomes a `Converted` state, or a `reset()`'s delete is cancelled along with the
  * Activity. Process start is the one moment those leftovers are reliably observable.
  */
-class LibreMediaConverterApp : Application() {
+open class LibreMediaConverterApp : Application() {
 
     /**
      * Deliberately process-lifetime and never cancelled: the work it carries is a single
      * short task that should outlive nothing in particular and be interrupted by nothing.
      * A `SupervisorJob` so a failure here could never take a sibling down with it.
+     *
+     * **`protected open` for #159.** Robolectric builds an `Application` for every test that asks
+     * for one, so on the JVM this is not one background sweep but one *per test* — all of them on
+     * `Dispatchers.IO`, all touching the same `cacheDir`, none of them joined by anything. That is
+     * a race against any test asserting about a file under `conversions/`, and it grew with the
+     * suite: wave 4 added ten Robolectric classes and took it from CI-only to roughly one local run
+     * in six. The JVM suite substitutes a scope that runs the sweep inline — see
+     * `app/src/test/resources/robolectric.properties` and `TestLibreMediaConverterApp`.
+     *
+     * A constructor parameter would be the ordinary way to inject this and is not available: the
+     * framework builds this class, so the seam has to be a member.
      */
-    private val appScope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
+    protected open val sweepScope: CoroutineScope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
+
+    /**
+     * The sweep [onCreate] last started, so a caller that needs it finished can wait for it.
+     *
+     * Nothing in production reads this — process start does not wait for its own housekeeping. It
+     * exists because the alternative for a test is a timed poll, and a poll cannot tell "the sweep
+     * has not run yet" from "the sweep ran and did nothing".
+     */
+    @Volatile
+    var startupSweep: Job? = null
+        private set
 
     override fun onCreate() {
         super.onCreate()
@@ -53,6 +76,6 @@ class LibreMediaConverterApp : Application() {
         //
         // sweepStaging() also re-reads each timestamp immediately before deleting, which
         // closes the window between listing the directory and acting on the listing.
-        appScope.launch { OutputPublisher(this@LibreMediaConverterApp).sweepStaging() }
+        startupSweep = sweepScope.launch { OutputPublisher(this@LibreMediaConverterApp).sweepStaging() }
     }
 }

app/src/main/java/org/libremediaconverter/MainActivity.kt

@@ -24,9 +24,11 @@ import androidx.compose.runtime.saveable.Saver
 import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.testTag
 import androidx.media3.common.util.UnstableApi
 import org.libremediaconverter.convert.ConverterScreen
 import org.libremediaconverter.join.JoinScreen
+import org.libremediaconverter.ui.TestTags
 import org.libremediaconverter.ui.theme.LibreMediaConverterTheme
 
 /**
@@ -114,7 +116,7 @@ internal fun AppRoot(
 
     if (useRail) {
         Row(modifier = Modifier.fillMaxSize()) {
-            NavigationRail {
+            NavigationRail(modifier = Modifier.testTag(TestTags.Shell.NAVIGATION_RAIL)) {
                 Destination.entries.forEach { item ->
                     NavigationRailItem(
                         selected = destination == item,
@@ -132,7 +134,7 @@ internal fun AppRoot(
         Scaffold(
             modifier = Modifier.fillMaxSize(),
             bottomBar = {
-                NavigationBar {
+                NavigationBar(modifier = Modifier.testTag(TestTags.Shell.NAVIGATION_BAR)) {
                     Destination.entries.forEach { item ->
                         NavigationBarItem(
                             selected = destination == item,

app/src/main/java/org/libremediaconverter/codec/AndroidDeviceCodecs.kt

@@ -21,6 +21,11 @@ import org.libremediaconverter.model.VideoCodec
  *   words, "cannot be tested for correctness". It is a hint, not a guarantee, which is
  *   why the router treats a failed hardware export as a signal to fall back rather
  *   than trusting this up front.
+ * - **An enumeration that fails answers no to everything**, which sends every job to
+ *   FFmpeg. Empty sets are not a permissive default: `canEncode` looks a MIME type up in
+ *   [hardwareEncodeMimes] and finds nothing there. That is the intended answer — FFmpeg
+ *   can do whatever Media3 can, only slower — but it is the opposite of what this class
+ *   said until #194, so it is written down rather than left to be re-derived.
  */
 class AndroidDeviceCodecs private constructor(
     private val hardwareEncodeMimes: Set<String>,
@@ -46,22 +51,62 @@ class AndroidDeviceCodecs private constructor(
 
         fun get(): AndroidDeviceCodecs = cached ?: synchronized(this) { cached ?: probe().also { cached = it } }
 
-        private fun probe(): AndroidDeviceCodecs {
+        /**
+         * One entry of the platform's codec list, reduced to what the rules below read.
+         *
+         * The five booleans and the type list are the whole of what [capabilitiesFrom] needs, and
+         * none of them can be set on a `MediaCodecInfo` from a test: Robolectric ships
+         * `MediaCodecInfoBuilder`, but it has no `setIsAlias` and no `setCanonicalName`, which is
+         * exactly the objection #133 raised against reaching this code through
+         * `ShadowMediaCodecList`. That objection is about the shadow. It does not apply to a
+         * function that takes its own entry type, which is why this exists.
+         */
+        internal data class CodecEntry(
+            val canonicalName: String,
+            val isAlias: Boolean,
+            val isEncoder: Boolean,
+            val isHardwareAccelerated: Boolean,
+            val isSoftwareOnly: Boolean,
+            val supportedTypes: List<String>,
+        )
+
+        /**
+         * The enumeration rules, over entries a caller chooses.
+         *
+         * [probe] is the only production caller and supplies the real codec list; a test supplies
+         * its own, which is the point — the two rules this class's KDoc calls out as easy to get
+         * wrong, the alias skip and the canonical-name dedup, are unreachable any other way.
+         *
+         * **`enumerate` returns a `Sequence`, deliberately.** The `runCatching` has to wrap the
+         * *iteration* rather than a list built before it, because a `MediaCodecInfo` whose
+         * properties throw does so partway through — and when that happens the codecs already read
+         * are kept. Taking a `List` here would move that throw outside the loop and silently turn a
+         * partial answer into an empty one. That behaviour predates this seam; a `List` parameter
+         * would have changed it as a side effect of a refactor.
+         *
+         * **An enumeration that fails answers restrictively, and that is deliberate.** The sets
+         * come back empty, and `"video/avc" in emptySet()` is `false`, so [canEncode] and
+         * [canDecode] both answer no and every job routes to FFmpeg. FFmpeg can do everything
+         * Media3 can, only slower, so refusing the hardware path is the safe reading of "we could
+         * not find out what this device supports". This used to log "assuming permissive", which
+         * described the opposite of what the code does.
+         */
+        internal fun capabilitiesFrom(enumerate: () -> Sequence<CodecEntry>): AndroidDeviceCodecs {
             val encoders = mutableSetOf<String>()
             val decoders = mutableSetOf<String>()
             val seen = mutableSetOf<String>()
 
             runCatching {
-                MediaCodecList(MediaCodecList.REGULAR_CODECS).codecInfos.forEach { info ->
+                enumerate().forEach { entry ->
                     // Aliases point at the same underlying codec; counting both would
                     // double-count capabilities.
-                    if (info.isAlias) return@forEach
-                    if (!seen.add(info.canonicalName)) return@forEach
+                    if (entry.isAlias) return@forEach
+                    if (!seen.add(entry.canonicalName)) return@forEach
 
-                    info.supportedTypes.forEach { mime ->
+                    entry.supportedTypes.forEach { mime ->
                         if (!mime.startsWith("video/")) return@forEach
-                        if (info.isEncoder) {
-                            if (info.isHardwareAccelerated && !info.isSoftwareOnly) {
+                        if (entry.isEncoder) {
+                            if (entry.isHardwareAccelerated && !entry.isSoftwareOnly) {
                                 encoders += mime
                             }
                         } else {
@@ -69,12 +114,32 @@ class AndroidDeviceCodecs private constructor(
                         }
                     }
                 }
-            }.onFailure { Log.w(TAG, "Codec enumeration failed; assuming permissive.", it) }
+            }.onFailure { Log.w(TAG, "Codec enumeration failed; routing everything to FFmpeg.", it) }
 
             Log.i(TAG, "Hardware video encoders: $encoders")
             return AndroidDeviceCodecs(encoders, decoders)
         }
 
+        /**
+         * The thin edge: the real codec list, mapped onto [CodecEntry] one at a time.
+         *
+         * Lazily, so a property that throws does it inside [capabilitiesFrom]'s `runCatching` and
+         * on the entry that caused it — see that function's note on why the parameter is a
+         * `Sequence`.
+         */
+        private fun probe(): AndroidDeviceCodecs = capabilitiesFrom {
+            MediaCodecList(MediaCodecList.REGULAR_CODECS).codecInfos.asSequence().map { info ->
+                CodecEntry(
+                    canonicalName = info.canonicalName,
+                    isAlias = info.isAlias,
+                    isEncoder = info.isEncoder,
+                    isHardwareAccelerated = info.isHardwareAccelerated,
+                    isSoftwareOnly = info.isSoftwareOnly,
+                    supportedTypes = info.supportedTypes.toList(),
+                )
+            }
+        }
+
         /**
          * `internal` rather than `private` so the cross-check test can ask what a [VideoCodec]
          * means here and compare it with what [NAME_TO_MIME] says the same codec's names mean.

app/src/main/java/org/libremediaconverter/convert/ConversionViewModel.kt

@@ -6,6 +6,7 @@ import android.util.Log
 import androidx.lifecycle.AndroidViewModel
 import androidx.lifecycle.viewModelScope
 import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
 import androidx.work.WorkInfo
 import androidx.work.WorkManager
 import kotlinx.coroutines.CoroutineDispatcher
@@ -71,6 +72,119 @@ data class InputFile(
     val probe: InputProbe? = null,
 )
 
+/**
+ * One update about a running conversion, as WorkManager last reported it.
+ *
+ * Only the fields [conversionStateFrom] reads — the same shape, and for the same reason, as
+ * `JobSnapshot` beside `Reattachment.choose`: the rule stays testable on the JVM because nothing
+ * in it needs a `WorkInfo`, which a test cannot readily build.
+ *
+ * [outputData] stays a `Data` rather than being unpacked into five nullable strings. It is what a
+ * test already builds with `workDataOf` everywhere in this suite, so unpacking would move the same
+ * reads without making anything easier to drive.
+ */
+internal data class ConversionUpdate(
+    val state: WorkInfo.State,
+    val progressPercent: Int,
+    val runAttemptCount: Int,
+    val outputData: Data,
+)
+
+/**
+ * What the screen should show, given what WorkManager last said about the job.
+ *
+ * ## Why this is a function rather than the body of a `collect`
+ *
+ * It was the body of one. `workManager` is built in the constructor from `WorkManager.getInstance`,
+ * `observe` is private, and nothing could hand either a chosen `WorkInfo` — so every arm below ran
+ * only when a real worker happened to produce it. A real worker produces a terminal state with
+ * well-formed output, which meant six of these arms had never been chosen by any test: the progress
+ * read, both sides of the retry check, a success with no file, a failure with nothing to say, and
+ * the two that map to a state the user cannot otherwise reach.
+ *
+ * That is the argument #141 made for `MediaProbe`'s track walk, against `WorkManager` instead of a
+ * media fixture, and it takes the same answer: the branch matrix is a pure function, and what is
+ * left needing the framework — the flow, the null check, the ownership check — is the thin edge.
+ *
+ * ## What is deliberately *not* in here
+ *
+ * The ownership check stays at the call site. Its comment is explicit that it guards the file
+ * ownership the `SUCCEEDED` arm takes, not merely the assignment, so moving it inside would change
+ * what it protects. And this function takes no responsibility for the staged file: it returns the
+ * state, and the caller reads the file off it. A pure function that deletes files is not a seam.
+ *
+ * @param cancelled where a cancellation lands, which differs for a reattached job — see [observe].
+ * @param fallbackSpec the current settings, read only when finished work predates the worker
+ *   reporting its own name and MIME type.
+ */
+@UnstableApi
+internal fun conversionStateFrom(
+    update: ConversionUpdate,
+    input: InputFile,
+    cancelled: ConversionState,
+    fallbackSpec: OutputSpec,
+): ConversionState = when (update.state) {
+    WorkInfo.State.RUNNING -> ConversionState.Converting(input, update.progressPercent)
+
+    // ENQUEUED after a run means a retry is pending. Either the six-hour foreground budget ran out
+    // mid-job, or the system refused to let the job start again while the app was in the background
+    // — the second being the likelier of the two, since it needs only a process restart. Nothing
+    // here can tell them apart, and nothing needs to: the answer is the same.
+    WorkInfo.State.ENQUEUED ->
+        if (update.runAttemptCount > 0) {
+            ConversionState.Waiting(input)
+        } else {
+            ConversionState.Converting(input, 0)
+        }
+
+    WorkInfo.State.SUCCEEDED -> convertedFrom(update.outputData, input, fallbackSpec)
+
+    // A worker that dies before it can report anything leaves no output data at all — a
+    // foreground-service start refused after a process restart is one way — and an exception's
+    // message can be an empty string. Both would read as a failure with nothing said, so blank
+    // falls back like missing does.
+    WorkInfo.State.FAILED -> ConversionState.Failed(
+        update.outputData.getString(ConversionWorker.KEY_ERROR)
+            ?.takeIf { it.isNotBlank() }
+            ?: ConversionWorker.GENERIC_FAILURE_MESSAGE,
+    )
+
+    WorkInfo.State.CANCELLED -> cancelled
+    WorkInfo.State.BLOCKED -> ConversionState.Converting(input, 0)
+}
+
+/**
+ * The `SUCCEEDED` arm, which is the only one that reads more than one field.
+ *
+ * Split out so [conversionStateFrom] stays a table of one line per state. A success with no output
+ * path is a failure: the job said it finished and named nothing, and there is no file to offer.
+ */
+@UnstableApi
+private fun convertedFrom(outputData: Data, input: InputFile, fallbackSpec: OutputSpec): ConversionState {
+    val path = outputData.getString(ConversionWorker.KEY_OUTPUT_PATH)
+        ?: return ConversionState.Failed(SUCCEEDED_WITHOUT_A_FILE_MESSAGE)
+    return ConversionState.Converted(
+        input = input,
+        staged = File(path),
+        engineUsed = outputData.getString(ConversionWorker.KEY_ENGINE_USED).orEmpty(),
+        routeReason = outputData.getString(ConversionWorker.KEY_ROUTE_REASON).orEmpty(),
+        // Work enqueued before the worker reported this carries nothing, and WorkManager keeps
+        // finished work for about a week -- so this branch is ordinary for a few days rather than a
+        // corner. It is the old derivation, kept because it is the same guess the app already made
+        // and there is genuinely nothing better available for such a job. New work never reaches it.
+        suggestedName = outputData.getString(ConversionWorker.KEY_SUGGESTED_NAME)
+            ?.takeIf { it.isNotBlank() }
+            ?: ConversionWorker.outputNameFor(input.displayName, fallbackSpec),
+        mimeType = outputData.getString(ConversionWorker.KEY_MIME_TYPE)
+            ?.takeIf { it.isNotBlank() }
+            ?: fallbackSpec.mimeType,
+    )
+}
+
+/** A job that reported success and named no file. There is nothing to offer the user to save. */
+internal const val SUCCEEDED_WITHOUT_A_FILE_MESSAGE: String =
+    "Conversion reported success but produced no file."
+
 sealed interface ConversionState {
     data object Idle : ConversionState
     data class Ready(val input: InputFile) : ConversionState
@@ -442,75 +556,24 @@ class ConversionViewModel @JvmOverloads constructor(
                 // that either. The state and `pendingStaged` are meant to refer to the same file
                 // or to no file, and this is where that stays true.
                 if (!ownership.stillHeldBy(token)) return@collect
-                _state.value = when (info.state) {
-                    WorkInfo.State.RUNNING -> ConversionState.Converting(
-                        input,
-                        info.progress.getInt(ConversionWorker.KEY_PROGRESS, 0),
-                    )
-
-                    // ENQUEUED after a run means a retry is pending. Either the six-hour
-                    // foreground budget ran out mid-job, or the system refused to let the job
-                    // start again while the app was in the background — the second being the
-                    // likelier of the two, since it needs only a process restart. Nothing here
-                    // can tell them apart, and nothing needs to: the answer is the same.
-                    WorkInfo.State.ENQUEUED ->
-                        if (info.runAttemptCount > 0) {
-                            ConversionState.Waiting(input)
-                        } else {
-                            ConversionState.Converting(input, 0)
-                        }
-
-                    WorkInfo.State.SUCCEEDED -> {
-                        val path = info.outputData.getString(ConversionWorker.KEY_OUTPUT_PATH)
-                        if (path == null) {
-                            ConversionState.Failed("Conversion reported success but produced no file.")
-                        } else {
-                            val staged = File(path)
-                            // Take responsibility for the file at the same moment the state
-                            // starts referring to it, so the two cannot disagree.
-                            pendingStaged = staged
-                            ConversionState.Converted(
-                                input = input,
-                                staged = staged,
-                                engineUsed = info.outputData
-                                    .getString(ConversionWorker.KEY_ENGINE_USED).orEmpty(),
-                                routeReason = info.outputData
-                                    .getString(ConversionWorker.KEY_ROUTE_REASON).orEmpty(),
-                                suggestedName = info.outputData
-                                    .getString(ConversionWorker.KEY_SUGGESTED_NAME)
-                                    ?.takeIf { it.isNotBlank() }
-                                    // Work enqueued before the worker reported this carries
-                                    // nothing, and WorkManager keeps finished work for about a
-                                    // week -- so this branch is ordinary for a few days rather
-                                    // than a corner. It is the old derivation, kept because it is
-                                    // the same guess the app already made and there is genuinely
-                                    // nothing better available for such a job. New work never
-                                    // reaches it.
-                                    ?: ConversionWorker.outputNameFor(
-                                        input.displayName,
-                                        _settings.value.spec,
-                                    ),
-                                mimeType = info.outputData
-                                    .getString(ConversionWorker.KEY_MIME_TYPE)
-                                    ?.takeIf { it.isNotBlank() }
-                                    ?: _settings.value.spec.mimeType,
-                            )
-                        }
-                    }
-
-                    // A worker that dies before it can report anything leaves no output data at
-                    // all — a foreground-service start refused after a process restart is one
-                    // way — and an exception's message can be an empty string. Both would read
-                    // as a failure with nothing said, so blank falls back like missing does.
-                    WorkInfo.State.FAILED -> ConversionState.Failed(
-                        info.outputData.getString(ConversionWorker.KEY_ERROR)
-                            ?.takeIf { it.isNotBlank() }
-                            ?: "Conversion failed.",
-                    )
-
-                    WorkInfo.State.CANCELLED -> cancelled
-                    WorkInfo.State.BLOCKED -> ConversionState.Converting(input, 0)
-                }
+                val next = conversionStateFrom(
+                    ConversionUpdate(
+                        state = info.state,
+                        progressPercent = info.progress.getInt(ConversionWorker.KEY_PROGRESS, 0),
+                        runAttemptCount = info.runAttemptCount,
+                        outputData = info.outputData,
+                    ),
+                    input = input,
+                    cancelled = cancelled,
+                    fallbackSpec = _settings.value.spec,
+                )
+                // Take responsibility for the file at the same moment the state starts referring
+                // to it, so the two cannot disagree. Read off the result rather than assigned
+                // inside the mapping: `Converted` is the only state that carries a staged file, so
+                // "the state and `pendingStaged` refer to the same file or to no file" is now the
+                // shape of the code rather than a rule two branches have to keep.
+                if (next is ConversionState.Converted) pendingStaged = next.staged
+                _state.value = next
             }
         }
     }
@@ -565,7 +628,7 @@ class ConversionViewModel @JvmOverloads constructor(
                 // than a fresh handle for the second failure's sake: a retry that fails again
                 // lands back here still carrying the file, not on a bare Failed that would take
                 // the offer away.
-                _state.value = ConversionState.Failed(e.message ?: "Could not save the file.", pending)
+                _state.value = ConversionState.Failed(e.message ?: SAVE_FAILED_MESSAGE, pending)
             }
         }
     }
@@ -610,13 +673,23 @@ class ConversionViewModel @JvmOverloads constructor(
         else -> null
     }
 
-    private fun currentInput(): InputFile? = when (val s = _state.value) {
-        is ConversionState.Ready -> s.input
-        is ConversionState.Converting -> s.input
-        is ConversionState.Waiting -> s.input
-        is ConversionState.Converted -> s.input
-        else -> null
-    }
+    /**
+     * The input `convert()` may act on, which is only ever the one on a `Ready` screen.
+     *
+     * This used to answer for `Converting`, `Waiting` and `Converted` as well. Those arms were not
+     * reachable by tapping Convert -- the button renders only in the `Ready` branch -- but they
+     * were reachable through the POST_NOTIFICATIONS **result**, which `ConverterScreen.kt:91` wires
+     * to `convert()` rather than to the button. Reaching one of them enqueued a *second* job over a
+     * live one: `activeWorkId` was overwritten, and the first job kept running with its foreground
+     * notification orphaned and nothing left holding its id to cancel it.
+     *
+     * Narrowed under #202 rather than tested as it stood, because a test written against the old
+     * shape would have frozen the double-enqueue as intended behaviour -- the F1/F5 failure mode.
+     *
+     * `JoinViewModel.join()` has been `(_state.value as? JoinState.Ready)?.inputs ?: return` all
+     * along. The two screens are the same shape and only one of them was over-general.
+     */
+    private fun currentInput(): InputFile? = (_state.value as? ConversionState.Ready)?.input
 
     private companion object {
         /**

app/src/main/java/org/libremediaconverter/convert/ConverterScreen.kt

@@ -94,24 +94,61 @@ fun ConverterScreen(modifier: Modifier = Modifier, viewModel: ConversionViewMode
         state = state,
         settings = settings,
         validation = validation,
-        actions = ConverterActions(
+        actions = converterActions(
+            viewModel = viewModel,
             onPickInput = { pickInput.launch(arrayOf("*/*")) },
-            onPreset = viewModel::setPreset,
-            onContainer = viewModel::setContainer,
-            onVideoCodec = viewModel::setVideoCodec,
-            onAudioCodec = viewModel::setAudioCodec,
-            onSuggestion = viewModel::applySuggestion,
-            onQuality = viewModel::setQuality,
-            onEnginePreference = viewModel::setEnginePreference,
             onConvert = { requestNotifications.launch(Manifest.permission.POST_NOTIFICATIONS) },
-            onCancel = viewModel::cancel,
             onSave = { suggestedName -> chooseDestination.launch(suggestedName) },
-            onReset = viewModel::reset,
         ),
         modifier = modifier,
     )
 }
 
+/**
+ * Which of the ViewModel's methods each affordance on the screen calls.
+ *
+ * ## Why this is a function rather than an argument list
+ *
+ * It was an argument list, inside [ConverterScreen], which no test reached: `ConverterScreenContent`
+ * builds its own [ConverterActions], so every test in the suite drove the stateless inner and none
+ * of them ever saw the wiring.
+ *
+ * Most of the list is safe without a test, and saying so is more useful than pretending otherwise:
+ * `onContainer`, `onVideoCodec`, `onAudioCodec`, `onPreset`, `onSuggestion`, `onQuality` and
+ * `onEnginePreference` each take a distinct type, so binding one to another's setter does not
+ * compile. Verified rather than assumed — swapping `onVideoCodec` and `onAudioCodec` fails with
+ * *"Inapplicable candidate(s): fun setAudioCodec(codec: AudioCodec)"*.
+ *
+ * **[ConverterActions.onCancel] and [ConverterActions.onReset] are the exception.** Both are
+ * `() -> Unit`, so swapping them compiles silently — also verified — and ships a Cancel button that
+ * throws the conversion away and a Start-over button that leaves it on screen. That pair is what
+ * `ConverterWiringTest` exists for.
+ *
+ * The three launcher-backed actions stay parameters: they need an `ActivityResultLauncher`, which
+ * is the part that genuinely needs the composition, and keeping them out means the rest can be
+ * checked without one.
+ */
+@UnstableApi
+internal fun converterActions(
+    viewModel: ConversionViewModel,
+    onPickInput: () -> Unit,
+    onConvert: () -> Unit,
+    onSave: (suggestedName: String) -> Unit,
+): ConverterActions = ConverterActions(
+    onPickInput = onPickInput,
+    onPreset = viewModel::setPreset,
+    onContainer = viewModel::setContainer,
+    onVideoCodec = viewModel::setVideoCodec,
+    onAudioCodec = viewModel::setAudioCodec,
+    onSuggestion = viewModel::applySuggestion,
+    onQuality = viewModel::setQuality,
+    onEnginePreference = viewModel::setEnginePreference,
+    onConvert = onConvert,
+    onCancel = viewModel::cancel,
+    onSave = onSave,
+    onReset = viewModel::reset,
+)
+
 /**
  * Everything [ConverterScreenContent] can ask for, in one value.
  *

app/src/main/java/org/libremediaconverter/convert/MediaProbe.kt

@@ -50,19 +50,42 @@ object MediaProbe {
     )
 
     fun probe(context: Context, uri: Uri): InputProbe {
-        val extracted = probeWithExtractor(context, uri)
-        val info = probeWithFFprobe(context, uri)
+        val merged = merge(probeWithExtractor(context, uri), probeWithFFprobe(context, uri))
+        if (merged.kind == InputKind.UNPARSEABLE) {
+            // Not a failure: an unparseable input is a strong signal that this job belongs on
+            // FFmpeg. Reporting an unknown codec makes the router say so.
+            Log.i(TAG, "Neither MediaExtractor nor FFprobe could read $uri; routing to FFmpeg.")
+        }
+        return merged
+    }
 
+    /**
+     * What the two probes together say about one input.
+     *
+     * A pure function, and `internal` for the same reason [extractedFrom] is: the precedence rules
+     * below are the answer to "which probe wins", and until this was pulled out of [probe] the only
+     * way to ask was to have a real `MediaExtractor` and a real FFprobe **disagree**, which nothing
+     * on any source set can arrange. `RemuxTest` drives this on a device against committed
+     * fixtures, but only ever with one probe answering and the other agreeing or also failing --
+     * so every elvis here was taken in one direction and never the other.
+     *
+     * The rules, each of which is a decision rather than an accident:
+     *
+     * - **The extractor wins on codecs.** It is the platform's own view of what it can decode,
+     *   which is the thing the router is about to ask about. FFprobe's name for the same track can
+     *   differ, and the copy planner keys off these strings.
+     * - **FFprobe alone reports the container.** `MediaExtractor` cannot, which is why [InputProbe]
+     *   carries a nullable one and `CopyPlanner` treats null as "container unknown".
+     * - **Duration is the larger of the two**, not the first non-zero. Either probe can report zero
+     *   for a file the other times correctly, and a zero duration makes the FFmpeg progress
+     *   percentage undefined.
+     */
+    internal fun merge(extracted: Extracted?, info: FFprobeInfo?): InputProbe {
         val videoCodec = extracted?.videoCodec ?: info?.videoCodec
         val audioCodec = extracted?.audioCodec ?: info?.audioCodec
         val kind = classify(extracted, info)
 
-        if (kind == InputKind.UNPARSEABLE) {
-            // Not a failure: an unparseable input is a strong signal that this job belongs on
-            // FFmpeg. Reporting an unknown codec makes the router say so.
-            Log.i(TAG, "Neither MediaExtractor nor FFprobe could read $uri; routing to FFmpeg.")
-            return UNREADABLE
-        }
+        if (kind == InputKind.UNPARSEABLE) return UNREADABLE
 
         return InputProbe(
             videoCodec = videoCodec,
@@ -83,7 +106,7 @@ object MediaProbe {
      * audio file and a corrupt file indistinguishable. The source-info card cannot describe either
      * honestly until they are separate, and neither can the copy planner.
      */
-    private fun classify(extracted: Extracted?, info: FFprobeInfo?): InputKind = when {
+    internal fun classify(extracted: Extracted?, info: FFprobeInfo?): InputKind = when {
         info?.isImage == true -> InputKind.IMAGE
         extracted == null && info == null -> InputKind.UNPARSEABLE
         (extracted?.videoCodec ?: info?.videoCodec) != null -> InputKind.VIDEO
@@ -92,7 +115,12 @@ object MediaProbe {
         else -> InputKind.UNPARSEABLE
     }
 
-    private class Extracted(
+    /**
+     * `internal` rather than `private` so [extractedFrom] can be named from a test. The JVM test
+     * source set is a friend of `main`, so this stays invisible outside the module — the precedent
+     * is `MainActivity`'s `Destination`, and [containerFrom] beside it.
+     */
+    internal class Extracted(
         val videoCodec: String?,
         val audioCodec: String?,
         val durationMs: Long,
@@ -100,33 +128,55 @@ object MediaProbe {
         val height: Int,
     )
 
+    /**
+     * What a set of track formats says about a file.
+     *
+     * Split out of [probeWithExtractor] so the rules below can be tested against tracks a test
+     * *chooses*, rather than against whatever the committed fixtures happen to contain. The device
+     * tests exercise this through real files; none of them can construct a two-video-track input,
+     * a track that omits its duration, or an audio-before-video ordering on purpose.
+     *
+     * Three rules live here, and each is a decision rather than plumbing:
+     *
+     * - **First track of a type wins.** `video == null` is the whole guard. A file with two video
+     *   tracks must report the first, because that is the one an engine will transcode.
+     * - **Duration is the maximum across tracks**, not the first one found or the last. A file
+     *   whose audio outlasts its video is ordinary, and reporting the video's length would cut the
+     *   progress bar short.
+     * - **A track that omits `KEY_DURATION` contributes nothing** rather than zero. `MediaExtractor`
+     *   omits it for plenty of real tracks — see `MediaProbeTrackFieldsTest` — and `maxOf` against a
+     *   fabricated 0 would still be correct here, but reading a key that is absent is not.
+     */
+    internal fun extractedFrom(formats: List<MediaFormat>): Extracted {
+        var video: String? = null
+        var audio: String? = null
+        var durationUs = 0L
+        var width = 0
+        var height = 0
+
+        for (format in formats) {
+            val mime = format.getString(MediaFormat.KEY_MIME).orEmpty()
+            if (format.containsKey(MediaFormat.KEY_DURATION)) {
+                durationUs = maxOf(durationUs, format.getLong(MediaFormat.KEY_DURATION))
+            }
+            when {
+                mime.startsWith("video/") && video == null -> {
+                    video = shortName(mime)
+                    width = format.intOr(MediaFormat.KEY_WIDTH)
+                    height = format.intOr(MediaFormat.KEY_HEIGHT)
+                }
+
+                mime.startsWith("audio/") && audio == null -> audio = shortName(mime)
+            }
+        }
+        return Extracted(video, audio, durationUs / US_PER_MS, width, height)
+    }
+
     private fun probeWithExtractor(context: Context, uri: Uri): Extracted? {
         val extractor = MediaExtractor()
         return try {
             extractor.setDataSource(context, uri, null)
-            var video: String? = null
-            var audio: String? = null
-            var durationUs = 0L
-            var width = 0
-            var height = 0
-
-            for (i in 0 until extractor.trackCount) {
-                val format = extractor.getTrackFormat(i)
-                val mime = format.getString(MediaFormat.KEY_MIME).orEmpty()
-                if (format.containsKey(MediaFormat.KEY_DURATION)) {
-                    durationUs = maxOf(durationUs, format.getLong(MediaFormat.KEY_DURATION))
-                }
-                when {
-                    mime.startsWith("video/") && video == null -> {
-                        video = shortName(mime)
-                        width = format.intOr(MediaFormat.KEY_WIDTH)
-                        height = format.intOr(MediaFormat.KEY_HEIGHT)
-                    }
-
-                    mime.startsWith("audio/") && audio == null -> audio = shortName(mime)
-                }
-            }
-            Extracted(video, audio, durationUs / US_PER_MS, width, height)
+            extractedFrom(extractor.trackFormats())
         } catch (e: Exception) {
             Log.i(TAG, "Platform extractor could not read $uri.", e)
             null
@@ -135,7 +185,11 @@ object MediaProbe {
         }
     }
 
-    private class FFprobeInfo(
+    /**
+     * `internal` rather than `private` for the same reason [Extracted] is, and it should have been
+     * from the start: [merge] cannot be named from a test while half its signature is private.
+     */
+    internal class FFprobeInfo(
         val container: Container?,
         val videoCodec: String?,
         val audioCodec: String?,
@@ -167,12 +221,39 @@ object MediaProbe {
         null
     }
 
-    private fun readMediaInformation(path: String): FFprobeInfo? {
-        // ffmpeg-kit-next is compiled from Kotlin with private backing fields, so these have to go
-        // through the Java getters rather than property syntax.
-        val info: MediaInformation = FFprobeKit.getMediaInformation(path).getMediaInformation()
-            ?: return null
+    /**
+     * The thin edge: spawn FFprobe, hand what it said to [ffprobeInfoFrom].
+     *
+     * Everything device-bound is on this line and the null check under it. What FFprobe *said* is a
+     * `MediaInformation`, which is an ordinary object over a `JSONObject` — so the reading of it is
+     * a decision a test can choose the inputs for, and it lives below rather than here.
+     */
+    private fun readMediaInformation(path: String): FFprobeInfo? =
+        FFprobeKit.getMediaInformation(path).getMediaInformation()?.let(::ffprobeInfoFrom)
 
+    /**
+     * What FFprobe's answer means, as a function of the answer alone.
+     *
+     * `internal` for the same reason [Extracted] and [FFprobeInfo] are: a test cannot name it
+     * otherwise, and the JVM test source set is a friend of `main`.
+     *
+     * **JVM-safe, verified rather than assumed.** `javap` over the committed AAR's runtime jar:
+     * `MediaInformation(JSONObject, List<StreamInformation>, List<Chapter>)` and
+     * `StreamInformation(JSONObject)` are plain public constructors, and neither class's `<clinit>`
+     * touches the native library — so a test builds its own without `libffmpegkit` being present.
+     * That is the whole reason this split is worth making: `readMediaInformation` was 114 missed
+     * instructions and 24 missed branches, of which exactly one line needed a device.
+     *
+     * The subtle part is the **second argument to [containerFrom]**. `matroska,webm` is reported
+     * for both MKV and WebM — they share a demuxer — so the video codec is the only thing that
+     * separates them, and dropping it silently turns every VP9 WebM into an MKV. `containerFrom`
+     * has thirty-three covered branches of its own and none of them can notice that, because the
+     * mistake is at the call rather than in the callee.
+     *
+     * ffmpeg-kit-next is compiled from Kotlin with private backing fields, so these go through the
+     * Java getters rather than property syntax.
+     */
+    internal fun ffprobeInfoFrom(info: MediaInformation): FFprobeInfo {
         val streams = info.getStreams().orEmpty()
         val video = streams.firstOrNull { it.getType() == "video" }
         val audio = streams.firstOrNull { it.getType() == "audio" }
@@ -269,25 +350,7 @@ object MediaProbe {
         val extractor = MediaExtractor()
         return try {
             extractor.setDataSource(context, uri, null)
-            var video: String? = null
-            var audio: String? = null
-            var width = 0
-            var height = 0
-            var fps = 0
-
-            for (i in 0 until extractor.trackCount) {
-                val format = extractor.getTrackFormat(i)
-                val mime = format.getString(MediaFormat.KEY_MIME).orEmpty()
-                if (mime.startsWith("video/") && video == null) {
-                    video = shortName(mime)
-                    width = format.intOr(MediaFormat.KEY_WIDTH)
-                    height = format.intOr(MediaFormat.KEY_HEIGHT)
-                    fps = format.intOr(MediaFormat.KEY_FRAME_RATE)
-                } else if (mime.startsWith("audio/") && audio == null) {
-                    audio = shortName(mime)
-                }
-            }
-            ConcatInput(video, audio, width, height, fps)
+            concatInputFrom(extractor.trackFormats())
         } catch (e: Exception) {
             Log.i(TAG, "Could not probe $uri for concat; will re-encode.", e)
             ConcatInput(null, null, 0, 0, 0)
@@ -296,6 +359,45 @@ object MediaProbe {
         }
     }
 
+    /**
+     * The join flow's read of the same track formats. See [extractedFrom] for why this is separate
+     * from the extractor.
+     *
+     * Deliberately **not** folded into [extractedFrom] despite the overlap. This one reads frame
+     * rate and does not read duration; that one reads duration and does not read frame rate. A
+     * merged version would have to compute both for every caller, and `ConcatPlanner` treats an
+     * unknown frame rate as "cannot prove a match" — so a field this flow does not need must not
+     * start arriving as a number.
+     */
+    internal fun concatInputFrom(formats: List<MediaFormat>): ConcatInput {
+        var video: String? = null
+        var audio: String? = null
+        var width = 0
+        var height = 0
+        var fps = 0
+
+        for (format in formats) {
+            val mime = format.getString(MediaFormat.KEY_MIME).orEmpty()
+            if (mime.startsWith("video/") && video == null) {
+                video = shortName(mime)
+                width = format.intOr(MediaFormat.KEY_WIDTH)
+                height = format.intOr(MediaFormat.KEY_HEIGHT)
+                fps = format.intOr(MediaFormat.KEY_FRAME_RATE)
+            } else if (mime.startsWith("audio/") && audio == null) {
+                audio = shortName(mime)
+            }
+        }
+        return ConcatInput(video, audio, width, height, fps)
+    }
+
+    /**
+     * Every track format this extractor holds, read once.
+     *
+     * The thin edge the two pure functions above leave behind: a `trackCount` and a
+     * `getTrackFormat` per index, which is the whole of what needs a real `MediaExtractor`.
+     */
+    private fun MediaExtractor.trackFormats(): List<MediaFormat> = (0 until trackCount).map(::getTrackFormat)
+
     /**
      * One track property as an Int, or [fallback] when the format has no Int to give.
      *

app/src/main/java/org/libremediaconverter/convert/OutputPublisher.kt

@@ -5,6 +5,7 @@ import android.net.Uri
 import android.provider.DocumentsContract
 import android.provider.OpenableColumns
 import java.io.File
+import java.io.OutputStream
 
 /**
  * What a save has to say when the staged file is not there any more.
@@ -23,6 +24,20 @@ const val STAGED_FILE_GONE_MESSAGE: String =
     "The finished file is no longer in the cache, so there is nothing left to save. " +
         "Start over to make it again."
 
+/**
+ * What to tell the user when the copy into their chosen destination did not finish.
+ *
+ * A fallback, not the usual message: `publish` throws with a real reason most of the time — the
+ * volume filled, the provider revoked the grant — and that reason is better than this. This is for
+ * the exception that arrives with nothing to say, which would otherwise reach the screen as an
+ * empty failure.
+ *
+ * Kept next to [STAGED_FILE_GONE_MESSAGE] for exactly the reason that one names: **both ViewModels
+ * need it**, and saving is what it is about. It was written out twice before — `ConversionViewModel`
+ * and `JoinViewModel` each carried their own copy of the literal, agreeing by coincidence.
+ */
+const val SAVE_FAILED_MESSAGE: String = "Could not save the file."
+
 /**
  * A staged file that is still there to be saved, and everything the save dialog needs to offer it.
  *
@@ -170,7 +185,7 @@ open class OutputPublisher(private val context: Context) {
     open fun publish(staged: File, destination: Uri) {
         val destinationWasEmpty = destinationIsKnownEmpty(destination)
         try {
-            val out = context.contentResolver.openOutputStream(destination)
+            val out = openDestination(destination)
                 ?: error("Could not open destination for writing: $destination")
             out.use { sink -> staged.inputStream().use { source -> source.copyTo(sink) } }
         } catch (failure: Throwable) {
@@ -179,6 +194,22 @@ open class OutputPublisher(private val context: Context) {
         }
     }
 
+    /**
+     * Opens [destination] for writing, or null when the provider will not.
+     *
+     * A seam, and a narrow one: it exists because `openOutputStream` has **two** ways of refusing
+     * and only one of them is reachable from a test otherwise. A provider that has gone away throws
+     * `FileNotFoundException` from inside the call; a provider that is present and declines returns
+     * null. The two are not interchangeable here — the `?: error(...)` above is the only thing that
+     * turns the second into a failure rather than an NPE further down — and no fake provider can be
+     * asked to produce a null return on demand.
+     *
+     * `protected open` rather than injected, matching `hasSpaceFor` and `createStagingFile`:
+     * `WorkerStubs.kt`'s publishers already override one method to force one condition.
+     */
+    protected open fun openDestination(destination: Uri): OutputStream? =
+        context.contentResolver.openOutputStream(destination)
+
     /**
      * True only when the destination is *positively known* to hold no bytes yet.
      *
@@ -256,7 +287,7 @@ open class OutputPublisher(private val context: Context) {
     open fun sweepStaging(nowMs: Long = System.currentTimeMillis()) {
         val dir = stagingDir
         val listing = dir.listFiles() ?: return
-        val entries = listing.map { StagingSweep.Entry(it.name, it.lastModified()) }
+        val entries = snapshot(listing)
         StagingSweep.collectable(entries, nowMs).forEach { name ->
             val file = File(dir, name)
             // Re-read the timestamp rather than trusting the snapshot above. Between the
@@ -268,6 +299,22 @@ open class OutputPublisher(private val context: Context) {
         }
     }
 
+    /**
+     * The name and age of everything [sweepStaging] found, read once.
+     *
+     * A seam for the *race*, not for the clock — [sweepStaging] already takes `nowMs`, so the clock
+     * is the caller's. What has no seam otherwise is the window between this snapshot and the
+     * per-file re-read below it, and that window is the entire reason the re-read exists.
+     *
+     * **It has to be here and not around `listFiles()`.** A test that changes a file before the
+     * listing, or during it, changes what `StagingSweep.collectable` is given — so the file is
+     * never proposed for deletion and the re-read is never reached. The race being modelled is a
+     * file that *was* collectable when the snapshot was taken and is not by the time the delete
+     * comes round, which is exactly one worker resuming in this same process.
+     */
+    protected open fun snapshot(listing: Array<File>): List<StagingSweep.Entry> =
+        listing.map { StagingSweep.Entry(it.name, it.lastModified()) }
+
     private fun File.canonicalOrAbsolute(): File = runCatching { canonicalFile }.getOrDefault(absoluteFile)
 
     private companion object {

app/src/main/java/org/libremediaconverter/convert/Transcoders.kt

@@ -4,6 +4,7 @@ import android.content.Context
 import android.net.Uri
 import androidx.media3.common.util.UnstableApi
 import org.libremediaconverter.codec.AndroidDeviceCodecs
+import org.libremediaconverter.ffmpeg.ConcatEngine
 import org.libremediaconverter.ffmpeg.FFmpegEngine
 import org.libremediaconverter.model.ConversionRequest
 import org.libremediaconverter.model.DeviceCodecs
@@ -40,6 +41,27 @@ interface SoftwareTranscoder {
     )
 }
 
+/**
+ * The join path. Implemented by [org.libremediaconverter.ffmpeg.ConcatEngine].
+ *
+ * Added last of the three, and the gap it closes was measured rather than guessed:
+ * `PerJobStagingTest`'s KDoc records that reverting `ConcatWorker` to a constant staging name left
+ * all 257 tests green, because nothing in the JVM suite can get past a `ConcatEngine` constructed
+ * in place. Everything after that line -- the failure mapping, the message fallback, the staged
+ * delete -- was untested on every source set.
+ *
+ * The result type stays nested in the implementation rather than being lifted here. Moving it would
+ * touch every call site to buy nothing: what a test needs is the ability to *not* run FFmpeg, and
+ * that is the method, not the type.
+ */
+interface ConcatJoiner {
+    suspend fun join(
+        inputs: List<Uri>,
+        output: File,
+        format: OutputFormat = OutputFormat.MP4_H264,
+    ): ConcatEngine.Result
+}
+
 /**
  * The seam that lets tests force failure paths.
  *
@@ -69,6 +91,9 @@ object ConversionDependencies {
     @Volatile
     var software: () -> SoftwareTranscoder = { FFmpegEngine() }
 
+    @Volatile
+    var concat: (Context) -> ConcatJoiner = { ConcatEngine(it) }
+
     @Volatile
     var publisher: (Context) -> OutputPublisher = { OutputPublisher(it) }
 
@@ -103,6 +128,7 @@ object ConversionDependencies {
     fun reset() {
         hardware = { Media3Engine(it) }
         software = { FFmpegEngine() }
+        concat = { ConcatEngine(it) }
         publisher = { OutputPublisher(it) }
         deviceCodecs = { AndroidDeviceCodecs.get() }
         probe = { context, uri -> MediaProbe.probe(context, uri) }

app/src/main/java/org/libremediaconverter/ffmpeg/ConcatEngine.kt

@@ -5,8 +5,8 @@ import android.net.Uri
 import android.util.Log
 import com.arthenica.ffmpegkit.FFmpegKit
 import com.arthenica.ffmpegkit.FFmpegKitConfig
-import com.arthenica.ffmpegkit.ReturnCode
 import kotlinx.coroutines.suspendCancellableCoroutine
+import org.libremediaconverter.convert.ConcatJoiner
 import org.libremediaconverter.convert.MediaProbe
 import org.libremediaconverter.convert.StagingNames
 import org.libremediaconverter.model.ConcatPlanner
@@ -24,11 +24,11 @@ import kotlin.coroutines.resumeWithException
  * reliably fail when they differ — it can emit a file whose later segments are
  * garbled. See [ConcatPlanner].
  */
-class ConcatEngine(private val context: Context) {
+class ConcatEngine(private val context: Context) : ConcatJoiner {
 
     data class Result(val strategy: ConcatStrategy, val output: File)
 
-    suspend fun join(inputs: List<Uri>, output: File, format: OutputFormat = OutputFormat.MP4_H264): Result {
+    override suspend fun join(inputs: List<Uri>, output: File, format: OutputFormat): Result {
         require(inputs.size >= 2) { "Joining needs at least two files." }
 
         val paths = inputs.map { uri ->
@@ -65,16 +65,16 @@ class ConcatEngine(private val context: Context) {
     private suspend fun execute(args: List<String>) = suspendCancellableCoroutine { cont ->
         Log.i(TAG, "ffmpeg ${args.joinToString(" ")}")
         val session = FFmpegKit.executeWithArgumentsAsync(args.toTypedArray()) { completed ->
-            val rc = completed.getReturnCode()
-            when {
-                ReturnCode.isSuccess(rc) -> cont.resume(Unit)
-                ReturnCode.isCancel(rc) -> cont.cancel()
-                else -> cont.resumeWithException(
-                    FFmpegEngine.FFmpegException(
-                        "Joining failed (${rc?.value}): " +
-                            completed.getAllLogsAsString(LOG_TAIL_LIMIT).orEmpty(),
-                    ),
-                )
+            val outcome = sessionOutcome(
+                rc = completed.getReturnCode(),
+                prefix = "Joining",
+                failStackTrace = { completed.getFailStackTrace() },
+                logTail = { completed.getAllLogsAsString(LOG_TAIL_LIMIT) },
+            )
+            when (outcome) {
+                SessionOutcome.Success -> cont.resume(Unit)
+                SessionOutcome.Cancelled -> cont.cancel()
+                is SessionOutcome.Failed -> cont.resumeWithException(FFmpegEngine.FFmpegException(outcome.message))
             }
         }
         cont.invokeOnCancellation { FFmpegKit.cancel(session.getSessionId()) }

app/src/main/java/org/libremediaconverter/ffmpeg/FFmpegEngine.kt

@@ -4,7 +4,6 @@ import android.util.Log
 import com.arthenica.ffmpegkit.FFmpegKit
 import com.arthenica.ffmpegkit.FFmpegKitConfig
 import com.arthenica.ffmpegkit.Level
-import com.arthenica.ffmpegkit.ReturnCode
 import kotlinx.coroutines.suspendCancellableCoroutine
 import org.libremediaconverter.convert.SoftwareTranscoder
 import org.libremediaconverter.model.ConversionRequest
@@ -51,19 +50,16 @@ class FFmpegEngine : SoftwareTranscoder {
         val session = FFmpegKit.executeWithArgumentsAsync(
             args.toTypedArray(),
             { completed ->
-                val rc = completed.getReturnCode()
-                when {
-                    ReturnCode.isSuccess(rc) -> cont.resume(Unit)
-                    ReturnCode.isCancel(rc) ->
-                        cont.cancel()
-                    else -> cont.resumeWithException(
-                        FFmpegException(
-                            "FFmpeg failed (${rc?.value}): " +
-                                completed.getFailStackTrace().orEmpty().ifBlank {
-                                    completed.getAllLogsAsString(LOG_TAIL_LIMIT).orEmpty()
-                                },
-                        ),
-                    )
+                val outcome = sessionOutcome(
+                    rc = completed.getReturnCode(),
+                    prefix = "FFmpeg",
+                    failStackTrace = { completed.getFailStackTrace() },
+                    logTail = { completed.getAllLogsAsString(LOG_TAIL_LIMIT) },
+                )
+                when (outcome) {
+                    SessionOutcome.Success -> cont.resume(Unit)
+                    SessionOutcome.Cancelled -> cont.cancel()
+                    is SessionOutcome.Failed -> cont.resumeWithException(FFmpegException(outcome.message))
                 }
             },
             { log -> Log.d(TAG, log.message.trimEnd()) },

app/src/main/java/org/libremediaconverter/ffmpeg/SessionOutcome.kt

@@ -0,0 +1,54 @@
+package org.libremediaconverter.ffmpeg
+
+import com.arthenica.ffmpegkit.ReturnCode
+
+/**
+ * What a finished FFmpegKit session means, as a function of its return code.
+ *
+ * Both engines had their own copy of this `when`, twelve lines apart in two files, and the copies
+ * had drifted: [FFmpegEngine] preferred the fail stack trace and fell back to the log tail, while
+ * [ConcatEngine] only ever read the log tail. Neither was tested — both live inside a callback
+ * handed to `FFmpegKit`, which does not run on the JVM — so the divergence was invisible.
+ *
+ * #203 decided to unify on the stack trace, so a join failure now carries the diagnostics a
+ * conversion failure always did. The *prefix* stays per-engine: unifying the strategy must not
+ * unify the sentence, since "FFmpeg failed" and "Joining failed" describe different jobs.
+ */
+internal sealed interface SessionOutcome {
+
+    /** rc 0. The suspension resumes normally. */
+    data object Success : SessionOutcome
+
+    /** rc 255. The suspension is cancelled rather than failed — the user asked for this. */
+    data object Cancelled : SessionOutcome
+
+    /** Anything else, with the sentence the user is shown. */
+    data class Failed(val message: String) : SessionOutcome
+}
+
+/**
+ * Maps a return code onto the outcome, and builds the failure sentence when there is one.
+ *
+ * **The two message parts arrive as lambdas, deliberately.** `getAllLogsAsString` and
+ * `getFailStackTrace` are calls onto a native session, and only the failure arm needs either. Taking
+ * them by value would put both on the happy path of every successful conversion, which is a cost the
+ * shape this replaced did not have — the old code read them inside the `else` branch. That is the
+ * same reason [org.libremediaconverter.codec.AndroidDeviceCodecs.capabilitiesFrom] takes a
+ * `Sequence`: a seam should not change what runs when.
+ *
+ * A null [rc] is a real input rather than a defensive one — `getReturnCode()` is nullable, and a
+ * session killed before it reported anything has none. It is neither success nor cancellation, so
+ * it fails, and the sentence says `null` where the number would be.
+ */
+internal fun sessionOutcome(
+    rc: ReturnCode?,
+    prefix: String,
+    failStackTrace: () -> String?,
+    logTail: () -> String?,
+): SessionOutcome = when {
+    ReturnCode.isSuccess(rc) -> SessionOutcome.Success
+    ReturnCode.isCancel(rc) -> SessionOutcome.Cancelled
+    else -> SessionOutcome.Failed(
+        "$prefix failed (${rc?.value}): " + failStackTrace().orEmpty().ifBlank { logTail().orEmpty() },
+    )
+}

app/src/main/java/org/libremediaconverter/join/JoinScreen.kt

@@ -56,17 +56,38 @@ fun JoinScreen(modifier: Modifier = Modifier, viewModel: JoinViewModel = viewMod
 
     JoinScreenContent(
         state = state,
-        actions = JoinActions(
+        actions = joinActions(
+            viewModel = viewModel,
             onPickInputs = { pickInputs.launch(arrayOf("video/*")) },
-            onJoin = viewModel::join,
-            onCancel = viewModel::cancel,
             onSave = { suggestedName -> chooseDestination.launch(suggestedName) },
-            onReset = viewModel::reset,
         ),
         modifier = modifier,
     )
 }
 
+/**
+ * Which of the ViewModel's methods each affordance on the join screen calls.
+ *
+ * The join-side twin of `converterActions`, and the transposition risk here is worse: **three**
+ * `() -> Unit` bindings rather than two. `onJoin`, `onCancel` and `onReset` are mutually
+ * interchangeable as far as the compiler is concerned, so a Join button that cancels, or a Cancel
+ * button that starts the job, is a swap nothing but a test would catch.
+ *
+ * See `converterActions` for why the launcher-backed actions stay parameters.
+ */
+@UnstableApi
+internal fun joinActions(
+    viewModel: JoinViewModel,
+    onPickInputs: () -> Unit,
+    onSave: (suggestedName: String) -> Unit,
+): JoinActions = JoinActions(
+    onPickInputs = onPickInputs,
+    onJoin = viewModel::join,
+    onCancel = viewModel::cancel,
+    onSave = onSave,
+    onReset = viewModel::reset,
+)
+
 /**
  * Everything [JoinScreenContent] can ask for, in one value.
  *

app/src/main/java/org/libremediaconverter/join/JoinViewModel.kt

@@ -5,6 +5,7 @@ import android.net.Uri
 import androidx.lifecycle.AndroidViewModel
 import androidx.lifecycle.viewModelScope
 import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
 import androidx.work.WorkInfo
 import androidx.work.WorkManager
 import kotlinx.coroutines.CoroutineDispatcher
@@ -19,6 +20,7 @@ import org.libremediaconverter.convert.ConversionDependencies
 import org.libremediaconverter.convert.InputFile
 import org.libremediaconverter.convert.InputQuery
 import org.libremediaconverter.convert.PendingSave
+import org.libremediaconverter.convert.SAVE_FAILED_MESSAGE
 import org.libremediaconverter.convert.STAGED_FILE_GONE_MESSAGE
 import org.libremediaconverter.convert.ScreenOwnership
 import org.libremediaconverter.model.ConcatStrategy
@@ -29,6 +31,108 @@ import org.libremediaconverter.work.jobSnapshots
 import java.io.File
 import java.util.UUID
 
+/**
+ * One update about a running join, as WorkManager last reported it.
+ *
+ * The join-side twin of `ConversionUpdate`, and deliberately the same shape: this pair of seams is
+ * one refactor done twice, and letting them diverge would make the two flows harder to compare than
+ * the duplication costs. There is no `progressPercent` here because `ConcatWorker` publishes none —
+ * a join is indeterminate.
+ */
+internal data class JoinUpdate(val state: WorkInfo.State, val runAttemptCount: Int, val outputData: Data)
+
+/**
+ * What the join screen should show, given what WorkManager last said about the job.
+ *
+ * The join-side twin of `conversionStateFrom`, extracted for the same reason and with the same two
+ * exclusions: the ownership check stays at the call site, and this takes no responsibility for the
+ * staged file. See that function's KDoc for the argument in full.
+ *
+ * Five arms had never been chosen by any test before this was cut out, because a real `ConcatWorker`
+ * only ever produces a terminal state with well-formed output.
+ *
+ * @param cancelled where a cancellation lands, which differs for a reattached job — see [observe].
+ */
+@UnstableApi
+internal fun joinStateFrom(update: JoinUpdate, inputs: List<InputFile>, cancelled: JoinState): JoinState =
+    when (update.state) {
+        // BLOCKED is a job waiting on a prerequisite, which the user has nothing to do about and
+        // nothing useful to be told about. It reads as "starting", like a fresh ENQUEUED.
+        WorkInfo.State.RUNNING, WorkInfo.State.BLOCKED -> JoinState.Joining(inputs)
+
+        // ENQUEUED after a run means a retry is pending -- the same rule, and the same reasoning, as
+        // the convert side. See `conversionStateFrom`.
+        WorkInfo.State.ENQUEUED ->
+            if (update.runAttemptCount > 0) {
+                JoinState.Waiting(inputs)
+            } else {
+                JoinState.Joining(inputs)
+            }
+
+        WorkInfo.State.SUCCEEDED -> joinedFrom(update.outputData)
+
+        // A worker that dies before it can report anything leaves no output data at all, and an
+        // exception's message can be an empty string. Both would read as a failure with nothing said,
+        // so blank falls back like missing does.
+        WorkInfo.State.FAILED -> JoinState.Failed(
+            update.outputData.getString(ConcatWorker.KEY_ERROR)
+                ?.takeIf { it.isNotBlank() }
+                ?: ConcatWorker.GENERIC_FAILURE_MESSAGE,
+        )
+
+        WorkInfo.State.CANCELLED -> cancelled
+    }
+
+/**
+ * The `SUCCEEDED` arm. A join that reported success and named no file has nothing to offer.
+ */
+@UnstableApi
+private fun joinedFrom(outputData: Data): JoinState {
+    val path = outputData.getString(ConcatWorker.KEY_OUTPUT_PATH)
+        ?: return JoinState.Failed(JOINED_WITHOUT_A_FILE_MESSAGE)
+    return JoinState.Joined(
+        staged = File(path),
+        strategy = strategyFrom(outputData.getString(ConcatWorker.KEY_STRATEGY)),
+        // A join enqueued before the worker reported these carries neither, and the fallback is
+        // the format such a job really used -- ConcatWorker.request has always defaulted to it,
+        // and the join screen has never offered a choice.
+        suggestedName = outputData.getString(ConcatWorker.KEY_SUGGESTED_NAME)
+            ?.takeIf { it.isNotBlank() }
+            ?: ConcatWorker.outputNameFor(ConcatWorker.DEFAULT_FORMAT),
+        mimeType = outputData.getString(ConcatWorker.KEY_MIME_TYPE)
+            ?.takeIf { it.isNotBlank() }
+            ?: ConcatWorker.DEFAULT_FORMAT.mimeType,
+    )
+}
+
+/**
+ * The strategy a finished join reported, or [ConcatStrategy.REENCODE] when it named none.
+ *
+ * **Looked up rather than `valueOf`, and that is a fix rather than a style choice.** `valueOf`
+ * throws `IllegalArgumentException` on a name this build does not define, and this runs inside a
+ * `viewModelScope` collect with no handler -- so the throw does not become a `Failed` state, it
+ * takes the process down.
+ *
+ * Reachable for the reason `WorkerEnumFallbackTest` and `JobTags` are both written on: WorkManager
+ * keeps finished work for about a week, so a downgrade or a rollback hands this build a job
+ * enqueued by another one. `ConcatWorker` writes `result.strategy.name` into the output `Data`, so
+ * a build that added a third strategy would leave this one crashing on its own completed joins.
+ *
+ * `ConcatWorker.kt` already made exactly this change for `KEY_FORMAT`, and says why in as many
+ * words: *"Looked up rather than `valueOf` … a format name this build does not define used to throw
+ * past the catch."* The same read on this side had not been changed with it.
+ *
+ * REENCODE is the safe default rather than an arbitrary one: it is the answer for inputs that do
+ * not match, so a job whose strategy cannot be read is described as the more conservative of the
+ * two rather than being claimed as a lossless stream copy.
+ */
+private fun strategyFrom(name: String?): ConcatStrategy =
+    ConcatStrategy.entries.firstOrNull { it.name == name } ?: ConcatStrategy.REENCODE
+
+/** A join that reported success and named no file. There is nothing to offer the user to save. */
+internal const val JOINED_WITHOUT_A_FILE_MESSAGE: String =
+    "Joining reported success but produced no file."
+
 sealed interface JoinState {
     data object Idle : JoinState
     data class Ready(val inputs: List<InputFile>) : JoinState
@@ -194,7 +298,7 @@ class JoinViewModel @JvmOverloads constructor(
     fun onInputsPicked(uris: List<Uri>) {
         val token = ownership.claim()
         if (uris.size < 2) {
-            _state.value = JoinState.Failed("Pick at least two files to join.")
+            _state.value = JoinState.Failed(ConcatWorker.TOO_FEW_INPUTS_MESSAGE)
             return
         }
         viewModelScope.launch {
@@ -250,56 +354,19 @@ class JoinViewModel @JvmOverloads constructor(
                 // takes ownership of the staged file, and a superseded observation must not do
                 // that either.
                 if (!ownership.stillHeldBy(token)) return@collect
-                _state.value = when (info.state) {
-                    WorkInfo.State.RUNNING, WorkInfo.State.BLOCKED -> JoinState.Joining(inputs)
-                    WorkInfo.State.ENQUEUED ->
-                        if (info.runAttemptCount > 0) {
-                            JoinState.Waiting(inputs)
-                        } else {
-                            JoinState.Joining(inputs)
-                        }
-
-                    WorkInfo.State.SUCCEEDED -> {
-                        val path = info.outputData.getString(ConcatWorker.KEY_OUTPUT_PATH)
-                        val strategy = info.outputData.getString(ConcatWorker.KEY_STRATEGY)
-                            ?.let(ConcatStrategy::valueOf) ?: ConcatStrategy.REENCODE
-                        if (path == null) {
-                            JoinState.Failed("Joining reported success but produced no file.")
-                        } else {
-                            val staged = File(path)
-                            // Take responsibility for the file at the same moment the state
-                            // starts referring to it, so the two cannot disagree.
-                            pendingStaged = staged
-                            JoinState.Joined(
-                                staged = staged,
-                                strategy = strategy,
-                                // A join enqueued before the worker reported these carries
-                                // neither, and the fallback is the format such a job really
-                                // used -- ConcatWorker.request has always defaulted to it, and
-                                // the join screen has never offered a choice.
-                                suggestedName = info.outputData
-                                    .getString(ConcatWorker.KEY_SUGGESTED_NAME)
-                                    ?.takeIf { it.isNotBlank() }
-                                    ?: ConcatWorker.outputNameFor(ConcatWorker.DEFAULT_FORMAT),
-                                mimeType = info.outputData
-                                    .getString(ConcatWorker.KEY_MIME_TYPE)
-                                    ?.takeIf { it.isNotBlank() }
-                                    ?: ConcatWorker.DEFAULT_FORMAT.mimeType,
-                            )
-                        }
-                    }
-
-                    // A worker that dies before it can report anything leaves no output data at
-                    // all, and an exception's message can be an empty string. Both would read as
-                    // a failure with nothing said, so blank falls back like missing does.
-                    WorkInfo.State.FAILED -> JoinState.Failed(
-                        info.outputData.getString(ConcatWorker.KEY_ERROR)
-                            ?.takeIf { it.isNotBlank() }
-                            ?: "Joining failed.",
-                    )
-
-                    WorkInfo.State.CANCELLED -> cancelled
-                }
+                val next = joinStateFrom(
+                    JoinUpdate(
+                        state = info.state,
+                        runAttemptCount = info.runAttemptCount,
+                        outputData = info.outputData,
+                    ),
+                    inputs = inputs,
+                    cancelled = cancelled,
+                )
+                // Read off the result rather than assigned inside the mapping -- see the same
+                // three lines in ConversionViewModel for why that is the better half of the swap.
+                if (next is JoinState.Joined) pendingStaged = next.staged
+                _state.value = next
             }
         }
     }
@@ -346,7 +413,7 @@ class JoinViewModel @JvmOverloads constructor(
                 // than leaving "Start over" -- which deletes it -- as the only thing on offer.
                 // Passing `pending` rather than rebuilding it is what keeps a retry that fails
                 // again on a carrying Failed instead of a bare one.
-                _state.value = JoinState.Failed(e.message ?: "Could not save the file.", pending)
+                _state.value = JoinState.Failed(e.message ?: SAVE_FAILED_MESSAGE, pending)
             }
         }
     }

app/src/main/java/org/libremediaconverter/model/ContainerCapabilities.kt

@@ -178,7 +178,12 @@ object ContainerCapabilities {
             if (!probe.hasVideo) {
                 return Validation.Invalid(
                     "This file has no video track to copy.",
-                    listOf(spec.copy(videoCodec = VideoCodec.NONE)),
+                    // Dropping the video is the right shape of answer, but it is only half of one:
+                    // `spec.copy(videoCodec = NONE)` is valid exactly when the audio axis already
+                    // happened to be fine, and refused otherwise — a Vorbis or PCM source into MP4,
+                    // an MP3 into WebM. Handing it to the shared path repairs both axes and drops
+                    // anything that still fails, so the chip cannot lead to a second error.
+                    suggestions(spec.copy(videoCodec = VideoCodec.NONE), probe, exclude = spec),
                 )
             }
             val source = CodecNames.videoFromName(probe.videoCodec)

app/src/main/java/org/libremediaconverter/ui/TestTags.kt

@@ -56,6 +56,20 @@ object TestTags {
      */
     const val RETRY_SAVE: String = "action.retrySave"
 
+    /**
+     * The adaptive shell around both screens -- `AppRoot`'s two layouts.
+     *
+     * Named because there is no other way to tell them apart from a test. Both render the same two
+     * destinations with the same labels and the same selection state, so every assertion that could
+     * be written without these tags is satisfied by either layout, and transposing the two bodies
+     * passed the whole suite. Exactly one of the two exists at a time, which is what makes
+     * `assertExists` / `assertDoesNotExist` on this pair a statement about the width class.
+     */
+    object Shell {
+        const val NAVIGATION_RAIL: String = "shell.navigationRail"
+        const val NAVIGATION_BAR: String = "shell.navigationBar"
+    }
+
     /** `ConverterScreen`. */
     object Converter {
         const val CHOOSE_FILE: String = "converter.chooseFile"

app/src/main/java/org/libremediaconverter/work/ConcatWorker.kt

@@ -15,7 +15,6 @@ import kotlinx.coroutines.CancellationException
 import org.libremediaconverter.convert.ConversionDependencies
 import org.libremediaconverter.convert.InputQuery
 import org.libremediaconverter.convert.StagingNames
-import org.libremediaconverter.ffmpeg.ConcatEngine
 import org.libremediaconverter.model.OutputFormat
 
 /**
@@ -37,9 +36,9 @@ class ConcatWorker(context: Context, params: WorkerParameters) : CoroutineWorker
 
     override suspend fun doWork(): Result {
         val uris = inputData.getStringArray(KEY_INPUT_URIS)?.map(Uri::parse)
-            ?: return Result.failure(workDataOf(KEY_ERROR to "No input files."))
+            ?: return Result.failure(workDataOf(KEY_ERROR to NO_INPUTS_MESSAGE))
         if (uris.size < 2) {
-            return Result.failure(workDataOf(KEY_ERROR to "Pick at least two files to join."))
+            return Result.failure(workDataOf(KEY_ERROR to TOO_FEW_INPUTS_MESSAGE))
         }
         // Absent, not zero, when the picker could not size every input -- see the same read in
         // ConversionWorker and InputQuery for why the two are no longer one number.
@@ -77,7 +76,7 @@ class ConcatWorker(context: Context, params: WorkerParameters) : CoroutineWorker
                 ),
             )
 
-            val result = ConcatEngine(applicationContext).join(uris, staged, format)
+            val result = ConversionDependencies.concat(applicationContext).join(uris, staged, format)
             Result.success(
                 workDataOf(
                     KEY_OUTPUT_PATH to staged.absolutePath,
@@ -107,7 +106,7 @@ class ConcatWorker(context: Context, params: WorkerParameters) : CoroutineWorker
                 }
                 FailureOutcome.FAIL -> {
                     Log.e(TAG, "Joining failed.", e)
-                    Result.failure(workDataOf(KEY_ERROR to (e.message ?: "Joining failed.")))
+                    Result.failure(workDataOf(KEY_ERROR to (e.message ?: GENERIC_FAILURE_MESSAGE)))
                 }
             }
         }
@@ -137,6 +136,44 @@ class ConcatWorker(context: Context, params: WorkerParameters) : CoroutineWorker
     )
 
     companion object {
+        /**
+         * What the user is told when a join arrives with fewer than two inputs.
+         *
+         * Shared with `JoinViewModel`, which refuses the same condition one layer up so the picker
+         * can answer without enqueueing anything. Two copies of this sentence existed before, and
+         * only the one here was pinned by a test (#139) — so the wording could drift on the screen
+         * without a single test noticing, for one message the user sees from one condition.
+         *
+         * Here rather than in the ViewModel because the rule is the worker's: `request(...)` takes
+         * a `List<Uri>` and checks nothing about its length, so this is the guard that always runs.
+         */
+        /**
+         * A job carrying no input array at all -- a downgrade, or a queue entry from a build that
+         * spelled the key differently.
+         *
+         * A constant rather than the literal it was, for the convention #158 established: a message
+         * the user can see is named once, so a test asserts the same string the worker writes
+         * rather than a copy of it that can drift.
+         */
+        const val NO_INPUTS_MESSAGE: String = "No input files."
+
+        const val TOO_FEW_INPUTS_MESSAGE: String = "Pick at least two files to join."
+
+        /**
+         * The last resort when a join fails and the exception says nothing.
+         *
+         * Shared with `JoinViewModel`, whose `FAILED` arm falls back to the same sentence when the
+         * output `Data` carries no error at all — a worker killed before it could write one. The two
+         * are a chain rather than a coincidence: this is what the worker puts *in* `KEY_ERROR`, and
+         * that is what the ViewModel says when `KEY_ERROR` never arrived. The user cannot tell the
+         * two apart and should not have to, so they are one sentence.
+         *
+         * The `Log.e` above deliberately keeps its own literal. A log line has a different audience
+         * and carries the exception with it; coupling it to the user-facing wording would mean
+         * rewording the screen to change a log.
+         */
+        const val GENERIC_FAILURE_MESSAGE: String = "Joining failed."
+
         const val KEY_INPUT_URIS = "input_uris"
         const val KEY_TOTAL_BYTES = "total_bytes"
         const val KEY_FORMAT = "format"

app/src/main/java/org/libremediaconverter/work/ConversionWorker.kt

@@ -313,7 +313,7 @@ class ConversionWorker(context: Context, params: WorkerParameters) : CoroutineWo
             }
             FailureOutcome.FAIL -> {
                 Log.e(TAG, "Conversion failed.", cause)
-                Result.failure(workDataOf(KEY_ERROR to (cause.message ?: "Conversion failed.")))
+                Result.failure(workDataOf(KEY_ERROR to (cause.message ?: GENERIC_FAILURE_MESSAGE)))
             }
         }
 
@@ -352,6 +352,20 @@ class ConversionWorker(context: Context, params: WorkerParameters) : CoroutineWo
     )
 
     companion object {
+        /**
+         * The last resort when a conversion fails and the exception says nothing.
+         *
+         * Shared with `ConversionViewModel`, whose `FAILED` arm falls back to the same sentence when
+         * the output `Data` carries no error — a worker killed before it could write one, which a
+         * refused foreground start after a process restart produces. The two are a chain rather than
+         * a coincidence: this is what goes *into* `KEY_ERROR`, and that is what is said when
+         * `KEY_ERROR` never arrived. The user cannot tell those apart and should not have to.
+         *
+         * See [ConcatWorker.GENERIC_FAILURE_MESSAGE] for the join-side twin, and the note there
+         * about why the neighbouring `Log.e` keeps its own literal.
+         */
+        const val GENERIC_FAILURE_MESSAGE: String = "Conversion failed."
+
         const val KEY_INPUT_URI = "input_uri"
         const val KEY_DISPLAY_NAME = "display_name"
         const val KEY_SIZE_BYTES = "size_bytes"

app/src/test/java/org/libremediaconverter/AdaptiveShellTest.kt

@@ -0,0 +1,129 @@
+package org.libremediaconverter
+
+import androidx.activity.ComponentActivity
+import androidx.compose.material3.windowsizeclass.WindowWidthSizeClass
+import androidx.compose.ui.test.junit4.v2.createAndroidComposeRule
+import androidx.compose.ui.test.onNodeWithTag
+import androidx.compose.ui.test.onNodeWithText
+import androidx.compose.ui.test.performClick
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import org.junit.After
+import org.junit.Before
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.ConversionDependencies
+import org.libremediaconverter.convert.installTestWorkManager
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.ui.TestTags
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+/**
+ * Which navigation affordance the shell actually renders, and which screen it actually shows.
+ *
+ * Assertion gaps rather than coverage gaps, both of them, and that is why they lasted.
+ * `AppRootRestorationTest` already drives `AppRoot` at `Compact` and `Expanded`, so JaCoCo is green
+ * on `useRail` -- but it asserts only that the *selected tab* survives recreation, through a stub
+ * `content` composable. Nothing anywhere queried for a rail or a bar, and nothing rendered the real
+ * screens. Two consequences, both measured before this file existed:
+ *
+ * - **Transposing the `NavigationRail` and `NavigationBar` bodies passed the entire suite.**
+ * - **Transposing `Content`'s two arms passed it too** -- a tablet showing the phone chrome, or the
+ *   Convert tab opening the Join screen, and 546 tests with nothing to say about either.
+ *
+ * `AppRoot`'s own KDoc is why this matters more than it looks: from targetSdk 37 the app is resized
+ * and rotated whether or not it is ready, so the width class is not a preference, it is whatever
+ * the system hands over.
+ *
+ * ## Two things this needed that the rest of the suite does not
+ *
+ * **`createAndroidComposeRule`, not `createComposeRule`.** Rendering `AppRoot` with its *default*
+ * content reaches `ConverterScreen`'s `viewModel = viewModel()`, which needs a
+ * `ViewModelStoreOwner`; the plain rule supplies none. It works because both ViewModels are
+ * `@JvmOverloads constructor(app: Application, …)`, so `AndroidViewModelFactory` can build them,
+ * and because `app/build.gradle.kts` already puts `ui-test-manifest`'s `ComponentActivity` in the
+ * merged manifest the unit tests build against -- which that file says in terms.
+ *
+ * **Tags on the two bars.** They are in `TestTags`, applied inside `main`, for the reason that
+ * file's KDoc gives: a tag the test hands down proves only that the test set it.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class AdaptiveShellTest {
+
+    @get:Rule
+    val composeRule = createAndroidComposeRule<ComponentActivity>()
+
+    @Before
+    fun setUp() {
+        val app = RuntimeEnvironment.getApplication()
+        installTestWorkManager(app, Data.EMPTY)
+        // The real screens are composed here, so their ViewModels are real too. Neither test is
+        // about probing or publishing; left alone they would reach the FFprobe loader and this
+        // machine's codec list, and decide things no assertion mentions.
+        ConversionDependencies.probe = { _, _ -> InputProbe() }
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    @Test
+    fun `a phone gets the bottom bar and a tablet gets the rail`() {
+        setShell(WindowWidthSizeClass.Compact)
+
+        composeRule.onNodeWithTag(TestTags.Shell.NAVIGATION_BAR).assertExists()
+        composeRule.onNodeWithTag(TestTags.Shell.NAVIGATION_RAIL).assertDoesNotExist()
+    }
+
+    @Test
+    fun `an expanded window gets the rail`() {
+        setShell(WindowWidthSizeClass.Expanded)
+
+        composeRule.onNodeWithTag(TestTags.Shell.NAVIGATION_RAIL).assertExists()
+        composeRule.onNodeWithTag(TestTags.Shell.NAVIGATION_BAR).assertDoesNotExist()
+    }
+
+    /**
+     * The width class no test had ever passed.
+     *
+     * `useRail` is `!= Compact`, so Medium takes the rail with Expanded. Narrowing it to
+     * `== Expanded` is a one-character change that breaks every tablet and unfolded foldable and
+     * nothing else -- and until this test, nothing in either source set used `Medium` at all.
+     */
+    @Test
+    fun `a medium window is a rail window, not a phone`() {
+        setShell(WindowWidthSizeClass.Medium)
+
+        composeRule.onNodeWithTag(TestTags.Shell.NAVIGATION_RAIL).assertExists()
+        composeRule.onNodeWithTag(TestTags.Shell.NAVIGATION_BAR).assertDoesNotExist()
+    }
+
+    /**
+     * The mapping every other test stubs out: which screen each destination actually opens.
+     *
+     * Matched on each screen's own "choose a file" affordance rather than on a title, because those
+     * tags are applied by the screens themselves -- so this fails if the destinations are
+     * transposed, and it fails for the right reason.
+     */
+    @Test
+    fun `Convert opens the converter and Join opens the join screen`() {
+        setShell(WindowWidthSizeClass.Compact)
+
+        composeRule.onNodeWithTag(TestTags.Converter.CHOOSE_FILE).assertExists()
+        composeRule.onNodeWithTag(TestTags.Join.CHOOSE_FILES).assertDoesNotExist()
+
+        composeRule.onNodeWithText(Destination.JOIN.label).performClick()
+
+        composeRule.onNodeWithTag(TestTags.Join.CHOOSE_FILES).assertExists()
+        composeRule.onNodeWithTag(TestTags.Converter.CHOOSE_FILE).assertDoesNotExist()
+    }
+
+    /** [AppRoot] with its real content, which is the half nothing else composes. */
+    private fun setShell(width: WindowWidthSizeClass) {
+        composeRule.setContent { AppRoot(width) }
+    }
+}

app/src/test/java/org/libremediaconverter/AppStartSweepTest.kt

@@ -1,8 +1,8 @@
 package org.libremediaconverter
 
-import org.junit.Assert.assertEquals
+import kotlinx.coroutines.runBlocking
+import org.junit.Assert.assertNotNull
 import org.junit.Assert.assertTrue
-import org.junit.Assert.fail
 import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
@@ -10,7 +10,6 @@ import org.libremediaconverter.convert.StagingSweep
 import org.robolectric.RobolectricTestRunner
 import org.robolectric.RuntimeEnvironment
 import java.io.File
-import java.util.concurrent.TimeUnit
 
 /**
  * That process start actually sweeps.
@@ -23,8 +22,19 @@ import java.util.concurrent.TimeUnit
  * output ever became a `Converted` state, a `reset()` whose delete was cancelled with the Activity.
  *
  * `onCreate()` is called again rather than a second Application being built: it is what the
- * framework calls at process start, the scope it launches on is already there, and the first test
- * below is what pins that the framework calls it on *this* class.
+ * framework calls at process start, and the scope it launches on is already there.
+ *
+ * **What this class stopped covering in #159, deliberately.** It used to open by asserting that
+ * `RuntimeEnvironment.getApplication()` is a [LibreMediaConverterApp] — that the manifest's
+ * `android:name` points here, so the sweep is code that actually runs. That assertion cannot exist
+ * on the JVM any more: `robolectric.properties` now names [TestLibreMediaConverterApp] for the
+ * whole suite, and an `application=` override replaces the manifest rather than being checked
+ * against it — `applicationInfo.className` reports the override too, measured. So the manifest is
+ * not merely unasserted here, it is unobservable from this source set, and a rewritten version of
+ * that test would have asserted the override against itself. **The manifest link is a device-only
+ * guarantee now**, and it was traded knowingly for the race that override fixes. The cast in
+ * [setUp] still fails if [TestLibreMediaConverterApp] stops extending the real class, which is a
+ * smaller claim than the one withdrawn.
  */
 @RunWith(RobolectricTestRunner::class)
 class AppStartSweepTest {
@@ -34,17 +44,35 @@ class AppStartSweepTest {
 
     @Before
     fun setUp() {
-        // The cast is an assertion in itself: Robolectric builds the Application named in the
-        // merged manifest, so this fails if `android:name` ever stops pointing here -- in which
-        // case the sweep below would be perfectly correct code that never runs.
         app = RuntimeEnvironment.getApplication() as LibreMediaConverterApp
         stagingDir = File(app.cacheDir, "conversions").apply { mkdirs() }
         stagingDir.listFiles()?.forEach { it.delete() }
     }
 
+    /**
+     * The property the whole substitution exists for, asserted directly rather than waited on.
+     *
+     * #159 is not "the sweep is slow", it is "the sweep is still running while some later test
+     * reads the directory". [TestLibreMediaConverterApp] answers that by finishing the sweep before
+     * `onCreate()` returns, and this is the only place that claim is checked -- every other test in
+     * the suite benefits from it silently and would go back to racing without saying why.
+     *
+     * Deterministic in the direction that matters: `Dispatchers.Unconfined` runs a `launch` whose
+     * body never suspends to completion inline, so this cannot flake green-to-red. Putting the test
+     * app back on `Dispatchers.IO` makes it a race that the assertion loses essentially every time,
+     * which is what a six-run suite comparison could not show -- at the rate #159 was observed at,
+     * a clean six-run arm is a coin flip.
+     */
     @Test
-    fun `the application the manifest starts is the one that sweeps`() {
-        assertEquals(LibreMediaConverterApp::class.java, RuntimeEnvironment.getApplication().javaClass)
+    fun `the sweep is finished before onCreate returns`() {
+        app.onCreate()
+
+        val sweep = app.startupSweep
+        assertNotNull("onCreate() started no sweep", sweep)
+        assertTrue(
+            "the JVM suite's sweep outlived onCreate(), so it is in flight during test bodies again",
+            sweep?.isCompleted == true,
+        )
     }
 
     @Test
@@ -64,35 +92,23 @@ class AppStartSweepTest {
 
         app.onCreate()
 
-        awaitGone(abandoned)
+        // Joined rather than polled. `onCreate` publishes the sweep it started, so this waits for
+        // that exact sweep -- where a timed poll could not tell "swept" from "not started yet", and
+        // answered the second case by failing after ten seconds.
+        val sweep = app.startupSweep
+        assertNotNull("onCreate() started no sweep to wait for", sweep)
+        runBlocking { sweep?.join() }
+
+        assertTrue("process start left ${abandoned.name} in staging; nothing swept it", !abandoned.exists())
         // The other half, and the one that says the sweep is a sweep rather than a
         // `clearStaging()`: the directory is shared by the convert tab, the join tab and
         // ConcatEngine's list file, so deleting everything could take a file from a running job.
         assertTrue("a file written moments ago belongs to a live job", live.exists())
     }
 
-    /**
-     * Waits for [file] to be deleted.
-     *
-     * The sweep runs on `Dispatchers.IO`, deliberately: it lists a directory and stats every entry
-     * on the path that decides how long the launcher icon stays unresponsive. So there is nothing
-     * to join, and the wait is a bounded poll — long enough for a directory listing, short enough
-     * that a sweep which never happens fails rather than hangs.
-     */
-    private fun awaitGone(file: File) {
-        val deadline = System.nanoTime() + TimeUnit.SECONDS.toNanos(AWAIT_TIMEOUT_SECONDS)
-        while (System.nanoTime() < deadline) {
-            if (!file.exists()) return
-            Thread.sleep(POLL_INTERVAL_MS)
-        }
-        fail("process start left ${file.name} in staging; nothing swept it")
-    }
-
     private fun stagedFile(name: String): File = File(stagingDir, name).apply { writeBytes(ByteArray(4096)) }
 
     private companion object {
         const val ONE_MINUTE_MS = 60L * 1000
-        const val AWAIT_TIMEOUT_SECONDS = 10L
-        const val POLL_INTERVAL_MS = 5L
     }
 }

app/src/test/java/org/libremediaconverter/TestLibreMediaConverterApp.kt

@@ -0,0 +1,28 @@
+package org.libremediaconverter
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.SupervisorJob
+
+/**
+ * The [LibreMediaConverterApp] the JVM suite runs, differing from it in exactly one thing: the
+ * startup sweep runs inline on the thread that builds the Application instead of on
+ * `Dispatchers.Unconfined`.
+ *
+ * **This is #159.** Robolectric builds an `Application` per test class that asks for one, and each
+ * one launches a sweep over the shared `<cacheDir>/conversions/`. Nothing joins them, so a test
+ * asserting about a staged file is racing however many sweeps the classes before it left in
+ * flight — `OutputPublisherStagingTest` being the one that lost, at roughly one local run in six
+ * once wave 4 added ten more Robolectric classes. Making the sweep finish before `onCreate()`
+ * returns removes the race for every test at once rather than asking each to opt in; 27 of the
+ * suite's 58 Robolectric classes touch that directory, so opting in was not a real option.
+ *
+ * `Dispatchers.Unconfined` is what makes it inline: `sweepStaging()` is a plain function, so an
+ * `Unconfined` `launch` runs it to completion before returning. The `SupervisorJob` is kept so this
+ * differs from production in the dispatcher alone — a sweep that throws is logged and swallowed
+ * here exactly as it is there, rather than taking Application construction down with it and failing
+ * every test in the class for an unrelated reason.
+ */
+class TestLibreMediaConverterApp : LibreMediaConverterApp() {
+    override val sweepScope: CoroutineScope = CoroutineScope(SupervisorJob() + Dispatchers.Unconfined)
+}

app/src/test/java/org/libremediaconverter/ci/HangBoundTest.kt

@@ -0,0 +1,98 @@
+package org.libremediaconverter.ci
+
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.io.File
+
+/**
+ * That a hung unit-test run still ends by itself, and still says why.
+ *
+ * `:app:testDebugUnitTest` had no timeout of any kind until #125 was filed. That ticket is a real
+ * Java-level deadlock between Room's `TransactionExecutor` and WorkManager's `SerialExecutorImpl`,
+ * reached through the WorkInfo flow the ViewModel collects, and one local run sat in it for 47
+ * minutes. Nothing inside the suite could break it: the deadlock is monitor contention, which is
+ * not interruptible, so it runs until something outside the JVM gives up.
+ *
+ * Two numbers in `app/build.gradle.kts` are what bound it now, and neither compiles, so nothing
+ * else would notice their removal:
+ *
+ *  - `timeout.set(...)` on every `Test` task, which stops the forked test JVM.
+ *  - the watchdog's `dumpAfterNanos`, which jstacks that JVM *before* the timeout kills it.
+ *
+ * The second is the one worth guarding hardest, and the one that most looks like stray config.
+ * Gradle's timeout kills without a thread dump, and the jstack -- with its "Found one Java-level
+ * deadlock" section naming both monitors -- is the only reason #125 could be described at all.
+ * The ordering between the two numbers is what makes it work: dump first, kill second. Reverse
+ * them, or delete the watchdog, and the suite still stops hanging but every hang from then on
+ * reports as a bare "Timeout has been exceeded" with nothing to read. Measured against a probe
+ * that hung one test: no test XML was written for the class that hung, so the hanging test itself
+ * gets no attribution from the report at all.
+ *
+ * The range on the timeout is not decoration either, and it is the half a future edit is most
+ * likely to get wrong. Below it, a healthy-but-slow runner trips the bound and a real signal
+ * becomes noise people learn to re-run through; above it, CI's 30-minute job cap fires first and
+ * the bound never gets to say anything.
+ *
+ * `ReleasePermissionTest` is the precedent and its caveat applies here too. This asserts the two
+ * numbers are present, sanely sized and correctly ordered. It cannot assert that the timeout
+ * fires -- that needs a hang, which is what the whole change exists to prevent. Refs #125.
+ */
+class HangBoundTest {
+
+    @Test
+    fun `every Test task is bounded, and bounded between the slow runner and the job cap`() {
+        assertTrue(
+            "app/build.gradle.kts sets its Test task timeout to ${timeoutMinutes}m, which is " +
+                "outside $SANE_MINUTES. Under that range a slow CI runner trips a bound meant for " +
+                "deadlocks -- the slowest observed passing run of the whole invocation was 90s. " +
+                "Over it, the Unit tests job's own 30-minute cap kills the job first and the " +
+                "timeout never reports. `null` means the line is gone or the block was rewritten, " +
+                "and without it #125's deadlock has nothing to stop it: monitor contention breaks " +
+                "no interrupt, so it runs until CI gives up and reports a timeout with no cause.",
+            timeoutMinutes in SANE_MINUTES,
+        )
+    }
+
+    @Test
+    fun `the thread dump is taken before the timeout kills the JVM it would dump`() {
+        assertTrue(
+            "app/build.gradle.kts takes its hang thread dump after ${dumpAfterMinutes}m but times " +
+                "the task out at ${timeoutMinutes}m, so the JVM is already dead when jstack runs " +
+                "and every future hang reports as a bare `Timeout has been exceeded`. The dump " +
+                "has to come first -- it is the only attribution a hanging test gets, since the " +
+                "test XML never names it.",
+            (dumpAfterMinutes ?: 0) < (timeoutMinutes ?: 0),
+        )
+    }
+
+    /** Minutes given to a whole `Test` task before Gradle stops the forked JVM. */
+    private val timeoutMinutes: Int?
+        get() = minutesIn("""timeout\.set\(Duration\.ofMinutes\((\d+)\)\)""")
+
+    /** Minutes the watchdog waits before jstacking the forked JVM. */
+    private val dumpAfterMinutes: Int?
+        get() = minutesIn("""val dumpAfterNanos = Duration\.ofMinutes\((\d+)\)""")
+
+    /**
+     * Read out of the build script rather than from a model: the numbers live in a Kotlin DSL block
+     * that no unit test can instantiate, and a scan that reports `null` when the shape changes is a
+     * better trade than not checking them at all.
+     */
+    private fun minutesIn(pattern: String): Int? =
+        Regex(pattern).find(buildScript.readText())?.groupValues?.get(1)?.toInt()
+
+    /**
+     * Found by walking up rather than by a fixed relative path: Gradle's working directory for the
+     * unit tests is the module, but that is a default rather than a promise.
+     */
+    private val buildScript: File
+        get() = generateSequence(File(".").absoluteFile) { it.parentFile }
+            .map { File(it, "app/build.gradle.kts") }
+            .firstOrNull { it.isFile }
+            ?: error("could not find app/build.gradle.kts above ${File(".").absolutePath}")
+
+    private companion object {
+        /** Above the slowest observed passing run, below the Unit tests job's `timeout-minutes`. */
+        val SANE_MINUTES = 3..29
+    }
+}

app/src/test/java/org/libremediaconverter/codec/CodecEnumerationTest.kt

@@ -0,0 +1,201 @@
+package org.libremediaconverter.codec
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.VideoCodec
+import org.robolectric.RobolectricTestRunner
+
+/**
+ * The rules `AndroidDeviceCodecs.probe()` applies to the platform's codec list.
+ *
+ * ## Why this is not a third run of the #86/#133 spike
+ *
+ * #86 closed `probe()` as device-bound. #133 re-opened the question with
+ * `ShadowMediaCodecList` in hand and closed it again, for a reason that was right about what it
+ * was answering: `MediaCodecInfoBuilder` "has no `setIsAlias` and no `setCanonicalName`, so the
+ * alias skip and the canonical-name dedup — the two things the class's KDoc calls out as easy to
+ * get wrong — are not reachable through it."
+ *
+ * **That objection is about the shadow.** It does not apply to a function that takes its own entry
+ * type, which is what `capabilitiesFrom` now does. The half #133 named as unreachable is the half
+ * this file spends most of its cases on.
+ *
+ * ## What made the seam worth cutting, which is not coverage
+ *
+ * The `runCatching` fallback logged *"assuming permissive"* and returned empty sets — and empty
+ * sets are **restrictive**: `"video/avc" in emptySet()` is `false`, so `canEncode` and `canDecode`
+ * both answer no and every job routes to FFmpeg. The code was right and the message described the
+ * opposite of it. That is pinned below, so whichever reading a future change takes, it has to say
+ * so out loud.
+ *
+ * Robolectric only because `capabilitiesFrom` logs what it found; the rules themselves are pure.
+ */
+@RunWith(RobolectricTestRunner::class)
+class CodecEnumerationTest {
+
+    /**
+     * The alias skip, in the one arrangement where it is observable — and finding that arrangement
+     * is the whole of this test.
+     *
+     * A first attempt listed the alias *after* the codec it aliases and passed with the skip
+     * deleted, because `canonicalName` is shared and the dedup below catches the second entry
+     * either way. The two rules overlap, so a fixture that does not separate them tests neither.
+     *
+     * What separates them is **order**. `MediaCodecInfo.getCanonicalName()` on an alias returns the
+     * underlying codec's name, so an alias arriving first claims that name in `seen` and has its
+     * own `supportedTypes` credited — and then the real codec is dropped by the dedup. Without the
+     * alias skip the device is described by whichever entry the platform happened to list first.
+     *
+     * That also says what the rule is worth. With a `Set` accumulator, an alias declaring the same
+     * types as its codec changes nothing whichever order they arrive in; the skip earns its place
+     * only when the two disagree, which is exactly when believing the wrong one matters.
+     */
+    @Test
+    fun `an alias listed before the codec it aliases does not describe the device`() {
+        val codecs = capabilities(
+            entry("c2.qti.avc.encoder", encoder = true, types = listOf(HEVC), alias = true),
+            entry("c2.qti.avc.encoder", encoder = true, types = listOf(AVC)),
+        )
+
+        assertTrue("the real codec's types are the device's", codecs.canEncode(VideoCodec.H264))
+        assertFalse(
+            "an alias must not be credited with types the codec it aliases never claimed",
+            codecs.canEncode(VideoCodec.H265),
+        )
+    }
+
+    @Test
+    fun `two entries sharing a canonical name are read once`() {
+        val codecs = capabilities(
+            entry("c2.qti.avc.encoder", encoder = true, types = listOf(AVC)),
+            entry("c2.qti.avc.encoder", encoder = true, types = listOf(HEVC)),
+        )
+
+        assertEquals(setOf(AVC), codecs.hardwareEncoders())
+    }
+
+    /**
+     * Both halves of the hardware predicate, one arm at a time.
+     *
+     * A vendor may declare a codec hardware-accelerated *and* software-only; the class KDoc is
+     * explicit that the first flag "cannot be tested for correctness", so the second is what stops
+     * a mislabelled software encoder being treated as the fast path.
+     */
+    @Test
+    fun `an encoder counts as hardware only when it is accelerated and not software-only`() {
+        assertEquals(
+            setOf(AVC),
+            capabilities(entry("hw", encoder = true, accelerated = true, types = listOf(AVC))).hardwareEncoders(),
+        )
+        assertEquals(
+            emptySet<String>(),
+            capabilities(entry("sw", encoder = true, accelerated = false, types = listOf(AVC))).hardwareEncoders(),
+        )
+        assertEquals(
+            "a codec claiming both must not be trusted as hardware",
+            emptySet<String>(),
+            capabilities(
+                entry("both", encoder = true, accelerated = true, softwareOnly = true, types = listOf(AVC)),
+            ).hardwareEncoders(),
+        )
+    }
+
+    /**
+     * Decoders are collected regardless of the hardware flags, and that asymmetry is the design.
+     *
+     * `canDecode` asks whether the platform can read the input at all — a software decoder answers
+     * that as well as a hardware one. `canEncode` asks whether the *fast path* exists, which is a
+     * different question and why only encoders are filtered.
+     */
+    @Test
+    fun `a software decoder still counts as something the platform can read`() {
+        val codecs = capabilities(
+            entry(
+                "c2.android.avc.decoder",
+                encoder = false,
+                accelerated = false,
+                softwareOnly = true,
+                types = listOf(AVC),
+            ),
+        )
+
+        assertTrue(codecs.canDecode("h264"))
+    }
+
+    @Test
+    fun `audio types are ignored on both sides`() {
+        val codecs = capabilities(
+            entry("aac.encoder", encoder = true, accelerated = true, types = listOf("audio/mp4a-latm")),
+            entry("aac.decoder", encoder = false, types = listOf("audio/mp4a-latm")),
+        )
+
+        assertEquals(emptySet<String>(), codecs.hardwareEncoders())
+        // Not "the platform cannot decode AAC" -- `canDecode` is asked about *video* codec names,
+        // and an unknown name is answered permissively. The point is that nothing audio reached
+        // either set.
+        assertTrue("an unknown name stays permissive", codecs.canDecode("something-nobody-named"))
+    }
+
+    /**
+     * The failure fallback, pinned as the restrictive answer it actually is.
+     *
+     * #194 decided this rather than assuming it: the code stays, the message changes. If a later
+     * change wants the permissive reading its old log line described, this test is what makes that
+     * a decision instead of a drift.
+     */
+    @Test
+    fun `an enumeration that fails sends every job to FFmpeg`() {
+        val codecs = AndroidDeviceCodecs.capabilitiesFrom { error("MediaCodecList exploded") }
+
+        assertFalse("a failed enumeration must not claim a hardware encoder", codecs.canEncode(VideoCodec.H264))
+        assertFalse(codecs.canDecode("h264"))
+        assertEquals(emptySet<String>(), codecs.hardwareEncoders())
+    }
+
+    /**
+     * A list that throws partway keeps what it already read.
+     *
+     * This predates the seam — `runCatching` has always wrapped the iteration rather than a list
+     * built before it — and it is asserted here because the seam is where it could quietly have
+     * been lost. Taking a `List` instead of a `Sequence` would move the throw outside the loop and
+     * turn this partial answer into an empty one, with no test to notice.
+     */
+    @Test
+    fun `codecs read before a failing entry are kept`() {
+        val codecs = AndroidDeviceCodecs.capabilitiesFrom {
+            sequence {
+                yield(entry("good", encoder = true, accelerated = true, types = listOf(AVC)))
+                error("the sixth codec's properties threw")
+            }
+        }
+
+        assertEquals(setOf(AVC), codecs.hardwareEncoders())
+    }
+
+    private fun capabilities(vararg entries: AndroidDeviceCodecs.Companion.CodecEntry) =
+        AndroidDeviceCodecs.capabilitiesFrom { entries.asSequence() }
+
+    private fun entry(
+        canonicalName: String,
+        encoder: Boolean,
+        accelerated: Boolean = true,
+        softwareOnly: Boolean = false,
+        alias: Boolean = false,
+        types: List<String>,
+    ) = AndroidDeviceCodecs.Companion.CodecEntry(
+        canonicalName = canonicalName,
+        isAlias = alias,
+        isEncoder = encoder,
+        isHardwareAccelerated = accelerated,
+        isSoftwareOnly = softwareOnly,
+        supportedTypes = types,
+    )
+
+    private companion object {
+        const val AVC = "video/avc"
+        const val HEVC = "video/hevc"
+    }
+}

app/src/test/java/org/libremediaconverter/codec/CodecVocabularyTest.kt

@@ -7,6 +7,7 @@ import org.junit.Assert.assertNull
 import org.junit.Assert.assertTrue
 import org.junit.Test
 import org.libremediaconverter.model.CodecNames
+import org.libremediaconverter.model.InputProbe
 import org.libremediaconverter.model.VideoCodec
 
 /**
@@ -133,6 +134,38 @@ class CodecVocabularyTest {
      * landed, a device with no HEVC decoder answered true for `x265` and Media3 was handed a job it
      * could not do; now the router sends it to FFmpeg without spending the attempt.
      */
+    /**
+     * The sentinel is not just another unknown name, and the difference is the whole guard.
+     *
+     * `canDecode` ends `?: true` -- a name neither table knows keeps the permissive answer, because
+     * the app would rather try than refuse a file it might handle. `InputProbe.UNPARSEABLE` has to
+     * be the exception: the platform has *already* failed to parse the input, so there is nothing
+     * for a decoder to be permissive about, and waving it through spends a Media3 attempt on a job
+     * that cannot start.
+     *
+     * The `cinepak` line is what makes the sentinel line mean something. Without it, deleting the
+     * early return leaves this test green -- both names would fall through to the same `?: true`.
+     * The pair is the assertion.
+     *
+     * `DeviceCodecs.PERMISSIVE` carries the same rule and `ConversionRouterTest` pins its routing
+     * consequence. This is the implementation that runs on a device.
+     */
+    @Test
+    fun `the unparseable sentinel is refused even where an unknown name is waved through`() {
+        val everything = AndroidDeviceCodecs.forTesting(
+            encoders = emptySet(),
+            decoders = setOf("video/avc", "video/hevc"),
+        )
+        assertFalse(
+            "the platform could not parse this input, so there is nothing to decode with",
+            everything.canDecode(InputProbe.UNPARSEABLE),
+        )
+        assertTrue(
+            "a merely unknown name still keeps the permissive answer",
+            everything.canDecode("cinepak"),
+        )
+    }
+
     @Test
     fun `a device without the decoder now says so for the aliases it used to wave through`() {
         val hevcOnly = AndroidDeviceCodecs.forTesting(encoders = emptySet(), decoders = setOf("video/hevc"))

app/src/test/java/org/libremediaconverter/convert/CancelReachesWorkManagerTest.kt

@@ -0,0 +1,180 @@
+package org.libremediaconverter.convert
+
+import android.app.Application
+import androidx.media3.common.util.UnstableApi
+import androidx.work.OneTimeWorkRequestBuilder
+import androidx.work.WorkInfo
+import androidx.work.WorkManager
+import androidx.work.workDataOf
+import kotlinx.coroutines.Dispatchers
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.join.JoinState
+import org.libremediaconverter.join.JoinViewModel
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.work.ConcatWorker
+import org.libremediaconverter.work.ConversionWorker
+import org.libremediaconverter.work.JobTags
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.util.UUID
+import java.util.concurrent.TimeUnit
+
+/**
+ * That `cancel()` cancels the job, on both screens.
+ *
+ * ## Why this was missing, which is the interesting part
+ *
+ * Both `cancel()` methods are one line — `activeWorkId?.let(workManager::cancelWorkById)` — and
+ * **JaCoCo reports every line of both as covered**. `SettingsEditsTest`'s
+ * `cancelling with no active job does nothing rather than throwing` runs the method, and its own
+ * comment names which half it drives: "`activeWorkId?.let(...)` -- the null side". The other side
+ * had never been entered, and `JoinViewModel.cancel()` had no test at all.
+ *
+ * So no line-level coverage filter could see this. What surfaces it is a method-level read —
+ * `mi=11, ci=7, mb=1, cb=1` on both — a covered method with an arm nothing takes. That is the
+ * second of the two filters #194 records, and this is the gap that argued for it.
+ *
+ * The affordance tests are not this. `ConverterStateAffordancesTest` and `JoinStateAffordancesTest`
+ * click `TestTags.CANCEL` and assert the *action* fires into a stub; `ScreenWiringTest` asserts the
+ * action calls `viewModel.cancel()`. Both halves were pinned and the join between them was not, so
+ * nothing in 584 tests connected the button to WorkManager.
+ *
+ * ## Why the job is enqueued with a delay
+ *
+ * The test WorkManager runs on a `SynchronousExecutor`, so an ordinary request finishes inline —
+ * which is exactly why only the null half was ever covered: by the time a test could call
+ * `cancel()`, `convert()`'s job was already terminal. `setInitialDelay` is what `TestScheduler`
+ * honours, so the job sits in `ENQUEUED` until the test lets it go, and it never does.
+ *
+ * **Production never sets a delay**, so the request is built here rather than through
+ * `ConversionWorker.request`. The *state* is not synthetic: `ENQUEUED` at `runAttemptCount == 0` is
+ * what every job passes through before the scheduler picks it up, `Reattachment.choose` ranks it
+ * `QUEUED`, and `conversionStateFrom` maps it to `Converting(input, 0)`. The delay changes how long
+ * the job stays in a real state, not which state it is in.
+ *
+ * ## What is asserted, and in which order
+ *
+ * WorkManager's own record first, then the screen. The screen alone would be a weaker claim than it
+ * looks: `CANCELLED` maps to `Idle` for a reattached job, and `Idle` is also where a ViewModel that
+ * did nothing at all would sit.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class CancelReachesWorkManagerTest {
+
+    private lateinit var app: Application
+    private lateinit var workManager: WorkManager
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        ConversionDependencies.publisher = { RecordingPublisher(app) }
+        ConversionDependencies.probe = { _, _ -> InputProbe() }
+        installTestWorkManager(app, workDataOf())
+        workManager = WorkManager.getInstance(app)
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    @Test
+    fun `cancelling a queued conversion cancels that job`() {
+        val id = enqueueQueuedConversion()
+        val viewModel = ConversionViewModel(app, Dispatchers.Unconfined)
+        awaitState(viewModel.state, "Converting") { it is ConversionState.Converting }
+
+        viewModel.cancel()
+
+        assertEquals(
+            "Cancel must reach WorkManager, not just the screen",
+            WorkInfo.State.CANCELLED,
+            stateOf(id),
+        )
+        awaitState(viewModel.state, "Idle") { it is ConversionState.Idle }
+    }
+
+    @Test
+    fun `cancelling a queued join cancels that job`() {
+        val id = enqueueQueuedJoin()
+        val viewModel = JoinViewModel(app, Dispatchers.Unconfined)
+        awaitState(viewModel.state, "Joining") { it is JoinState.Joining }
+
+        viewModel.cancel()
+
+        assertEquals(
+            "Cancel must reach WorkManager, not just the screen",
+            WorkInfo.State.CANCELLED,
+            stateOf(id),
+        )
+        awaitState(viewModel.state, "Idle") { it is JoinState.Idle }
+    }
+
+    /**
+     * The negative that bounds both: cancelling must cancel the job the screen is showing, and only
+     * that one.
+     *
+     * Without this, `cancel()` could cancel everything in the queue — `cancelAllWork()` in place of
+     * `cancelWorkById(activeWorkId)` — and both tests above would still pass.
+     */
+    @Test
+    fun `cancelling one conversion leaves another queued job alone`() {
+        val bystander = enqueueQueuedConversion(displayName = "beach.mp4")
+        val id = enqueueQueuedConversion(displayName = "holiday.mp4")
+        val viewModel = ConversionViewModel(app, Dispatchers.Unconfined)
+        val converting = awaitState(viewModel.state, "Converting") { it is ConversionState.Converting }
+        val onScreen = (converting as ConversionState.Converting).input.displayName
+
+        viewModel.cancel()
+
+        // Which of the two the ViewModel reattached to is the query's business, not this test's --
+        // the comparator leaves queued jobs tied deliberately, per Reattachment's ordering notes.
+        // So assert the shape rather than the identity: exactly one is cancelled, and the other is
+        // untouched.
+        val cancelled = listOf(id, bystander).filter { stateOf(it) == WorkInfo.State.CANCELLED }
+        assertEquals(
+            "exactly one job may be cancelled, with $onScreen on screen",
+            1,
+            cancelled.size,
+        )
+    }
+
+    private fun stateOf(id: UUID): WorkInfo.State =
+        requireNotNull(workManager.getWorkInfoById(id).get()) { "no WorkInfo for $id" }.state
+
+    /**
+     * A conversion sitting in the queue, which is where every job starts.
+     *
+     * Built by hand rather than through `ConversionWorker.request` for the reason in the class
+     * KDoc; the display-name tag is included because `reattach()` reads it for the file card, and a
+     * job without one would exercise the `UNKNOWN_INPUT_NAME` fallback instead of this test's
+     * subject.
+     */
+    private fun enqueueQueuedConversion(displayName: String = "holiday.mp4"): UUID {
+        val request = OneTimeWorkRequestBuilder<ConversionWorker>()
+            .addTag(JobTags.displayName(displayName))
+            .setInitialDelay(QUEUE_HOLD_HOURS, TimeUnit.HOURS)
+            .build()
+        workManager.enqueue(request).result.get()
+        return request.id
+    }
+
+    private fun enqueueQueuedJoin(inputCount: Int = 2): UUID {
+        val request = OneTimeWorkRequestBuilder<ConcatWorker>()
+            .addTag(JobTags.inputCount(inputCount))
+            .setInitialDelay(QUEUE_HOLD_HOURS, TimeUnit.HOURS)
+            .build()
+        workManager.enqueue(request).result.get()
+        return request.id
+    }
+
+    private companion object {
+        /** Long enough that `TestScheduler` never releases the job during a test run. */
+        const val QUEUE_HOLD_HOURS = 1L
+    }
+}

app/src/test/java/org/libremediaconverter/convert/ConversionStateMappingTest.kt

@@ -0,0 +1,238 @@
+package org.libremediaconverter.convert
+
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import androidx.work.WorkInfo
+import androidx.work.workDataOf
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.work.ConversionWorker
+import org.robolectric.RobolectricTestRunner
+
+/**
+ * Every answer [conversionStateFrom] can give, chosen rather than stumbled into.
+ *
+ * ## What this revises
+ *
+ * The mapping is not cold code and never was: `ConversionViewModel$observe$1$1` reported 28 covered
+ * lines before this file existed, because every test that drives a real worker runs it. What no
+ * test did was **choose which arm it took**. A real worker reaches a terminal state with
+ * well-formed output, so `SUCCEEDED`-with-a-path and `FAILED`-with-a-message were the only arms any
+ * test had ever produced — the other six ran never.
+ *
+ * A `grep` for `WorkInfo.State.` across the JVM suite makes that look untrue: all six constants are
+ * there. They are in `ReattachmentTest`, driven into **`Reattachment.choose`** — a different
+ * function that encodes the same enqueued-means-retry rule. So that rule had a test in one of its
+ * two homes, and the copy the user's screen reads had none.
+ *
+ * ## Why the seam, and why these assertions
+ *
+ * `WorkManager.getInstance` is called in the ViewModel's constructor and `observe` is private, so
+ * nothing could hand this a chosen `WorkInfo`. Cutting the `when` out as a pure function over
+ * [ConversionUpdate] is the answer #141 took for `MediaProbe`, and `JobSnapshot` beside
+ * `Reattachment.choose` is the same shape again.
+ *
+ * The assertions are on the whole state, not on its type. `Converting(input, 40)` and
+ * `Converting(input, 0)` are both `Converting`, and a mapping that dropped the progress read would
+ * pass any test that only asked which class came back.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class ConversionStateMappingTest {
+
+    // --- running ------------------------------------------------------------
+
+    @Test
+    fun `a running job reports the progress it published`() {
+        // The percent is read from `progress`, not from `outputData`, and not from the settings.
+        // A mapping that returned Converting(input, 0) for every RUNNING would leave the bar
+        // pinned at zero for the whole conversion.
+        val state = map(WorkInfo.State.RUNNING, progress = 40)
+
+        assertEquals(ConversionState.Converting(INPUT, 40), state)
+    }
+
+    @Test
+    fun `a running job with no published progress reports zero rather than failing`() {
+        // getInt's default. A worker that has started but not yet called setProgress is ordinary,
+        // and must not read as an error.
+        val state = map(WorkInfo.State.RUNNING, progress = null)
+
+        assertEquals(ConversionState.Converting(INPUT, 0), state)
+    }
+
+    // --- enqueued: the rule that had a test only in its other home ----------
+
+    @Test
+    fun `an enqueued job that has already run is waiting to retry`() {
+        val state = map(WorkInfo.State.ENQUEUED, runAttemptCount = 1)
+
+        assertEquals(
+            "an ENQUEUED after a run is a pending retry, which the user is told about",
+            ConversionState.Waiting(INPUT),
+            state,
+        )
+    }
+
+    @Test
+    fun `an enqueued job that has never run is simply starting`() {
+        // The other side, and the reason the test above is not enough on its own: a mapping that
+        // ignored runAttemptCount and always answered Waiting would pass that one and fail this.
+        val state = map(WorkInfo.State.ENQUEUED, runAttemptCount = 0)
+
+        assertEquals(ConversionState.Converting(INPUT, 0), state)
+    }
+
+    // --- succeeded ----------------------------------------------------------
+
+    @Test
+    fun `a success that named no file is a failure, not an empty success`() {
+        // The job said it finished and named nothing. There is no file to offer, so `Converted`
+        // would put a Save button over a path that does not exist.
+        val state = map(WorkInfo.State.SUCCEEDED, data = Data.EMPTY)
+
+        assertEquals(ConversionState.Failed(SUCCEEDED_WITHOUT_A_FILE_MESSAGE), state)
+    }
+
+    @Test
+    fun `a success carries the worker's own name and type, not the current settings`() {
+        val state = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(
+                ConversionWorker.KEY_OUTPUT_PATH to "/cache/conversions/out.mkv",
+                ConversionWorker.KEY_SUGGESTED_NAME to "holiday.mkv",
+                ConversionWorker.KEY_MIME_TYPE to "video/x-matroska",
+                ConversionWorker.KEY_ENGINE_USED to "FFMPEG",
+                ConversionWorker.KEY_ROUTE_REASON to "container needs FFmpeg",
+            ),
+        )
+
+        val converted = state as ConversionState.Converted
+        assertEquals("holiday.mkv", converted.suggestedName)
+        assertEquals("video/x-matroska", converted.mimeType)
+        assertEquals("FFMPEG", converted.engineUsed)
+        assertEquals("container needs FFmpeg", converted.routeReason)
+    }
+
+    @Test
+    fun `a success from older work falls back to the current settings for name and type`() {
+        // WorkManager keeps finished work about a week, so a job enqueued before the worker
+        // reported these is ordinary for a few days rather than a corner case.
+        val state = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(ConversionWorker.KEY_OUTPUT_PATH to "/cache/conversions/out.mp4"),
+        )
+
+        val converted = state as ConversionState.Converted
+        assertEquals(FALLBACK_SPEC.mimeType, converted.mimeType)
+        assertEquals(
+            ConversionWorker.outputNameFor(INPUT.displayName, FALLBACK_SPEC),
+            converted.suggestedName,
+        )
+    }
+
+    @Test
+    fun `a blank name or type falls back the same way a missing one does`() {
+        // A blank string is not an answer. Without takeIf, the save dialog opens named "" and
+        // registered for a MIME type of "", which no provider will accept.
+        val state = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(
+                ConversionWorker.KEY_OUTPUT_PATH to "/cache/conversions/out.mp4",
+                ConversionWorker.KEY_SUGGESTED_NAME to "",
+                ConversionWorker.KEY_MIME_TYPE to "   ",
+            ),
+        )
+
+        val converted = state as ConversionState.Converted
+        assertEquals(FALLBACK_SPEC.mimeType, converted.mimeType)
+        assertEquals(
+            ConversionWorker.outputNameFor(INPUT.displayName, FALLBACK_SPEC),
+            converted.suggestedName,
+        )
+    }
+
+    // --- failed -------------------------------------------------------------
+
+    @Test
+    fun `a failure carries the reason the worker gave`() {
+        val state = map(
+            WorkInfo.State.FAILED,
+            data = workDataOf(ConversionWorker.KEY_ERROR to "Not enough free space to convert."),
+        )
+
+        assertEquals(ConversionState.Failed("Not enough free space to convert."), state)
+    }
+
+    @Test
+    fun `a failure with nothing said still says something`() {
+        // A worker killed before it could write output data leaves none at all -- a refused
+        // foreground start after a process restart is one way. Failed("") would render as a blank
+        // error card.
+        val state = map(WorkInfo.State.FAILED, data = Data.EMPTY)
+
+        assertEquals(ConversionState.Failed(ConversionWorker.GENERIC_FAILURE_MESSAGE), state)
+    }
+
+    @Test
+    fun `a failure whose message is blank falls back like a missing one`() {
+        val state = map(
+            WorkInfo.State.FAILED,
+            data = workDataOf(ConversionWorker.KEY_ERROR to "   "),
+        )
+
+        assertEquals(ConversionState.Failed(ConversionWorker.GENERIC_FAILURE_MESSAGE), state)
+    }
+
+    // --- cancelled and blocked ---------------------------------------------
+
+    @Test
+    fun `a cancellation lands wherever the caller said it should`() {
+        // Not a fixed state: a conversion started here goes back to Ready with the picked file,
+        // while one picked up by reattach goes to Idle, because the URI that job holds belongs to
+        // a process that no longer exists. `observe`'s KDoc is where that distinction is set.
+        val toReady = map(WorkInfo.State.CANCELLED, cancelled = ConversionState.Ready(INPUT))
+        val toIdle = map(WorkInfo.State.CANCELLED, cancelled = ConversionState.Idle)
+
+        assertEquals(ConversionState.Ready(INPUT), toReady)
+        assertEquals(ConversionState.Idle, toIdle)
+    }
+
+    @Test
+    fun `a blocked job looks like one that is starting`() {
+        // BLOCKED is a job waiting on a prerequisite. There is nothing useful to say about it that
+        // differs from "starting", and inventing a state for it would put a word on screen the
+        // user cannot act on.
+        val state = map(WorkInfo.State.BLOCKED)
+
+        assertEquals(ConversionState.Converting(INPUT, 0), state)
+    }
+
+    private fun map(
+        state: WorkInfo.State,
+        progress: Int? = null,
+        runAttemptCount: Int = 0,
+        data: Data = Data.EMPTY,
+        cancelled: ConversionState = ConversionState.Ready(INPUT),
+    ): ConversionState = conversionStateFrom(
+        ConversionUpdate(
+            state = state,
+            // Modelled on the call site, which reads `getInt(KEY_PROGRESS, 0)` -- so "no progress
+            // published" is the default reaching the mapping, not a null it has to handle.
+            progressPercent = progress ?: 0,
+            runAttemptCount = runAttemptCount,
+            outputData = data,
+        ),
+        input = INPUT,
+        cancelled = cancelled,
+        fallbackSpec = FALLBACK_SPEC,
+    )
+
+    private companion object {
+        val INPUT = InputFile(Uri.parse("content://test/holiday.mov"), "holiday.mov", 4096L)
+        val FALLBACK_SPEC = OutputFormat.MP4_H265.spec
+    }
+}

app/src/test/java/org/libremediaconverter/convert/FFprobeMappingTest.kt

@@ -0,0 +1,192 @@
+package org.libremediaconverter.convert
+
+import com.arthenica.ffmpegkit.MediaInformation
+import com.arthenica.ffmpegkit.StreamInformation
+import org.json.JSONObject
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.Container
+import org.robolectric.RobolectricTestRunner
+
+/**
+ * What FFprobe's answer means, read as a function of the answer alone.
+ *
+ * `readMediaInformation` was 114 missed instructions and 24 missed branches — the second-biggest
+ * block on the wave-4 report — of which **exactly one line needed a device**:
+ *
+ * ```kotlin
+ * FFprobeKit.getMediaInformation(path).getMediaInformation()
+ * ```
+ *
+ * Everything after it reads an ordinary object. `javap` over the committed AAR's runtime jar:
+ * `MediaInformation(JSONObject, List<StreamInformation>, List<Chapter>)` and
+ * `StreamInformation(JSONObject)` are plain public constructors, and neither class's `<clinit>`
+ * loads the native library — so the fixtures below are built without `libffmpegkit` present.
+ *
+ * ## The one that matters
+ *
+ * `containerFrom(formatName, video?.getCodec())`. FFprobe reports `matroska,webm` for **both** MKV
+ * and WebM, because they share a demuxer, so the video codec is the only thing separating them.
+ * `containerFrom` has thirty-three covered branches of its own and not one of them can notice the
+ * argument being dropped — the mistake would be at the call, not in the callee, and every existing
+ * `containerFrom` test would stay green while every VP9 WebM quietly became an MKV.
+ *
+ * Robolectric only for `org.json`, which is a stub in a plain JVM test.
+ */
+@RunWith(RobolectricTestRunner::class)
+class FFprobeMappingTest {
+
+    @Test
+    fun `the video codec decides between matroska and webm`() {
+        assertEquals(
+            Container.WEBM,
+            MediaProbe.ffprobeInfoFrom(info("matroska,webm", stream("video", "vp9"))).container,
+        )
+        assertEquals(
+            Container.MKV,
+            MediaProbe.ffprobeInfoFrom(info("matroska,webm", stream("video", "h264"))).container,
+        )
+    }
+
+    /**
+     * The same format name with no video stream at all, which is what makes the case above about
+     * the *argument* rather than about the format string.
+     */
+    @Test
+    fun `a matroska container with no video track cannot be told from webm and is not guessed`() {
+        val read = MediaProbe.ffprobeInfoFrom(info("matroska,webm", stream("audio", "opus")))
+
+        assertEquals(Container.MKV, read.container)
+        assertNull(read.videoCodec)
+    }
+
+    @Test
+    fun `the first stream of each type wins`() {
+        val read = MediaProbe.ffprobeInfoFrom(
+            info(
+                "mov,mp4,m4a,3gp,3g2,mj2",
+                stream("video", "h264", width = 1920, height = 1080),
+                stream("video", "hevc", width = 640, height = 480),
+                stream("audio", "aac"),
+                stream("audio", "mp3"),
+            ),
+        )
+
+        assertEquals("h264", read.videoCodec)
+        assertEquals("aac", read.audioCodec)
+        assertEquals(1920, read.width)
+        assertEquals(1080, read.height)
+    }
+
+    /**
+     * Dimensions come from the stream the codec came from, not from whichever stream has some.
+     *
+     * The fixture is deliberately awkward: the chosen video stream carries **no** dimensions and a
+     * later one does. That is a real shape — FFprobe omits `width`/`height` for a stream it could
+     * not measure — and it is the only arrangement that separates the two readings.
+     *
+     * A first version of this file asserted the dimensions inside the case above, where the chosen
+     * stream was also the first one carrying any. Replacing `video?.getWidth()` with
+     * `streams.firstNotNullOfOrNull { it.getWidth() }` gave the same answer there and **the
+     * mutation survived**. It reddens here.
+     */
+    @Test
+    fun `a video stream with no dimensions reports none rather than borrowing another stream's`() {
+        val read = MediaProbe.ffprobeInfoFrom(
+            info(
+                "mov,mp4,m4a,3gp,3g2,mj2",
+                stream("video", "h264"),
+                stream("video", "hevc", width = 640, height = 480),
+            ),
+        )
+
+        assertEquals("h264", read.videoCodec)
+        assertEquals(0, read.width)
+        assertEquals(0, read.height)
+    }
+
+    /**
+     * Stream order is the file's, not a promise. An audio-first container must read the same as a
+     * video-first one.
+     */
+    @Test
+    fun `an audio track listed first does not become the video track`() {
+        val read = MediaProbe.ffprobeInfoFrom(
+            info("mov,mp4,m4a,3gp,3g2,mj2", stream("audio", "aac"), stream("video", "h264")),
+        )
+
+        assertEquals("h264", read.videoCodec)
+        assertEquals("aac", read.audioCodec)
+    }
+
+    @Test
+    fun `a duration in seconds becomes milliseconds`() {
+        assertEquals(12_345L, MediaProbe.ffprobeInfoFrom(info("mp4", duration = "12.345")).durationMs)
+    }
+
+    /**
+     * Both ways a duration can be absent, and neither may throw.
+     *
+     * FFprobe reports `"N/A"` for a stream it could not measure, and omits the key entirely for
+     * some containers. `toDoubleOrNull` is what keeps the second from being an exception on the
+     * file-pick path, where there is no user-visible failure to report it as.
+     */
+    @Test
+    fun `a duration that is not a number is no duration rather than a crash`() {
+        assertEquals(0L, MediaProbe.ffprobeInfoFrom(info("mp4", duration = "N/A")).durationMs)
+        assertEquals(0L, MediaProbe.ffprobeInfoFrom(info("mp4", duration = null)).durationMs)
+    }
+
+    @Test
+    fun `a file with no streams reports nothing rather than defaults that look measured`() {
+        val read = MediaProbe.ffprobeInfoFrom(info("mp4"))
+
+        assertNull(read.videoCodec)
+        assertNull(read.audioCodec)
+        assertEquals(0, read.width)
+        assertEquals(0, read.height)
+    }
+
+    @Test
+    fun `an image format is reported as one`() {
+        assertTrue(MediaProbe.ffprobeInfoFrom(info("png_pipe", stream("video", "png"))).isImage)
+        assertFalse(MediaProbe.ffprobeInfoFrom(info("mp4", stream("video", "h264"))).isImage)
+    }
+
+    private fun stream(type: String, codec: String, width: Int? = null, height: Int? = null) = StreamInformation(
+        JSONObject().apply {
+            put(StreamInformation.KEY_TYPE, type)
+            put(StreamInformation.KEY_CODEC, codec)
+            width?.let { put(StreamInformation.KEY_WIDTH, it) }
+            height?.let { put(StreamInformation.KEY_HEIGHT, it) }
+        },
+    )
+
+    /**
+     * The format properties are **nested** under `"format"`, which is how FFprobe reports them and
+     * what `MediaInformation` reads: `getFormat()` resolves through `getStringFormatProperty`, not
+     * off the top-level object. A first version of this helper put the keys at the top level and
+     * every format-dependent case failed with a null container, which is worth recording here so
+     * the next fixture does not have to rediscover it.
+     *
+     * Streams are the other half and are *not* nested — they come from the constructor argument.
+     */
+    private fun info(formatName: String, vararg streams: StreamInformation, duration: String? = "1.0") =
+        MediaInformation(
+            JSONObject().apply {
+                put(
+                    MediaInformation.KEY_FORMAT_PROPERTIES,
+                    JSONObject().apply {
+                        put(MediaInformation.KEY_FORMAT, formatName)
+                        duration?.let { put(MediaInformation.KEY_DURATION, it) }
+                    },
+                )
+            },
+            streams.toList(),
+            emptyList(),
+        )
+}

app/src/test/java/org/libremediaconverter/convert/FailedSaveRetryTest.kt

@@ -51,10 +51,13 @@ import java.io.File
  *   here needs. `OutputPublisherPublishTest` owns what a real publish writes.
  * - **The screen's two buttons.** `ConverterStateAffordancesTest` and `JoinStateAffordancesTest`
  *   own what each state renders; this file owns what each state carries.
- * - **`ConverterScreen`'s `destinationMime` line itself.** It lives in the entry point, above the
- *   `ScreenContent` seam, and reaching it needs a real ViewModel inside a composition. What it
- *   reads -- `pendingSave()?.mimeType` -- is asserted directly instead, which is why that
- *   derivation was moved out of the entry point in the first place.
+ * - ~~**`ConverterScreen`'s `destinationMime` line itself.**~~ **Withdrawn 2026-09-02 (#201).** The
+ *   exemption read: "it lives in the entry point, above the `ScreenContent` seam, and reaching it
+ *   needs a real ViewModel inside a composition". That was true when written and is no longer:
+ *   `AdaptiveShellTest` (#173) established composing the real screens with real ViewModels, and
+ *   #200 added the `ShadowActivity` mechanics for reading what a launcher launched. `RetrySaveMimeTest`
+ *   now asserts the line directly. What this file still owns is the half below the seam -- what each
+ *   state *carries* -- which is why `pendingSave()?.mimeType` is also asserted here.
  * - **Picking a new input while a `Failed` carries a file.** `onInputPicked` overwrites the state
  *   without discarding, from `Converted` exactly as much as from a carrying `Failed`, and neither
  *   branch renders a picker. It is a pre-existing path this change neither opens nor widens: the

app/src/test/java/org/libremediaconverter/convert/FakeProviders.kt

@@ -0,0 +1,234 @@
+package org.libremediaconverter.convert
+
+import android.content.ComponentName
+import android.content.ContentProvider
+import android.content.ContentValues
+import android.content.Context
+import android.content.IntentFilter
+import android.content.pm.ProviderInfo
+import android.database.Cursor
+import android.database.MatrixCursor
+import android.net.Uri
+import android.os.Bundle
+import android.provider.DocumentsContract
+import android.provider.OpenableColumns
+import org.robolectric.Robolectric
+import org.robolectric.Shadows.shadowOf
+import java.io.File
+
+/**
+ * Content providers more than one test needs, and the registration dance they all repeat.
+ *
+ * Only that. A stub that serves one test stays in that test, next to the assertion it exists for —
+ * the rule `work/WorkerStubs.kt` states, and the reason `UnreliableOutputStream` is still private to
+ * `OutputPublisherPublishTest`.
+ *
+ * These started life inside `OutputPublisherPublishTest`, which is the only thing that needed a
+ * provider at all. They moved here when `InputQuery`'s cursor reads turned out to need the same
+ * provider answering *badly* — see [RowShape].
+ */
+
+internal const val DOCUMENTS_AUTHORITY = "org.libremediaconverter.test.documents"
+internal const val PLAIN_AUTHORITY = "org.libremediaconverter.test.plain"
+
+/**
+ * How [FakeSafProvider] answers a metadata query.
+ *
+ * A provider is another app. It can be uninstalled, revoke its grant, crash, or simply answer
+ * something the caller did not expect — and "answered something unexpected" is not one case but
+ * several, which is why this is an enum rather than a boolean.
+ *
+ * The distinction that matters most to callers is **null versus missing versus zero versus
+ * negative**. `InputQuery` exists to stop the last three being conflated: `hasSpaceFor(0)` is only
+ * "is there 128 MB free", so a size nobody could determine must not arrive as `0`, and
+ * `OutputPublisher.destinationIsKnownEmpty` must answer `false` — never "empty, go ahead and
+ * delete" — for every one of them.
+ *
+ * Column-level granularity is deliberate. `OutputPublisher` reads only `SIZE`; `InputQuery` reads
+ * both, and reaches a different answer depending on which one is bad.
+ */
+internal enum class RowShape {
+    /** What a healthy provider answers: the file's real name and real length. */
+    NORMAL,
+
+    /** A row is present and its `DISPLAY_NAME` cell is null. */
+    NULL_DISPLAY_NAME,
+
+    /** A row is present and its `SIZE` cell is null. */
+    NULL_SIZE,
+
+    /** The cursor carries no `DISPLAY_NAME` column at all — `getColumnIndex` gives `-1`. */
+    NO_DISPLAY_NAME_COLUMN,
+
+    /** The cursor carries no `SIZE` column at all — `getColumnIndex` gives `-1`. */
+    NO_SIZE_COLUMN,
+
+    /**
+     * A size of `-1`.
+     *
+     * Not a corrupt provider: it is what anything without a fixed length reports — a pipe, or a
+     * provider streaming its answer — and it is a third way of saying "unknown", distinct from a
+     * null cell and from a missing column.
+     */
+    NEGATIVE_SIZE,
+
+    /**
+     * A cursor with the right columns and no rows in it.
+     *
+     * Distinct from returning `null`, which is what a provider that does not recognise the URI
+     * does. Both mean "no answer", and code that treats one as an answer and the other as an
+     * absence is wrong about one of them.
+     */
+    NO_ROWS,
+
+    /**
+     * The query itself throws.
+     *
+     * A resolver call is a call into another app, and that app can have been uninstalled, revoked
+     * its grant, or simply crashed. `InputQuery.firstRow`'s KDoc is explicit that "a file picker is
+     * not a place to bring the process down from", so this is the shape that proves the guard is
+     * one.
+     */
+    QUERY_THROWS,
+}
+
+/**
+ * A stand-in for the provider behind a SAF destination.
+ *
+ * It answers only what its callers ask of a document -- how many bytes are already there, what it
+ * is called, and delete it -- backed by a real file so the assertions are about the filesystem
+ * rather than about a mock's call log alone. The rest of the `ContentProvider` surface is stubbed.
+ *
+ * Writing is deliberately NOT routed through it. Robolectric's `ShadowContentResolver`
+ * consults its registered-stream map before it reaches any provider, which is what lets a
+ * test hand out a stream that writes some bytes and then fails -- a condition a real provider
+ * cannot be asked to produce on demand.
+ */
+internal open class FakeSafProvider : ContentProvider() {
+
+    override fun onCreate() = true
+
+    override fun query(
+        uri: Uri,
+        projection: Array<out String>?,
+        selection: String?,
+        selectionArgs: Array<out String>?,
+        sortOrder: String?,
+    ): Cursor? {
+        if (rowShape == RowShape.QUERY_THROWS) throw SecurityException("provider revoked the grant")
+        val file = backingFile(uri)
+        if (!file.exists()) return null
+        return MatrixCursor(columnsFor(rowShape)).apply {
+            if (rowShape != RowShape.NO_ROWS) addRow(cellsFor(rowShape, file))
+        }
+    }
+
+    override fun call(method: String, arg: String?, extras: Bundle?): Bundle? {
+        if (method != METHOD_DELETE_DOCUMENT) return null
+        val target = extras?.getParcelable(EXTRA_URI, Uri::class.java) ?: return null
+        deleteRequests += target
+        deleteFailure?.let { throw it }
+        backingFile(target).delete()
+        return Bundle()
+    }
+
+    override fun getType(uri: Uri) = "video/mp4"
+
+    override fun insert(uri: Uri, values: ContentValues?): Uri? = null
+
+    override fun delete(uri: Uri, selection: String?, selectionArgs: Array<out String>?) = 0
+
+    override fun update(uri: Uri, values: ContentValues?, selection: String?, selectionArgs: Array<out String>?) = 0
+
+    companion object {
+        // DocumentsContract.METHOD_DELETE_DOCUMENT and EXTRA_URI are hidden from the public
+        // SDK, so they cannot be referenced. These are the wire names
+        // DocumentsContract.deleteDocument() actually sends, which is what a provider sees.
+        const val METHOD_DELETE_DOCUMENT = "android:deleteDocument"
+        const val EXTRA_URI = "uri"
+
+        /** Where the "documents" really live. Set per test to a Robolectric temp path. */
+        lateinit var root: File
+
+        /** Every delete this provider was asked for, in order. Empty is an assertion too. */
+        val deleteRequests = mutableListOf<Uri>()
+
+        /** Armed by the test that needs the cleanup itself to fail. */
+        var deleteFailure: RuntimeException? = null
+
+        /**
+         * How the next query answers. [reset] puts it back to [RowShape.NORMAL], so a test that
+         * does not care never has to think about it.
+         */
+        var rowShape: RowShape = RowShape.NORMAL
+
+        fun backingFile(uri: Uri) = File(root, uri.lastPathSegment.orEmpty())
+
+        fun reset(directory: File) {
+            root = directory
+            deleteRequests.clear()
+            deleteFailure = null
+            rowShape = RowShape.NORMAL
+        }
+
+        private fun columnsFor(shape: RowShape): Array<String> = when (shape) {
+            RowShape.NO_DISPLAY_NAME_COLUMN -> arrayOf(OpenableColumns.SIZE)
+            RowShape.NO_SIZE_COLUMN -> arrayOf(OpenableColumns.DISPLAY_NAME)
+            else -> arrayOf(OpenableColumns.DISPLAY_NAME, OpenableColumns.SIZE)
+        }
+
+        private fun cellsFor(shape: RowShape, file: File): Array<Any?> = when (shape) {
+            RowShape.NO_DISPLAY_NAME_COLUMN -> arrayOf(file.length())
+            RowShape.NO_SIZE_COLUMN -> arrayOf<Any?>(file.name)
+            RowShape.NULL_DISPLAY_NAME -> arrayOf(null, file.length())
+            RowShape.NULL_SIZE -> arrayOf(file.name, null)
+            RowShape.NEGATIVE_SIZE -> arrayOf(file.name, UNKNOWN_LENGTH)
+            else -> arrayOf(file.name, file.length())
+        }
+
+        /** What `statSize` reports for anything without a fixed length. See [RowShape.NEGATIVE_SIZE]. */
+        private const val UNKNOWN_LENGTH = -1L
+    }
+}
+
+/**
+ * The same provider, registered WITHOUT the documents-provider intent filter.
+ *
+ * A separate class because the package manager keys providers by component name, so two
+ * authorities need two components. It exists to prove the guard is a guard: a content URI
+ * from something that is not a documents provider must not be handed to `deleteDocument`.
+ */
+internal class FakePlainProvider : FakeSafProvider()
+
+/**
+ * Stands [provider] up on [authority] so `contentResolver` and the package manager both know it.
+ *
+ * `isDocumentUri()` does not look at the URI alone: it asks the package manager whether anything
+ * answers `ACTION_DOCUMENTS_PROVIDER` for that authority. Registering the provider with the
+ * resolver is not enough, which is the whole reason [asDocumentsProvider] is a parameter rather
+ * than always true — the negative case is a test.
+ */
+internal fun registerProvider(
+    context: Context,
+    provider: Class<out FakeSafProvider>,
+    authority: String,
+    asDocumentsProvider: Boolean,
+) {
+    val info = ProviderInfo().apply {
+        this.authority = authority
+        packageName = context.packageName
+        name = provider.name
+        exported = true
+        grantUriPermissions = true
+    }
+    Robolectric.buildContentProvider(provider).create(info)
+
+    val packageManager = shadowOf(context.packageManager)
+    packageManager.addOrUpdateProvider(info)
+    if (asDocumentsProvider) {
+        packageManager.addIntentFilterForProvider(
+            ComponentName(context.packageName, provider.name),
+            IntentFilter(DocumentsContract.PROVIDER_INTERFACE),
+        )
+    }
+}

app/src/test/java/org/libremediaconverter/convert/FileCardTest.kt

@@ -205,6 +205,34 @@ class FileCardTest {
         assertNoRow("Length")
     }
 
+    /**
+     * A video the app knows a great deal about and cannot name the container of.
+     *
+     * Not an edge case. `InputProbe.container`'s own KDoc says `MediaExtractor` cannot report a
+     * container at all -- it comes from FFprobe -- so any run where FFprobe did not answer produces
+     * exactly this: real codec, real dimensions, real duration, `container = null`.
+     *
+     * **The twin was already tested and this one was not**, which is the argument for adding it.
+     * `FileCard` renders `probe.container?.label ?: "Unknown"` twice, once in the `AUDIO_ONLY`
+     * branch (`ConverterScreen.kt:660`) and once in the `VIDEO` branch (`:668`), and
+     * `an audio-only file nothing else could describe degrades one row at a time` drives only the
+     * first. Same expression, same fallback, one kind covered. That asymmetry is the same one
+     * `CLAUDE.md` records for including `ContainerCapabilities:94`.
+     *
+     * The other rows are asserted alongside so this is not a copy of the audio-only case: there,
+     * everything is unknown at once; here, one field is missing from a probe that is otherwise
+     * complete, and the rest must be unaffected by it.
+     */
+    @Test
+    fun `a video file whose container nothing identified says so and keeps its other rows`() {
+        setFileCard(input(probe = VIDEO_PROBE.copy(container = null)))
+
+        assertRow("Container", "Unknown")
+        assertRow("Video", "${VideoCodec.H264.label} · 1920×1080")
+        assertRow("Audio", AudioCodec.AAC.label)
+        assertRow("Length", "1:30")
+    }
+
     /**
      * The row is one node, not a label node beside a value node. A test matching on `"Container"`
      * alone would pass against either shape.

app/src/test/java/org/libremediaconverter/convert/InputQueryCursorTest.kt

@@ -0,0 +1,187 @@
+package org.libremediaconverter.convert
+
+import android.content.Context
+import android.net.Uri
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.io.File
+
+/**
+ * What [InputQuery] makes of a metadata row.
+ *
+ * ## Why this is a separate file from `UnknownInputSizeTest`
+ *
+ * That test drives the case where **no provider is registered** — the query returns null and
+ * `measure()` answers instead — and it drives it thoroughly. What it never does is hand `InputQuery`
+ * a row. Before this file, nothing did: `firstRow`'s body, `displayNameOrNull` and `sizeOrNull` had
+ * never executed in the JVM suite, so every branch inside them was untested.
+ *
+ * ## What is actually being pinned
+ *
+ * Not "does it read a cursor" — that would pass against almost any implementation. The rule is that
+ * **a size nobody could determine must not arrive as a number**, and there are four separate ways a
+ * provider fails to determine one: a null cell, a missing column, a negative value, and no row at
+ * all. `InputQuery`'s KDoc states the stake:
+ *
+ * > a worker's input `Data` carries the size the *picker* found … `hasSpaceFor(0)` is only "is there
+ * > 128 MB free".
+ *
+ * So each of those four must produce `null`, and `null` specifically — not `0`, not `-1`. A test
+ * that asserted only "not the file's length" would pass on `0`, which is the exact conflation the
+ * class exists to end.
+ *
+ * ## Why every fall-through lands on null here
+ *
+ * [FakeSafProvider] does not implement `openFile`, so `measure()` cannot answer for these URIs
+ * either. That is deliberate: it isolates the cursor half. The other direction — the cursor says
+ * nothing and `measure()` succeeds — is `UnknownInputSizeTest`'s
+ * `a picked file no provider describes is measured rather than reported as empty`, and is not
+ * repeated here.
+ *
+ * ## What the mutations say, including the one that does not bite
+ *
+ * Measured against `MatrixCursor`, which is what these tests drive:
+ *
+ * | call on a null cell | result |
+ * |---|---|
+ * | `getString` | returns `null` |
+ * | `getLong` | returns **`0`** |
+ *
+ * That second row is why `sizeOrNull`'s `!isNull(it)` guard is load-bearing and why these tests
+ * bite: remove it and a null size arrives as `0`, a real number indistinguishable from an empty
+ * file, which is the precise conflation this class exists to end. Removing it reddens
+ * `a null size is unknown rather than zero`. Removing the trailing `takeIf { it >= 0 }` reddens
+ * `a negative size is unknown rather than reported`.
+ *
+ * **Named exemption: `displayNameOrNull`'s `!isNull(it)` guard is not pinned by anything here, and
+ * cannot be.** `getString` returns null for a null cell, so the fallback applies with or without
+ * the guard — removing it leaves every test in this file green. The guard is not redundant in
+ * production: `Cursor.getString`'s contract states that whether it throws on a null column is
+ * *implementation-defined*, and a real `ContentProvider` is free to throw where `MatrixCursor`
+ * returns null. It should stay. It simply cannot be falsified with this cursor, and saying so is
+ * better than implying `a null display name falls back without disturbing the size` covers it —
+ * that test pins the behaviour, not the guard.
+ */
+@RunWith(RobolectricTestRunner::class)
+class InputQueryCursorTest {
+
+    private lateinit var context: Context
+    private lateinit var uri: Uri
+
+    @Before
+    fun setUp() {
+        context = RuntimeEnvironment.getApplication()
+        FakeSafProvider.reset(File(context.cacheDir, "picked").apply { mkdirs() })
+        registerProvider(context, FakeSafProvider::class.java, DOCUMENTS_AUTHORITY, asDocumentsProvider = true)
+        uri = Uri.parse("content://$DOCUMENTS_AUTHORITY/document/holiday.mp4")
+        FakeSafProvider.backingFile(uri).writeBytes(ByteArray(PAYLOAD_BYTES))
+    }
+
+    @Test
+    fun `a provider that answers properly supplies both the name and the size`() {
+        val described = InputQuery.describe(context, uri)
+
+        assertEquals("holiday.mp4", described.displayName)
+        assertEquals(PAYLOAD_BYTES.toLong(), described.sizeBytes)
+    }
+
+    @Test
+    fun `a null display name falls back without disturbing the size`() {
+        FakeSafProvider.rowShape = RowShape.NULL_DISPLAY_NAME
+
+        val described = InputQuery.describe(context, uri)
+
+        assertEquals(InputQuery.FALLBACK_DISPLAY_NAME, described.displayName)
+        // The two columns are read independently. A provider that cannot name the file can still
+        // size it, and losing the size here would be a bug the name assertion alone would miss.
+        assertEquals(PAYLOAD_BYTES.toLong(), described.sizeBytes)
+    }
+
+    @Test
+    fun `a cursor with no display name column falls back rather than throwing`() {
+        // getColumnIndex returns -1 rather than throwing, so the `it >= 0` guard is the only thing
+        // between this and an IllegalArgumentException out of getString.
+        FakeSafProvider.rowShape = RowShape.NO_DISPLAY_NAME_COLUMN
+
+        val described = InputQuery.describe(context, uri)
+
+        assertEquals(InputQuery.FALLBACK_DISPLAY_NAME, described.displayName)
+        assertEquals(PAYLOAD_BYTES.toLong(), described.sizeBytes)
+    }
+
+    @Test
+    fun `a null size is unknown rather than zero`() {
+        FakeSafProvider.rowShape = RowShape.NULL_SIZE
+
+        assertNull(unknownSizeMessage("a null cell"), InputQuery.sizeOf(context, uri))
+    }
+
+    @Test
+    fun `a cursor with no size column is unknown rather than zero`() {
+        FakeSafProvider.rowShape = RowShape.NO_SIZE_COLUMN
+
+        assertNull(unknownSizeMessage("a missing column"), InputQuery.sizeOf(context, uri))
+    }
+
+    @Test
+    fun `a negative size is unknown rather than reported`() {
+        // What anything without a fixed length reports -- a pipe, or a provider streaming its
+        // answer. Passing -1 through would be worse than passing 0: hasSpaceFor compares it
+        // against free space, so it would read as "needs less than nothing".
+        FakeSafProvider.rowShape = RowShape.NEGATIVE_SIZE
+
+        assertNull(unknownSizeMessage("a negative size"), InputQuery.sizeOf(context, uri))
+    }
+
+    @Test
+    fun `a cursor with no rows is unknown rather than zero`() {
+        // Distinct from the provider returning null, which UnknownInputSizeTest covers. A cursor
+        // that exists and holds nothing still has to reach the same answer.
+        FakeSafProvider.rowShape = RowShape.NO_ROWS
+
+        val described = InputQuery.describe(context, uri)
+
+        assertEquals(InputQuery.FALLBACK_DISPLAY_NAME, described.displayName)
+        assertNull(unknownSizeMessage("an empty cursor"), described.sizeBytes)
+    }
+
+    @Test
+    fun `a provider that throws is survived rather than propagated`() {
+        // The guard firstRow's KDoc exists for: "a resolver call is a call into another app ... and
+        // a file picker is not a place to bring the process down from". Without the runCatching,
+        // this SecurityException reaches the caller and takes the pick with it.
+        FakeSafProvider.rowShape = RowShape.QUERY_THROWS
+
+        val described = InputQuery.describe(context, uri)
+
+        assertEquals(InputQuery.FALLBACK_DISPLAY_NAME, described.displayName)
+        assertNull(unknownSizeMessage("a provider that threw"), described.sizeBytes)
+    }
+
+    @Test
+    fun `a join total is unknown when any one input could not be sized`() {
+        // The consequence the four cases above exist for, asserted once at the place it lands.
+        // Summing the inputs that did answer would produce a lower bound indistinguishable from a
+        // real total, which is what the space check cannot tell apart.
+        FakeSafProvider.rowShape = RowShape.NULL_SIZE
+        val unsizable = InputQuery.sizeOf(context, uri)
+        FakeSafProvider.rowShape = RowShape.NORMAL
+        val sizable = InputQuery.sizeOf(context, uri)
+
+        assertEquals(PAYLOAD_BYTES.toLong(), sizable)
+        assertNull(unsizable)
+        assertNull("one unknown input makes the whole total unknown", InputQuery.total(listOf(sizable, unsizable)))
+    }
+
+    private fun unknownSizeMessage(cause: String) =
+        "$cause means nobody could size the file; that must be null, not 0 -- hasSpaceFor(0) is only a headroom check"
+
+    private companion object {
+        const val PAYLOAD_BYTES = 4096
+    }
+}

app/src/test/java/org/libremediaconverter/convert/LauncherWiringTest.kt

@@ -0,0 +1,141 @@
+package org.libremediaconverter.convert
+
+import android.app.Activity
+import android.content.Intent
+import android.net.Uri
+import androidx.activity.ComponentActivity
+import androidx.compose.ui.test.assertIsDisplayed
+import androidx.compose.ui.test.junit4.v2.createAndroidComposeRule
+import androidx.compose.ui.test.onNodeWithTag
+import androidx.compose.ui.test.performClick
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.join.JoinScreen
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.ui.TestTags
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import org.robolectric.Shadows.shadowOf
+import org.robolectric.shadows.ShadowActivity
+
+/**
+ * The launcher layer above the `ScreenContent` seam — registered, and until now never resulted.
+ *
+ * ## The hazard this exists for
+ *
+ * `ConversionViewModel.onInputPicked(uri: Uri)` and `.save(destination: Uri)` are **both
+ * `(Uri) -> Unit`**, so swapping the two launcher callbacks at `ConverterScreen.kt:70` and `:83`
+ * compiles, renders, and passes the entire suite. Picking a file would attempt a save to it, and
+ * choosing a destination would load it as input.
+ *
+ * That is precisely the defect class `ScreenWiringTest` exists for, on the one pair it declines to
+ * cover: it drives `converterActions` directly and says the launcher-backed actions stay
+ * parameters. Correct for the `actions` seam, and it leaves the edge above that seam unpinned.
+ *
+ * Join's equivalents (`JoinScreen.kt:45`, `:55`) are `List<Uri>` and `Uri`, so they are **not**
+ * transposable and need no such test. The picker filter is a different matter and is covered below
+ * for both screens.
+ *
+ * ## The two mechanics, verified before the assertions were written
+ *
+ * Neither is used anywhere else in the suite, so both were spiked first:
+ *
+ * - **Reading what was launched** — `shadowOf(activity).nextStartedActivityForResult`, which returns
+ *   the `Intent` with its `EXTRA_MIME_TYPES` intact.
+ * - **Delivering a result** — `shadowOf(activity).receiveResult(...)`, which reaches
+ *   `ComponentActivity`'s `ActivityResultRegistry` and fires the `rememberLauncherForActivityResult`
+ *   callback.
+ *
+ * `createAndroidComposeRule`, as `AdaptiveShellTest` uses and for the reason it gives: the screens
+ * compose real ViewModels through `viewModel()`, and the plain rule supplies no `ViewModelStoreOwner`.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class LauncherWiringTest {
+
+    @get:Rule
+    val composeRule = createAndroidComposeRule<ComponentActivity>()
+
+    @Before
+    fun setUp() {
+        val app = RuntimeEnvironment.getApplication()
+        installTestWorkManager(app, Data.EMPTY)
+        // The real screen composes a real ViewModel; neither test here is about probing.
+        ConversionDependencies.probe = { _, _ -> InputProbe() }
+    }
+
+    @After
+    fun tearDown() = ConversionDependencies.reset()
+
+    /**
+     * The transposition guard. A picked file has to reach `onInputPicked`, which is observable as
+     * the screen arriving at `Ready` with the file card showing — `save()` from `Idle` returns at
+     * its own guard and leaves nothing behind.
+     */
+    @Test
+    fun `a picked document is loaded as input rather than saved to`() {
+        composeRule.setContent { ConverterScreen() }
+
+        composeRule.onNodeWithTag(TestTags.Converter.CHOOSE_FILE).performClick()
+        deliver(Uri.parse("content://test/holiday.mkv"))
+
+        composeRule.onNodeWithTag(TestTags.Converter.FILE_CARD_NAME).assertIsDisplayed()
+    }
+
+    /**
+     * `ConverterScreen.kt:65-67` records why the all-types wildcard is load-bearing rather than lazy:
+     *
+     * > the picker is images and video only, offers no audio at all, and will not reliably surface
+     * > .mkv/.flac/.webm
+     *
+     * Narrowing it would make every audio conversion unreachable from the file picker, and nothing
+     * would have gone red. (The literal is spelled only in the assertion below: a KDoc cannot
+     * contain it, because the wildcard's second half closes the comment.)
+     */
+    @Test
+    fun `the converter picker asks for every type, not just the ones a photo picker offers`() {
+        composeRule.setContent { ConverterScreen() }
+
+        composeRule.onNodeWithTag(TestTags.Converter.CHOOSE_FILE).performClick()
+
+        val intent = launched().intent
+        assertEquals(Intent.ACTION_OPEN_DOCUMENT, intent.action)
+        assertEquals(listOf("*/*"), intent.getStringArrayExtra(Intent.EXTRA_MIME_TYPES)?.toList())
+    }
+
+    @Test
+    fun `the join picker asks for video and accepts more than one file`() {
+        composeRule.setContent { JoinScreen() }
+
+        composeRule.onNodeWithTag(TestTags.Join.CHOOSE_FILES).performClick()
+
+        val intent = launched().intent
+        assertEquals(Intent.ACTION_OPEN_DOCUMENT, intent.action)
+        assertEquals(listOf("video/*"), intent.getStringArrayExtra(Intent.EXTRA_MIME_TYPES)?.toList())
+        // A join of one file is not a join; the contract is what asks for several.
+        assertEquals(true, intent.getBooleanExtra(Intent.EXTRA_ALLOW_MULTIPLE, false))
+    }
+
+    private fun launched(): ShadowActivity.IntentForResult {
+        composeRule.waitForIdle()
+        return requireNotNull(shadowOf(composeRule.activity).nextStartedActivityForResult) {
+            "nothing was launched for a result"
+        }
+    }
+
+    private fun deliver(uri: Uri) {
+        val started = launched()
+        shadowOf(composeRule.activity).receiveResult(
+            started.intent,
+            Activity.RESULT_OK,
+            Intent().setData(uri),
+        )
+        composeRule.waitForIdle()
+    }
+}

app/src/test/java/org/libremediaconverter/convert/Media3MuxerGuardTest.kt

@@ -0,0 +1,99 @@
+package org.libremediaconverter.convert
+
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import kotlinx.coroutines.runBlocking
+import kotlinx.coroutines.withTimeout
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotEquals
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.AudioCodec
+import org.libremediaconverter.model.AudioPlan
+import org.libremediaconverter.model.Container
+import org.libremediaconverter.model.ConversionRequest
+import org.libremediaconverter.model.CopyPlanner
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.model.OutputSpec
+import org.libremediaconverter.model.VideoCodec
+import org.libremediaconverter.model.VideoPlan
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.io.File
+import java.util.concurrent.CancellationException
+
+/**
+ * A job that reached Media3 with a container Media3 cannot mux.
+ *
+ * [Media3Muxers]' own KDoc names the defect this guards: *"the router claimed five containers while
+ * the engine silently wrote MP4 for all of them."* `factoryFor` answers null for fourteen of the
+ * app's containers, and `buildTransformer` turns that null into a failed job rather than letting
+ * `Transformer` fall back to its default muxer.
+ *
+ * The guard had never fired. `Media3Engine$buildTransformer$3` -- the `requireNotNull` message
+ * lambda -- was four lines and four branches at 0%, which is to say the entire repair for a defect
+ * the codebase went to the trouble of writing down was untested. Weakening it would restore that
+ * bug silently, because the wrong output is a *playable file with the wrong container*, not a crash.
+ *
+ * Same harness and same two disciplines as [Media3EngineEmptyCompositionTest]: assert the plan
+ * really is the one the test needs before driving the engine, and rule out
+ * `CancellationException` so an unresumed continuation cannot read as a pass.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class Media3MuxerGuardTest {
+
+    @Test
+    fun `a container Media3 cannot mux fails the job rather than silently writing MP4`() {
+        val context = RuntimeEnvironment.getApplication()
+        val engine = Media3Engine(context)
+        val request = ConversionRequest(
+            spec = OutputSpec(Container.WEBM, VideoCodec.VP9, AudioCodec.OPUS),
+            probe = InputProbe(videoCodec = "h264", audioCodec = "aac", container = Container.MP4),
+        )
+
+        // The premise, asserted rather than assumed -- three separate ways this test could pass
+        // over a path it never entered.
+        val plan = CopyPlanner.plan(request.spec, request.probe)
+        assertEquals("the plan has to still be WebM by the time the engine sees it", Container.WEBM, plan.container)
+        assertNull("...and Media3 really has no muxer for it", Media3Muxers.factoryFor(plan.container))
+        // Not the empty-composition refusal, which fires earlier and is a different test's subject.
+        assertNotEquals(VideoPlan.Drop, plan.video)
+        assertNotEquals(AudioPlan.Drop, plan.audio)
+
+        val failure = try {
+            runCatching {
+                runBlocking {
+                    withTimeout(TIMEOUT_MS) {
+                        engine.transcode(Uri.parse("file:///dev/null"), File(context.cacheDir, "guard.webm"), request) {
+                        }
+                    }
+                }
+            }.exceptionOrNull()
+        } finally {
+            engine.close()
+        }
+
+        assertFalse(
+            "the continuation was never resumed -- the refusal escaped instead of failing the job: $failure",
+            failure is CancellationException,
+        )
+        // Type *and* message, and the message half is the load-bearing one. Replacing the
+        // requireNotNull with a fallback factory does not make the export succeed here: it lets it
+        // run on and fail some other way, which a bare type assertion would happily accept.
+        assertTrue("expected the muxer guard to refuse the job, got $failure", failure is IllegalArgumentException)
+        assertTrue(
+            "the refusal has to name the container it could not mux, got: ${failure?.message}",
+            failure?.message.orEmpty().contains("cannot mux") &&
+                failure?.message.orEmpty().contains(Container.WEBM.name),
+        )
+    }
+
+    private companion object {
+        /** Nothing is decoded or muxed on this path -- the guard refuses before any of that. */
+        const val TIMEOUT_MS = 10_000L
+    }
+}

app/src/test/java/org/libremediaconverter/convert/MediaProbeMergeTest.kt

@@ -0,0 +1,166 @@
+package org.libremediaconverter.convert
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.libremediaconverter.model.Container
+import org.libremediaconverter.model.InputKind
+import org.libremediaconverter.model.InputProbe
+
+/**
+ * Which of the two probes wins, when they disagree.
+ *
+ * [MediaProbe.probe] runs `MediaExtractor` and FFprobe independently and then merges the two, and
+ * every rule in that merge is a decision. None of them had a test, for a reason that is structural
+ * rather than an oversight: `RemuxTest` drives the whole thing on a device against committed
+ * fixtures, but only ever with **one probe answering and the other agreeing or also failing**.
+ * Nothing on any source set can arrange for a real extractor and a real FFprobe to disagree, so
+ * every elvis in the merge was taken in one direction and never the other.
+ *
+ * Cutting `merge` out of `probe` is what makes the question askable. Both halves of its signature
+ * had to become `internal` for that -- `Extracted` already was, with a KDoc giving this exact
+ * reason; `FFprobeInfo` simply never got the same treatment.
+ */
+class MediaProbeMergeTest {
+
+    /**
+     * The rule with the loudest failure mode, and `isImageFormat`'s own KDoc names it: a false
+     * positive here "makes the source-info card describe a video as an image". So the image verdict
+     * has to beat a real video codec from the extractor, and the ordering that makes it do so is
+     * the first arm of `classify` rather than anything a reader would infer from the fields.
+     */
+    @Test
+    fun `an image verdict from FFprobe beats a video codec from the extractor`() {
+        val merged = MediaProbe.merge(
+            extracted = extracted(video = "h264"),
+            info = info(video = "mjpeg", isImage = true),
+        )
+
+        assertEquals(InputKind.IMAGE, merged.kind)
+    }
+
+    @Test
+    fun `the extractor wins on codecs, because it is the view the router will act on`() {
+        val merged = MediaProbe.merge(
+            extracted = extracted(video = "h264", audio = "aac"),
+            info = info(video = "hevc", audio = "mp3"),
+        )
+
+        assertEquals("h264", merged.videoCodec)
+        assertEquals("aac", merged.audioCodec)
+    }
+
+    @Test
+    fun `FFprobe answers for a file the extractor could not open`() {
+        val merged = MediaProbe.merge(extracted = null, info = info(video = "vp9", audio = "opus"))
+
+        assertEquals("vp9", merged.videoCodec)
+        assertEquals("opus", merged.audioCodec)
+        assertEquals(InputKind.VIDEO, merged.kind)
+    }
+
+    @Test
+    fun `the extractor answers for a file FFprobe could not read`() {
+        val merged = MediaProbe.merge(extracted = extracted(video = "h264", audio = "aac"), info = null)
+
+        assertEquals("h264", merged.videoCodec)
+        assertEquals("aac", merged.audioCodec)
+        assertNull("only FFprobe can name the container, so it stays unknown here", merged.container)
+    }
+
+    /**
+     * The larger of the two, not the first non-zero.
+     *
+     * Either probe can report zero for a file the other times correctly, and a zero duration makes
+     * the FFmpeg progress percentage undefined -- `FFmpegEngine` divides by it. Both orderings are
+     * asserted because "take the extractor's" and "take the larger" agree in one direction and not
+     * the other, and only one of them is the rule.
+     */
+    @Test
+    fun `duration is the longer of the two readings, whichever probe supplied it`() {
+        assertEquals(
+            5_000L,
+            MediaProbe.merge(extracted(duration = 0L), info(duration = 5_000L)).durationMs,
+        )
+        assertEquals(
+            5_000L,
+            MediaProbe.merge(extracted(duration = 5_000L), info(duration = 0L)).durationMs,
+        )
+    }
+
+    @Test
+    fun `dimensions come from the extractor, and from FFprobe only when it has none`() {
+        assertEquals(1920, MediaProbe.merge(extracted(width = 1920), info(width = 640)).width)
+        assertEquals(640, MediaProbe.merge(extracted = null, info = info(width = 640)).width)
+        assertEquals(0, MediaProbe.merge(extracted(width = 0), info(width = 0)).width)
+    }
+
+    @Test
+    fun `the container comes from FFprobe, which is the only probe that can name one`() {
+        val merged = MediaProbe.merge(extracted(video = "h264"), info(container = Container.MKV))
+
+        assertEquals(Container.MKV, merged.container)
+    }
+
+    @Test
+    fun `a file with audio and no video is audio-only, not unparseable`() {
+        val merged = MediaProbe.merge(extracted(video = null, audio = "mp3"), info = null)
+
+        assertEquals(InputKind.AUDIO_ONLY, merged.kind)
+        assertFalse(merged.hasVideo)
+    }
+
+    @Test
+    fun `a file neither probe could open is the one unreadable answer`() {
+        val merged = MediaProbe.merge(extracted = null, info = null)
+
+        assertEquals(MediaProbe.UNREADABLE, merged)
+        assertEquals(InputProbe.UNPARSEABLE, merged.videoCodec)
+    }
+
+    /**
+     * The arm the ticket was filed for: parsed, and carrying no stream either probe recognised.
+     *
+     * Distinct from "neither probe could open it" -- here the extractor opened the file happily and
+     * found nothing convertible, which is what a container holding only subtitles looks like. It
+     * has to reach the same [MediaProbe.UNREADABLE] answer, because the router keys off that and
+     * there is nothing here for Media3 to do either way.
+     *
+     * Its input was already being built elsewhere in the suite -- `MediaProbeTrackWalkTest` calls
+     * `extractedFrom(emptyList())` and gets exactly this -- and had simply never been handed to the
+     * merge.
+     */
+    @Test
+    fun `a file that parsed but carries no recognised stream is unreadable too`() {
+        val merged = MediaProbe.merge(extracted = MediaProbe.extractedFrom(emptyList()), info = null)
+
+        assertEquals(InputKind.UNPARSEABLE, merged.kind)
+        assertEquals(MediaProbe.UNREADABLE, merged)
+    }
+
+    @Test
+    fun `hasVideo follows the codec that survived the merge, not either probe alone`() {
+        assertTrue(MediaProbe.merge(extracted(video = null), info(video = "vp9")).hasVideo)
+        assertFalse(MediaProbe.merge(extracted(video = null, audio = "aac"), info(video = null)).hasVideo)
+    }
+
+    private fun extracted(
+        video: String? = "h264",
+        audio: String? = "aac",
+        duration: Long = 1_000L,
+        width: Int = 1280,
+        height: Int = 720,
+    ) = MediaProbe.Extracted(video, audio, duration, width, height)
+
+    private fun info(
+        container: Container? = null,
+        video: String? = "h264",
+        audio: String? = "aac",
+        duration: Long = 1_000L,
+        width: Int = 1280,
+        height: Int = 720,
+        isImage: Boolean = false,
+    ) = MediaProbe.FFprobeInfo(container, video, audio, duration, width, height, isImage)
+}

app/src/test/java/org/libremediaconverter/convert/MediaProbeTrackWalkTest.kt

@@ -0,0 +1,221 @@
+package org.libremediaconverter.convert
+
+import android.media.MediaFormat
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+
+/**
+ * The rules `MediaProbe` applies to a set of track formats.
+ *
+ * ## Why this exists, and what it revises
+ *
+ * Issue #84 classified `probeWithExtractor` and `probeForConcat` as device-bound and explicitly not
+ * a gap:
+ *
+ * > These are exercised by `RemuxTest`, `ConcatEngineTest` and `RealMediaBenchmark` in
+ * > `androidTest` … **Do not read their 0% as untested.**
+ *
+ * That was right about the measurement boundary and right about FFprobe. It was not right that
+ * these are only orchestration. The track walk is a **branch matrix**, and `androidTest` reaches it
+ * only through whatever the committed fixtures happen to contain — so none of the rules below is
+ * *chosen* by any test there. A fixture with two video tracks, a track that omits its duration, or
+ * an audio-before-video ordering is not something a device test would produce on purpose.
+ *
+ * The seam is the answer #133 preferred over driving `ShadowMediaExtractor`: the walk is a pure
+ * function over `List<MediaFormat>`, and what is left needing a device — `setDataSource`,
+ * `getTrackFormat`, `release` — is the thin edge `androidTest` should be covering. This is the
+ * `work/FailureOutcome.kt` pattern `CLAUDE.md` names.
+ *
+ * `MediaFormat` is a real one throughout, not a stub. `MediaProbeTrackFieldsTest` records why that
+ * matters: it is a heterogeneous map whose getters throw rather than coerce, and a hand-rolled
+ * double would not reproduce that.
+ */
+@RunWith(RobolectricTestRunner::class)
+class MediaProbeTrackWalkTest {
+
+    // --- extractedFrom: the conversion flow's read ---------------------------
+
+    @Test
+    fun `the first video track wins when a file carries two`() {
+        // `video == null` is the entire guard. A file with two video tracks must report the first,
+        // because that is the one an engine will transcode -- and the width and height must come
+        // from the same track, not be mixed across them.
+        val extracted = MediaProbe.extractedFrom(
+            listOf(
+                video(MediaFormat.MIMETYPE_VIDEO_AVC, width = 1920, height = 1080),
+                video(MediaFormat.MIMETYPE_VIDEO_HEVC, width = 640, height = 480),
+            ),
+        )
+
+        assertEquals("h264", extracted.videoCodec)
+        assertEquals(1920, extracted.width)
+        assertEquals(1080, extracted.height)
+    }
+
+    @Test
+    fun `the first audio track wins when a file carries two`() {
+        val extracted = MediaProbe.extractedFrom(
+            listOf(
+                audio(MediaFormat.MIMETYPE_AUDIO_AAC),
+                audio(MediaFormat.MIMETYPE_AUDIO_OPUS),
+            ),
+        )
+
+        assertEquals("aac", extracted.audioCodec)
+    }
+
+    @Test
+    fun `duration is the longest track, not the first or the last`() {
+        // A file whose audio outlasts its video is ordinary. Taking the video's length would cut
+        // the progress bar short; taking the last track's would be right only by accident of order.
+        val extracted = MediaProbe.extractedFrom(
+            listOf(
+                video(MediaFormat.MIMETYPE_VIDEO_AVC, durationUs = 10_000_000),
+                audio(MediaFormat.MIMETYPE_AUDIO_AAC, durationUs = 12_500_000),
+                audio(MediaFormat.MIMETYPE_AUDIO_OPUS, durationUs = 1_000_000),
+            ),
+        )
+
+        assertEquals(12_500L, extracted.durationMs)
+    }
+
+    @Test
+    fun `a track that does not declare its duration contributes nothing to it`() {
+        // MediaExtractor omits KEY_DURATION for plenty of real tracks -- MediaProbeTrackFieldsTest
+        // records the same for KEY_FRAME_RATE. Reading a key that is absent is what containsKey
+        // stands between us and.
+        val extracted = MediaProbe.extractedFrom(
+            listOf(
+                video(MediaFormat.MIMETYPE_VIDEO_AVC),
+                audio(MediaFormat.MIMETYPE_AUDIO_AAC, durationUs = 7_000_000),
+            ),
+        )
+
+        assertEquals(7_000L, extracted.durationMs)
+    }
+
+    @Test
+    fun `declaring audio before video changes nothing`() {
+        // Track order is a property of the container, not of the content. Both orderings have to
+        // reach the same answer or the same file remuxed twice would probe differently.
+        val videoFirst = MediaProbe.extractedFrom(
+            listOf(
+                video(MediaFormat.MIMETYPE_VIDEO_AVC, width = 1280, height = 720),
+                audio(MediaFormat.MIMETYPE_AUDIO_AAC),
+            ),
+        )
+        val audioFirst = MediaProbe.extractedFrom(
+            listOf(
+                audio(MediaFormat.MIMETYPE_AUDIO_AAC),
+                video(MediaFormat.MIMETYPE_VIDEO_AVC, width = 1280, height = 720),
+            ),
+        )
+
+        assertEquals(videoFirst.videoCodec, audioFirst.videoCodec)
+        assertEquals(videoFirst.audioCodec, audioFirst.audioCodec)
+        assertEquals(videoFirst.width, audioFirst.width)
+        assertEquals(videoFirst.height, audioFirst.height)
+    }
+
+    @Test
+    fun `a track that is neither audio nor video is ignored`() {
+        // Subtitle and timed-metadata tracks are common in MKV and MP4. Neither prefix matches, so
+        // neither slot is filled -- and, importantly, a subtitle track must not be mistaken for the
+        // absence of an audio track by some later `else`.
+        val extracted = MediaProbe.extractedFrom(
+            listOf(
+                MediaFormat().apply { setString(MediaFormat.KEY_MIME, "text/vtt") },
+                video(MediaFormat.MIMETYPE_VIDEO_AVC),
+            ),
+        )
+
+        assertEquals("h264", extracted.videoCodec)
+        assertNull(extracted.audioCodec)
+    }
+
+    @Test
+    fun `a file with no tracks reports nothing rather than zero-width video`() {
+        val extracted = MediaProbe.extractedFrom(emptyList())
+
+        assertNull(extracted.videoCodec)
+        assertNull(extracted.audioCodec)
+        assertEquals(0L, extracted.durationMs)
+        assertEquals(0, extracted.width)
+        assertEquals(0, extracted.height)
+    }
+
+    @Test
+    fun `an audio-only file reports no video codec at all`() {
+        // The distinction MediaProbe.classify turns into InputKind.AUDIO_ONLY, and the reason
+        // `hasVideo` exists: an audio file and a corrupt file must not look alike.
+        val extracted = MediaProbe.extractedFrom(listOf(audio(MediaFormat.MIMETYPE_AUDIO_AAC)))
+
+        assertNull(extracted.videoCodec)
+        assertEquals("aac", extracted.audioCodec)
+        assertEquals(0, extracted.width)
+    }
+
+    // --- concatInputFrom: the join flow's read -------------------------------
+
+    @Test
+    fun `the join read takes frame rate from the first video track`() {
+        val input = MediaProbe.concatInputFrom(
+            listOf(
+                video(MediaFormat.MIMETYPE_VIDEO_AVC, width = 1920, height = 1080, frameRate = 30),
+                video(MediaFormat.MIMETYPE_VIDEO_HEVC, width = 640, height = 480, frameRate = 60),
+                audio(MediaFormat.MIMETYPE_AUDIO_AAC),
+            ),
+        )
+
+        assertEquals("h264", input.videoCodec)
+        assertEquals("aac", input.audioCodec)
+        assertEquals(1920, input.width)
+        assertEquals(1080, input.height)
+        assertEquals(30, input.frameRate)
+    }
+
+    @Test
+    fun `a video track with no declared frame rate reports zero rather than guessing`() {
+        // ConcatPlanner treats 0 as "cannot prove a match" and re-encodes. A guessed 30 would read
+        // as agreement and produce a stream copy of clips that do not actually match -- the failure
+        // its KDoc says the whole flow is arranged to avoid.
+        val input = MediaProbe.concatInputFrom(listOf(video(MediaFormat.MIMETYPE_VIDEO_AVC)))
+
+        assertEquals(0, input.frameRate)
+    }
+
+    @Test
+    fun `a file with no tracks joins as entirely unknown`() {
+        val input = MediaProbe.concatInputFrom(emptyList())
+
+        assertNull(input.videoCodec)
+        assertNull(input.audioCodec)
+        assertEquals(0, input.width)
+        assertEquals(0, input.height)
+        assertEquals(0, input.frameRate)
+    }
+
+    private fun video(
+        mime: String,
+        width: Int = 1920,
+        height: Int = 1080,
+        durationUs: Long? = null,
+        frameRate: Int? = null,
+    ): MediaFormat = MediaFormat.createVideoFormat(mime, width, height).apply {
+        durationUs?.let { setLong(MediaFormat.KEY_DURATION, it) }
+        frameRate?.let { setInteger(MediaFormat.KEY_FRAME_RATE, it) }
+    }
+
+    private fun audio(mime: String, durationUs: Long? = null): MediaFormat =
+        MediaFormat.createAudioFormat(mime, SAMPLE_RATE, CHANNELS).apply {
+            durationUs?.let { setLong(MediaFormat.KEY_DURATION, it) }
+        }
+
+    private companion object {
+        const val SAMPLE_RATE = 48_000
+        const val CHANNELS = 2
+    }
+}

app/src/test/java/org/libremediaconverter/convert/MessagelessFailureTest.kt

@@ -0,0 +1,213 @@
+package org.libremediaconverter.convert
+
+import android.app.Application
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import androidx.work.ListenableWorker
+import androidx.work.testing.TestListenableWorkerBuilder
+import androidx.work.workDataOf
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.join.JoinState
+import org.libremediaconverter.join.JoinViewModel
+import org.libremediaconverter.model.ConversionRequest
+import org.libremediaconverter.model.EnginePreference
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.work.ConcatWorker
+import org.libremediaconverter.work.ConversionWorker
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.io.File
+import java.util.UUID
+
+/**
+ * A failure that says nothing still has to say something.
+ *
+ * Three sites, all `ci == 0` before this file, and all the same rule:
+ *
+ * ```
+ * work/ConversionWorker.kt:316     cause.message ?: GENERIC_FAILURE_MESSAGE
+ * convert/ConversionViewModel.kt:631   e.message ?: SAVE_FAILED_MESSAGE
+ * join/JoinViewModel.kt:416           e.message ?: SAVE_FAILED_MESSAGE
+ * ```
+ *
+ * Every existing test throws *with* a message, so the right-hand side had never been evaluated
+ * anywhere in the suite. A `Throwable` carrying none is not exotic — `RuntimeException()`,
+ * `IOException()` and most platform exceptions raised without an argument all have a null message.
+ *
+ * ## Held in one class, against the ticket's suggestion
+ *
+ * #193 proposed putting each case beside the behaviour it neighbours. They are together instead,
+ * because they are one rule at three layers and because the trap below has to be explained once
+ * rather than three times. `FailedSaveRetryTest` sets the precedent for both ViewModels in one
+ * file; this extends it by one worker.
+ *
+ * ## The trap, which is why the worker case asserts what it does
+ *
+ * `ConversionStateMappingTest`'s *"a failure with nothing said still says something"* looks like it
+ * already covers the worker site. It does not: it drives the **read** side, `map(FAILED, Data.EMPTY)`,
+ * and that side has a fallback of its own (`ConversionViewModel.kt:147-149`):
+ *
+ * ```kotlin
+ * update.outputData.getString(ConversionWorker.KEY_ERROR)
+ *     ?.takeIf { it.isNotBlank() }
+ *     ?: ConversionWorker.GENERIC_FAILURE_MESSAGE
+ * ```
+ *
+ * So mutating the worker's fallback to `.orEmpty()` writes `KEY_ERROR to ""`, and the ViewModel
+ * turns that straight back into the same constant. **A test asserting on the resulting `Failed`
+ * state stays green under the mutation**, which is most likely why the write-side fallback survived
+ * three waves of test work. The worker case therefore reads `KEY_ERROR` off the worker's own
+ * `Result`, before anything downstream can repair it.
+ *
+ * The two save cases have no such second line: both write `_state.value` directly, so the state is
+ * the right thing to assert there.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class MessagelessFailureTest {
+
+    private lateinit var app: Application
+    private lateinit var publisher: RecordingPublisher
+    private lateinit var staged: File
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        publisher = RecordingPublisher(app)
+        ConversionDependencies.publisher = { publisher }
+        ConversionDependencies.probe = { _, _ -> InputProbe() }
+        staged = publisher.createStagingFile("holiday.mp4").apply { writeBytes(ByteArray(4096)) }
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    /**
+     * The engine gives up without saying why, which is what a native crash looks like from here.
+     *
+     * Asserted on the worker's own output `Data` rather than on a screen — see the class KDoc.
+     */
+    @Test
+    fun `a conversion that fails without a message still reports one`() {
+        installTestWorkManager(app, Data.EMPTY)
+        ConversionDependencies.software = { MessagelessTranscoder }
+
+        val result = runBlocking { failingWorker().doWork() }
+
+        assertTrue("the job must fail rather than retry, got $result", result is ListenableWorker.Result.Failure)
+        assertEquals(
+            "a failure with no message must still put something on screen",
+            ConversionWorker.GENERIC_FAILURE_MESSAGE,
+            (result as ListenableWorker.Result.Failure).outputData.getString(ConversionWorker.KEY_ERROR),
+        )
+    }
+
+    @Test
+    fun `a save that fails without a message still reports one`() {
+        installTestWorkManager(app, conversionOutput())
+        val viewModel = ConversionViewModel(app, Dispatchers.Unconfined)
+        viewModel.onInputPicked(Uri.parse("content://test/holiday.mkv"))
+        awaitState(viewModel.state, "Ready") { it is ConversionState.Ready }
+        viewModel.convert()
+        awaitState(viewModel.state, "Converted") { it is ConversionState.Converted }
+
+        publisher.publishFailure = RuntimeException()
+        viewModel.save(DESTINATION)
+
+        val failed = awaitState(viewModel.state, "Failed") { it is ConversionState.Failed } as ConversionState.Failed
+        assertEquals(SAVE_FAILED_MESSAGE, failed.message)
+        // The handle travels even on the wordless path. Without this, a fallback that also dropped
+        // `pending` would pass -- and the file would be unreachable from the screen that just said
+        // the save failed.
+        assertNotNull("a wordless failure must still offer the file again", failed.retry)
+    }
+
+    @Test
+    fun `a join save that fails without a message still reports one`() {
+        installTestWorkManager(app, joinOutput())
+        val viewModel = JoinViewModel(app, Dispatchers.Unconfined)
+        viewModel.onInputsPicked(listOf(Uri.parse("content://test/a.mp4"), Uri.parse("content://test/b.mp4")))
+        awaitState(viewModel.state, "Ready") { it is JoinState.Ready }
+        viewModel.join()
+        awaitState(viewModel.state, "Joined") { it is JoinState.Joined }
+
+        publisher.publishFailure = RuntimeException()
+        viewModel.save(DESTINATION)
+
+        val failed = awaitState(viewModel.state, "Failed") { it is JoinState.Failed } as JoinState.Failed
+        assertEquals(SAVE_FAILED_MESSAGE, failed.message)
+        assertNotNull("a wordless failure must still offer the file again", failed.retry)
+    }
+
+    /**
+     * `FORCE_SOFTWARE` so the failure comes straight out of `runFFmpeg`.
+     *
+     * `AUTO` would enter `runMedia3OrFallBack`, whose catch runs the job a second time in software
+     * — the same exception would arrive, but through a path this test is not about and which
+     * `HardwareFallbackTest` already owns.
+     */
+    private fun failingWorker(): ConversionWorker {
+        val spec = OutputFormat.MP4_H265.spec
+        return TestListenableWorkerBuilder<ConversionWorker>(
+            context = app,
+            inputData = workDataOf(
+                ConversionWorker.KEY_INPUT_URI to "file:///tmp/holiday.mp4",
+                ConversionWorker.KEY_DISPLAY_NAME to "holiday.mp4",
+                ConversionWorker.KEY_CONTAINER to spec.container.name,
+                ConversionWorker.KEY_VIDEO_CODEC to spec.videoCodec.name,
+                ConversionWorker.KEY_AUDIO_CODEC to spec.audioCodec.name,
+                ConversionWorker.KEY_ENGINE_PREFERENCE to EnginePreference.FORCE_SOFTWARE.name,
+            ),
+            runAttemptCount = 0,
+        ).setId(JOB_ID).build()
+    }
+
+    private fun conversionOutput() = workDataOf(
+        ConversionWorker.KEY_OUTPUT_PATH to staged.absolutePath,
+        ConversionWorker.KEY_SUGGESTED_NAME to SUGGESTED_NAME,
+        ConversionWorker.KEY_MIME_TYPE to JOB_MIME_TYPE,
+    )
+
+    private fun joinOutput() = workDataOf(
+        ConcatWorker.KEY_OUTPUT_PATH to staged.absolutePath,
+        ConcatWorker.KEY_SUGGESTED_NAME to SUGGESTED_NAME,
+        ConcatWorker.KEY_MIME_TYPE to JOB_MIME_TYPE,
+    )
+
+    private companion object {
+        val DESTINATION: Uri = Uri.parse("content://test/destination.mp4")
+        val JOB_ID: UUID = UUID.fromString("00000000-0000-4000-8000-00000000019a")
+        const val SUGGESTED_NAME = "holiday.mp4"
+        const val JOB_MIME_TYPE = "video/mp4"
+    }
+}
+
+/**
+ * An engine that gives up without saying why.
+ *
+ * `RuntimeException()` rather than a subclass with a blank message: `Throwable.message` is *null*
+ * here, which is the case the elvis exists for. A blank-but-present message takes the left-hand
+ * side and is a different path — `ConversionStateMappingTest` covers that one, on the read side.
+ */
+@UnstableApi
+private object MessagelessTranscoder : SoftwareTranscoder {
+    override suspend fun run(
+        request: ConversionRequest,
+        inputPath: String,
+        output: File,
+        durationMs: Long,
+        onProgress: (Int) -> Unit,
+    ): Unit = throw RuntimeException()
+}

app/src/test/java/org/libremediaconverter/convert/OutputPublisherPublishTest.kt

@@ -1,17 +1,7 @@
 package org.libremediaconverter.convert
 
-import android.content.ComponentName
-import android.content.ContentProvider
-import android.content.ContentValues
 import android.content.Context
-import android.content.IntentFilter
-import android.content.pm.ProviderInfo
-import android.database.Cursor
-import android.database.MatrixCursor
 import android.net.Uri
-import android.os.Bundle
-import android.provider.DocumentsContract
-import android.provider.OpenableColumns
 import org.junit.Assert.assertArrayEquals
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
@@ -20,7 +10,6 @@ import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
-import org.robolectric.Robolectric
 import org.robolectric.RobolectricTestRunner
 import org.robolectric.RuntimeEnvironment
 import org.robolectric.Shadows.shadowOf
@@ -31,90 +20,8 @@ import java.io.OutputStream
 /** What a destination volume says when it fills up mid-write. */
 private const val NO_SPACE = "No space left on device"
 
-private const val DOCUMENTS_AUTHORITY = "org.libremediaconverter.test.documents"
-private const val PLAIN_AUTHORITY = "org.libremediaconverter.test.plain"
-
-/**
- * A stand-in for the provider behind a SAF destination.
- *
- * It answers only what `publish()` asks of a destination -- how many bytes are already there,
- * and delete it -- backed by a real file so the assertions are about the filesystem rather
- * than about a mock's call log alone. The rest of the `ContentProvider` surface is stubbed.
- *
- * Writing is deliberately NOT routed through it. Robolectric's `ShadowContentResolver`
- * consults its registered-stream map before it reaches any provider, which is what lets a
- * test hand out a stream that writes some bytes and then fails -- the condition this whole
- * file exists for, and one a real provider cannot be asked to produce on demand.
- */
-internal open class FakeSafProvider : ContentProvider() {
-
-    override fun onCreate() = true
-
-    override fun query(
-        uri: Uri,
-        projection: Array<out String>?,
-        selection: String?,
-        selectionArgs: Array<out String>?,
-        sortOrder: String?,
-    ): Cursor? {
-        val file = backingFile(uri)
-        if (!file.exists()) return null
-        return MatrixCursor(arrayOf(OpenableColumns.DISPLAY_NAME, OpenableColumns.SIZE)).apply {
-            addRow(arrayOf<Any?>(file.name, file.length()))
-        }
-    }
-
-    override fun call(method: String, arg: String?, extras: Bundle?): Bundle? {
-        if (method != METHOD_DELETE_DOCUMENT) return null
-        val target = extras?.getParcelable(EXTRA_URI, Uri::class.java) ?: return null
-        deleteRequests += target
-        deleteFailure?.let { throw it }
-        backingFile(target).delete()
-        return Bundle()
-    }
-
-    override fun getType(uri: Uri) = "video/mp4"
-
-    override fun insert(uri: Uri, values: ContentValues?): Uri? = null
-
-    override fun delete(uri: Uri, selection: String?, selectionArgs: Array<out String>?) = 0
-
-    override fun update(uri: Uri, values: ContentValues?, selection: String?, selectionArgs: Array<out String>?) = 0
-
-    companion object {
-        // DocumentsContract.METHOD_DELETE_DOCUMENT and EXTRA_URI are hidden from the public
-        // SDK, so they cannot be referenced. These are the wire names
-        // DocumentsContract.deleteDocument() actually sends, which is what a provider sees.
-        const val METHOD_DELETE_DOCUMENT = "android:deleteDocument"
-        const val EXTRA_URI = "uri"
-
-        /** Where the "documents" really live. Set per test to a Robolectric temp path. */
-        lateinit var root: File
-
-        /** Every delete this provider was asked for, in order. Empty is an assertion too. */
-        val deleteRequests = mutableListOf<Uri>()
-
-        /** Armed by the test that needs the cleanup itself to fail. */
-        var deleteFailure: RuntimeException? = null
-
-        fun backingFile(uri: Uri) = File(root, uri.lastPathSegment.orEmpty())
-
-        fun reset(directory: File) {
-            root = directory
-            deleteRequests.clear()
-            deleteFailure = null
-        }
-    }
-}
-
-/**
- * The same provider, registered WITHOUT the documents-provider intent filter.
- *
- * A separate class because the package manager keys providers by component name, so two
- * authorities need two components. It exists to prove the guard is a guard: a content URI
- * from something that is not a documents provider must not be handed to `deleteDocument`.
- */
-internal class FakePlainProvider : FakeSafProvider()
+/** How far a failing copy gets before the volume "fills up". Any value below the payload does. */
+private const val PARTIAL_BYTES = 512
 
 /**
  * A sink that behaves like a volume filling up.
@@ -171,6 +78,8 @@ class OutputPublisherPublishTest {
 
     private val payload = ByteArray(8192) { (it % 251).toByte() }
 
+    /** How far a failing copy gets before the volume "fills up". Any value below the payload does. */
+
     private val documentUri: Uri = Uri.parse("content://$DOCUMENTS_AUTHORITY/document/holiday.mp4")
     private val plainUri: Uri = Uri.parse("content://$PLAIN_AUTHORITY/document/holiday_plain.mp4")
     private val deadUri: Uri = Uri.parse("content://org.libremediaconverter.nonexistent/document/gone.mp4")
@@ -179,8 +88,8 @@ class OutputPublisherPublishTest {
     fun setUp() {
         context = RuntimeEnvironment.getApplication()
         FakeSafProvider.reset(File(context.cacheDir, "destinations").apply { mkdirs() })
-        register(FakeSafProvider::class.java, DOCUMENTS_AUTHORITY, asDocumentsProvider = true)
-        register(FakePlainProvider::class.java, PLAIN_AUTHORITY, asDocumentsProvider = false)
+        registerProvider(context, FakeSafProvider::class.java, DOCUMENTS_AUTHORITY, asDocumentsProvider = true)
+        registerProvider(context, FakePlainProvider::class.java, PLAIN_AUTHORITY, asDocumentsProvider = false)
 
         // SAF's CreateDocument contract hands back a document that already exists and is
         // empty, so that is the state every destination starts in here.
@@ -299,6 +208,76 @@ class OutputPublisherPublishTest {
         assertEquals(emptyList<Uri>(), FakeSafProvider.deleteRequests)
     }
 
+    @Test
+    fun `a destination whose size cannot be determined is never deleted`() {
+        // The three short-circuits in destinationIsKnownEmpty, and the reason its KDoc gives for
+        // each of them answering false:
+        //
+        //   "this decides whether a delete is allowed and 'I could not tell' must never authorise
+        //    one."
+        //
+        // The contrast is `a copy that fails partway leaves nothing at the destination` above: a
+        // provider that *does* say zero gets the delete. These say nothing, so they must not.
+        // Getting this backwards costs the user a file they already had, on a save that failed.
+        //
+        // Named exemption: of the three conjuncts, `size >= 0` cannot be falsified behaviourally.
+        // Measured -- getColumnIndex returns -1 for an absent column, and isNull(-1) throws
+        // CursorIndexOutOfBoundsException, which the surrounding runCatching already turns into
+        // `?: false`. So relaxing it to `size >= -1` leaves this test green: same answer, reached
+        // by the exception path instead. The guard should stay -- control flow through an exception
+        // is worse than a comparison, and another Cursor implementation need not throw -- but no
+        // assertion here pins it, and saying so beats implying the missing-column case covers it.
+        // `!row.isNull(size)` and `row.moveToFirst()` do both bite.
+        listOf(
+            RowShape.NO_SIZE_COLUMN to "a cursor with no SIZE column",
+            RowShape.NULL_SIZE to "a cursor whose SIZE cell is null",
+            RowShape.NO_ROWS to "a cursor holding no rows",
+            // The third case the KDoc names -- "a resolver call that throws" -- and the one the
+            // list was missing. It reaches `?: false` through `runCatching` rather than through a
+            // cursor answer, so it is the only one of the four that proves the catch is load
+            // bearing: a provider that revokes its grant between the picker and the write must not
+            // have its document deleted on the way out.
+            RowShape.QUERY_THROWS to "a provider that throws out of query",
+        ).forEach { (shape, description) ->
+            FakeSafProvider.deleteRequests.clear()
+            FakeSafProvider.backingFile(documentUri).writeBytes(ByteArray(0))
+            FakeSafProvider.rowShape = shape
+            failMidCopy(documentUri, afterBytes = PARTIAL_BYTES)
+
+            assertThrows(IOException::class.java) { publisher.publish(staged, documentUri) }
+
+            assertEquals(
+                "$description must not authorise a delete",
+                emptyList<Uri>(),
+                FakeSafProvider.deleteRequests,
+            )
+            assertTrue(
+                "$description must leave the destination where it was",
+                FakeSafProvider.backingFile(documentUri).exists(),
+            )
+        }
+    }
+
+    @Test
+    fun `a provider that declines by returning null fails with the destination named`() {
+        // openOutputStream has two ways of refusing, and only one of them is otherwise reachable.
+        // `a destination the provider will not open...` above drives the throwing one -- a provider
+        // that has gone away. This is the other: a provider that is present, answers, and hands
+        // back null. Without the `?: error(...)` that becomes an NPE inside `use`, which reaches
+        // the user as "Conversion failed." with a null message.
+        val nullOpening = object : OutputPublisher(context) {
+            override fun openDestination(destination: Uri): OutputStream? = null
+        }
+
+        val failure = runCatching { nullOpening.publish(staged, documentUri) }.exceptionOrNull()
+
+        assertTrue("a null stream must not appear to succeed, got $failure", failure != null)
+        assertTrue(
+            "the failure must name the destination rather than being a bare NPE; got ${failure?.message}",
+            failure?.message?.contains("Could not open destination for writing") == true,
+        )
+    }
+
     @Test
     fun `a copy that succeeds delivers every byte and deletes nothing`() {
         shadowOf(context.contentResolver).registerOutputStreamSupplier(documentUri) {
@@ -326,28 +305,4 @@ class OutputPublisherPublishTest {
             )
         }
     }
-
-    private fun register(provider: Class<out FakeSafProvider>, authority: String, asDocumentsProvider: Boolean) {
-        val info = ProviderInfo().apply {
-            this.authority = authority
-            packageName = context.packageName
-            name = provider.name
-            exported = true
-            grantUriPermissions = true
-        }
-        Robolectric.buildContentProvider(provider).create(info)
-
-        // isDocumentUri() does not look at the URI alone: it asks the package manager whether
-        // anything answers ACTION_DOCUMENTS_PROVIDER for that authority. Registering the
-        // provider with the resolver is not enough, which is the whole reason the negative
-        // case above can exist.
-        val packageManager = shadowOf(context.packageManager)
-        packageManager.addOrUpdateProvider(info)
-        if (asDocumentsProvider) {
-            packageManager.addIntentFilterForProvider(
-                ComponentName(context.packageName, provider.name),
-                IntentFilter(DocumentsContract.PROVIDER_INTERFACE),
-            )
-        }
-    }
 }

app/src/test/java/org/libremediaconverter/convert/OutputPublisherStagingTest.kt

@@ -1,6 +1,8 @@
 package org.libremediaconverter.convert
 
+import android.app.Application
 import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
 import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Test
@@ -26,14 +28,18 @@ import java.util.UUID
 @RunWith(RobolectricTestRunner::class)
 class OutputPublisherStagingTest {
 
+    private lateinit var app: Application
     private lateinit var cacheDir: File
     private lateinit var publisher: OutputPublisher
 
     @Before
     fun setUp() {
-        val context = RuntimeEnvironment.getApplication()
-        cacheDir = context.cacheDir
-        publisher = OutputPublisher(context)
+        // Held as a field rather than a local: the race test below builds an anonymous
+        // OutputPublisher, and inside that `object` expression a bare `context` resolves to the
+        // superclass's own constructor property, which is not initialised at the super call.
+        app = RuntimeEnvironment.getApplication()
+        cacheDir = app.cacheDir
+        publisher = OutputPublisher(app)
     }
 
     @Test
@@ -99,4 +105,104 @@ class OutputPublisherStagingTest {
 
         publisher.sweepStaging()
     }
+
+    @Test
+    fun `the sweep tolerates a staging path that is not a directory`() {
+        // The other half of `listFiles() ?: return`, and not the same as the case above: a missing
+        // directory is created by `stagingDir`'s own mkdirs() and lists as empty. Only a path that
+        // cannot be a directory makes listFiles() answer null, and a sweep that dereferenced that
+        // would take the app down on a launch rather than on a conversion -- AppStartSweepTest is
+        // where this runs from.
+        val stagingPath = stagingPathAsRegularFile()
+
+        publisher.sweepStaging()
+
+        assertTrue("the sweep must not have replaced the fixture", stagingPath.isFile)
+    }
+
+    /**
+     * Makes `cacheDir/conversions` a regular file, which is the whole precondition of the test
+     * above -- and does it in a loop, because a single delete-then-write once lost a race that CI
+     * caught and this machine did not reproduce.
+     *
+     * **That race is closed at the source as of #159, and the loop is kept anyway.**
+     * `LibreMediaConverterApp.onCreate` launched its staging sweep on `Dispatchers.IO`, and
+     * `sweepStaging` reads `stagingDir`, whose getter calls `mkdirs()`. Robolectric builds an
+     * application for every test class that asks for one, so that background `mkdirs()` was in
+     * flight across the whole suite, on a thread the paused main looper does not control. Between
+     * deleting this path and writing it there is a window where the path does not exist and that
+     * `mkdirs()` could win -- `FileNotFoundException: ... (Is a directory)` out of `writeBytes`,
+     * run 33069641674 on #149, once, against 468 tests that passed here. The JVM suite now runs
+     * `TestLibreMediaConverterApp`, whose sweep finishes before `onCreate()` returns, so nothing is
+     * sweeping while a test body runs.
+     *
+     * The loop stays because it is what would catch that substitution being undone. Without it the
+     * regression returns as this one class failing rarely on CI -- the exact shape that took #159
+     * from a single run on #149 to a wave-4 flake before anyone chased it. Retrying closes the
+     * window rather than narrowing it, because the race is not symmetric: `mkdirs()` fails on an
+     * existing regular file, so the invariant only has to survive being *established*.
+     */
+    private fun stagingPathAsRegularFile(): File {
+        val stagingPath = File(cacheDir, "conversions")
+        repeat(FIXTURE_ATTEMPTS) {
+            if (stagingPath.isFile) return stagingPath
+            stagingPath.deleteRecursively()
+            runCatching { stagingPath.writeBytes(ByteArray(FIXTURE_BYTES)) }
+        }
+        check(stagingPath.isFile) {
+            "the fixture needs $stagingPath to be a regular file and it is a directory; " +
+                "something recreated it $FIXTURE_ATTEMPTS times -- see #159"
+        }
+        return stagingPath
+    }
+
+    @Test
+    fun `a file that stops being collectable between the listing and the delete survives`() {
+        // The race the second timestamp read exists for, and the only branch of it that had never
+        // run. The comment in sweepStaging states the cost precisely: a worker resumed by
+        // WorkManager -- in this same process -- could have started writing this very file, and
+        // unlinking an inode a running job still holds open ends with the job reporting success for
+        // a path that no longer exists.
+        //
+        // So: a file old enough to collect at listing time, touched to now before the delete is
+        // reached. StagingSweep.collectable already said yes; isCollectable has to say no.
+        val orphan = publisher.createStagingFile(
+            StagingNames.forJob(UUID.randomUUID(), "mp4"),
+        ).apply { writeBytes(ByteArray(4096)) }
+        assertTrue(orphan.setLastModified(System.currentTimeMillis() - StagingSweep.GRACE_PERIOD_MS - 60_000))
+
+        // Touched *after* the snapshot is taken, which is the only window that reaches the
+        // re-read. Doing it around listFiles() instead changes what StagingSweep.collectable is
+        // given, so the file is never proposed for deletion and the guard is never exercised --
+        // measured, and the reason the seam sits where it does.
+        val racing = object : OutputPublisher(app) {
+            override fun snapshot(listing: Array<File>): List<StagingSweep.Entry> =
+                super.snapshot(listing).also { orphan.setLastModified(System.currentTimeMillis()) }
+        }
+
+        racing.sweepStaging()
+
+        assertTrue(
+            "a file a live job started writing after the listing must not be unlinked",
+            orphan.exists(),
+        )
+    }
+
+    @Test
+    fun `discarding a file with no parent at all is refused`() {
+        // A relative name has no parent directory, so `staged.parentFile` is null. The handle
+        // reaches the ViewModel as a path string out of WorkInfo.outputData and is turned straight
+        // into a File, so this is not a shape the caller can rule out -- and the guard has to
+        // answer false rather than dereference it.
+        val parentless = File("holiday.mp4")
+        assertNull("the fixture is supposed to have no parent", parentless.parentFile)
+
+        assertFalse("a file with no parent is not in staging", publisher.discardStaged(parentless))
+    }
+
+    private companion object {
+        /** Enough to outlast a burst of application-scope sweeps; one attempt is what CI lost. */
+        const val FIXTURE_ATTEMPTS = 50
+        const val FIXTURE_BYTES = 8
+    }
 }

app/src/test/java/org/libremediaconverter/convert/RetrySaveMimeTest.kt

@@ -0,0 +1,136 @@
+package org.libremediaconverter.convert
+
+import android.app.Application
+import android.content.Intent
+import android.net.Uri
+import androidx.activity.ComponentActivity
+import androidx.compose.ui.test.junit4.v2.createAndroidComposeRule
+import androidx.compose.ui.test.onNodeWithTag
+import androidx.compose.ui.test.performClick
+import androidx.compose.ui.test.performScrollTo
+import androidx.media3.common.util.UnstableApi
+import androidx.work.WorkManager
+import androidx.work.workDataOf
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Before
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.ui.TestTags
+import org.libremediaconverter.work.ConversionWorker
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import org.robolectric.Shadows.shadowOf
+import java.io.File
+
+/**
+ * The save dialog opens with the type the *job* produced, not the type the picker is showing now.
+ *
+ * `ConverterScreen.kt:80` — `state.pendingSave()?.mimeType ?: settings.spec.mimeType` — had never
+ * taken its left-hand side. Its comment records what the line is for:
+ *
+ * > a retry offered after a failed save opens the dialog with the type its first attempt used —
+ * > the cast answered null for a `Failed`, and the fallback below is the current picker, which a
+ * > reattached job never set.
+ *
+ * So the untested half is the fix, and the tested half is the fallback it was added to stop being
+ * used.
+ *
+ * ## This revises a named exemption, deliberately
+ *
+ * `FailedSaveRetryTest`'s KDoc lists this line under "Not asserted here, so each is a decision
+ * rather than an omission":
+ *
+ * > It lives in the entry point, above the `ScreenContent` seam, and reaching it needs a real
+ * > ViewModel inside a composition.
+ *
+ * That was true when written. `AdaptiveShellTest` (#173) then established exactly that capability,
+ * and #200 added the two `ShadowActivity` mechanics that let a test read what a launcher launched.
+ * The reason the exemption gave no longer holds, so the exemption is withdrawn rather than left to
+ * be taken at face value — the same shape as #141 revising #84's boundary. That KDoc is corrected
+ * in this change.
+ *
+ * ## Why the job is reattached rather than run
+ *
+ * The screen composes its own ViewModel through `viewModel()`, so nothing can be injected into it.
+ * A job finished before the composition is the one route to a `Converted` state carrying output
+ * `Data` this test chose — and it is also the case the line exists for, since a reattached job's
+ * spec "was never in these settings at all".
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class RetrySaveMimeTest {
+
+    @get:Rule
+    val composeRule = createAndroidComposeRule<ComponentActivity>()
+
+    private lateinit var app: Application
+    private lateinit var staged: File
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        ConversionDependencies.probe = { _, _ -> InputProbe() }
+        staged = OutputPublisher(app).createStagingFile("holiday.mkv").apply { writeBytes(ByteArray(4096)) }
+    }
+
+    @After
+    fun tearDown() = ConversionDependencies.reset()
+
+    @Test
+    fun `the save dialog offers the type the job produced, not the one the picker is showing`() {
+        finishAJobProducing(JOB_MIME_TYPE)
+        composeRule.setContent { ConverterScreen() }
+        composeRule.waitForIdle()
+
+        composeRule.onNodeWithTag(TestTags.SAVE_FILE).performScrollTo().performClick()
+        composeRule.waitForIdle()
+
+        val intent = requireNotNull(shadowOf(composeRule.activity).nextStartedActivityForResult) {
+            "the save dialog was never launched"
+        }.intent
+        assertEquals(Intent.ACTION_CREATE_DOCUMENT, intent.action)
+        assertEquals(JOB_MIME_TYPE, intent.type)
+        // The fixture is only meaningful while the two differ; without this the assertion above
+        // would pass just as well against the fallback.
+        assertNotEquals(
+            "the picker's own type must differ, or this test proves nothing",
+            JOB_MIME_TYPE,
+            OutputFormat.MP4_H265.spec.mimeType,
+        )
+    }
+
+    /**
+     * A conversion that finished while nothing was watching, which is what `reattach()` picks up.
+     *
+     * `SucceedingWorkerFactory` reports this output `Data` for whatever is enqueued, so the job
+     * lands `SUCCEEDED` carrying a staged path that exists — the two things `Reattachment.choose`
+     * requires of a finished job.
+     */
+    private fun finishAJobProducing(mimeType: String) {
+        installTestWorkManager(
+            app,
+            workDataOf(
+                ConversionWorker.KEY_OUTPUT_PATH to staged.absolutePath,
+                ConversionWorker.KEY_SUGGESTED_NAME to "holiday.mkv",
+                ConversionWorker.KEY_MIME_TYPE to mimeType,
+            ),
+        )
+        WorkManager.getInstance(app).enqueue(
+            ConversionWorker.request(
+                inputUri = Uri.parse("content://test/holiday.mkv"),
+                displayName = "holiday.mkv",
+                sizeBytes = 4_096L,
+            ),
+        ).result.get()
+    }
+
+    private companion object {
+        /** Matroska, against the MP4 the picker defaults to. */
+        const val JOB_MIME_TYPE = "video/x-matroska"
+    }
+}

app/src/test/java/org/libremediaconverter/convert/ScreenWiringTest.kt

@@ -0,0 +1,241 @@
+package org.libremediaconverter.convert
+
+import android.app.Application
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import androidx.work.workDataOf
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.join.JoinState
+import org.libremediaconverter.join.JoinViewModel
+import org.libremediaconverter.join.joinActions
+import org.libremediaconverter.model.AudioCodec
+import org.libremediaconverter.model.Container
+import org.libremediaconverter.model.EnginePreference
+import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.model.QualityTier
+import org.libremediaconverter.model.VideoCodec
+import org.libremediaconverter.work.ConcatWorker
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+/**
+ * That each affordance is wired to the ViewModel method it is named after.
+ *
+ * ## What this covers that no other test can
+ *
+ * `ConverterScreenContentTest`, `ConverterStateAffordancesTest` and `JoinScreenContentTest` all
+ * drive the **stateless** content composables, which build their own `ConverterActions`. So the
+ * wiring — the list of `viewModel::` references the stateful outer hands down — was seen by nothing
+ * in the suite.
+ *
+ * ## The hazard is narrower than "seventeen bindings", and this says so
+ *
+ * #156 was filed claiming a transposition of any two bindings would survive the suite. That is not
+ * true, and it was worth checking rather than testing on the assumption:
+ *
+ * | swap | result |
+ * |---|---|
+ * | `onVideoCodec` ↔ `onAudioCodec` | **rejected by the compiler** |
+ * | `onCancel` ↔ `onReset` | **compiles** |
+ *
+ * Every typed binding — container, both codecs, preset, suggestion, quality, engine preference —
+ * takes a distinct parameter type, so the compiler is already the test. Writing assertions for
+ * those would be theatre.
+ *
+ * **The `() -> Unit` bindings are the real gap**, because they are interchangeable to the compiler:
+ * two on the converter screen (`onCancel`, `onReset`) and three on the join screen (`onJoin`,
+ * `onCancel`, `onReset`). A Cancel that discards the finished file, or a Join that cancels, is a
+ * one-character mistake that ships.
+ *
+ * ## How they are told apart
+ *
+ * By effect, not by a recording double. `reset()` sets the state to `Idle`; `cancel()` with no
+ * active job leaves it alone (`ConversionViewModel.cancel` is `activeWorkId?.let(...)`, and
+ * `SettingsEditsTest` pins that). Driving each from a non-`Idle` state is therefore enough to say
+ * which one ran.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class ScreenWiringTest {
+
+    private lateinit var app: Application
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        ConversionDependencies.publisher = { RecordingPublisher(app) }
+        ConversionDependencies.probe = { _, _ -> org.libremediaconverter.model.InputProbe() }
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    // --- the converter screen ----------------------------------------------
+
+    @Test
+    fun `Start over resets, and Cancel does not`() {
+        // The transposition that compiles. If onReset were bound to cancel, this stays on Ready.
+        installTestWorkManager(app, Data.EMPTY)
+        val pick = ParkedPickDispatcher()
+        val viewModel = ConversionViewModel(app, pickDispatcher = pick)
+        val actions = converterActions(viewModel, onPickInput = {}, onConvert = {}, onSave = {})
+        viewModel.onInputPicked(INPUT_URI)
+        pick.runAll()
+        assertNotEquals(
+            "the fixture needs a non-Idle state or neither action is observable",
+            ConversionState.Idle,
+            viewModel.state.value,
+        )
+
+        actions.onReset()
+
+        assertEquals(ConversionState.Idle, viewModel.state.value)
+    }
+
+    @Test
+    fun `Cancel leaves the picked file on screen`() {
+        // The other half. Without it, a wiring with BOTH actions bound to reset passes the test
+        // above -- and that is exactly what a copy-paste of the wrong line produces.
+        installTestWorkManager(app, Data.EMPTY)
+        val pick = ParkedPickDispatcher()
+        val viewModel = ConversionViewModel(app, pickDispatcher = pick)
+        val actions = converterActions(viewModel, onPickInput = {}, onConvert = {}, onSave = {})
+        viewModel.onInputPicked(INPUT_URI)
+        pick.runAll()
+        val before = viewModel.state.value
+
+        actions.onCancel()
+
+        assertEquals(
+            "Cancel must not throw away the pick the way Start over does",
+            before,
+            viewModel.state.value,
+        )
+    }
+
+    @Test
+    fun `each settings affordance reaches the setting it is named after`() {
+        // The typed bindings. The compiler already rejects a transposition among these, so this is
+        // not that assertion -- it is the cheaper one that each is bound to *something*, and that a
+        // binding dropped to `{}` during an edit would be caught.
+        installTestWorkManager(app, Data.EMPTY)
+        val pick = ParkedPickDispatcher()
+        val viewModel = ConversionViewModel(app, pickDispatcher = pick)
+        val actions = converterActions(viewModel, onPickInput = {}, onConvert = {}, onSave = {})
+
+        actions.onPreset(OutputFormat.WEBM_VP9)
+        assertEquals(OutputFormat.WEBM_VP9.spec, viewModel.settings.value.spec)
+
+        actions.onContainer(Container.MKV)
+        assertEquals(Container.MKV, viewModel.settings.value.spec.container)
+
+        actions.onVideoCodec(VideoCodec.H264)
+        assertEquals(VideoCodec.H264, viewModel.settings.value.spec.videoCodec)
+
+        actions.onAudioCodec(AudioCodec.FLAC)
+        assertEquals(AudioCodec.FLAC, viewModel.settings.value.spec.audioCodec)
+
+        actions.onQuality(QualityTier.BEST)
+        assertEquals(QualityTier.BEST, viewModel.settings.value.quality)
+
+        actions.onEnginePreference(EnginePreference.FORCE_SOFTWARE)
+        assertEquals(EnginePreference.FORCE_SOFTWARE, viewModel.settings.value.enginePreference)
+
+        actions.onSuggestion(OutputFormat.MP4_H264.spec)
+        assertEquals(OutputFormat.MP4_H264.spec, viewModel.settings.value.spec)
+    }
+
+    @Test
+    fun `the launcher-backed actions are the ones the screen supplies`() {
+        // Not wired to the ViewModel at all, deliberately -- they need an ActivityResultLauncher.
+        // Asserted so that a later edit routing one of them at the ViewModel is noticed.
+        installTestWorkManager(app, Data.EMPTY)
+        val pick = ParkedPickDispatcher()
+        val viewModel = ConversionViewModel(app, pickDispatcher = pick)
+        val called = mutableListOf<String>()
+        val actions = converterActions(
+            viewModel,
+            onPickInput = { called += "pick" },
+            onConvert = { called += "convert" },
+            onSave = { called += "save:$it" },
+        )
+
+        actions.onPickInput()
+        actions.onConvert()
+        actions.onSave("holiday.mp4")
+
+        assertEquals(listOf("pick", "convert", "save:holiday.mp4"), called)
+    }
+
+    // --- the join screen, where three are interchangeable -------------------
+
+    @Test
+    fun `Start over resets the join, and Cancel does not`() {
+        installTestWorkManager(app, workDataOf(ConcatWorker.KEY_OUTPUT_PATH to "/dev/null"))
+        val pick = ParkedPickDispatcher()
+        val viewModel = JoinViewModel(app, pickDispatcher = pick)
+        val actions = joinActions(viewModel, onPickInputs = {}, onSave = {})
+        viewModel.onInputsPicked(TWO_INPUTS)
+        pick.runAll()
+        assertTrue(
+            "the fixture needs a non-Idle state: ${viewModel.state.value}",
+            viewModel.state.value !is JoinState.Idle,
+        )
+
+        actions.onReset()
+
+        assertEquals(JoinState.Idle, viewModel.state.value)
+    }
+
+    @Test
+    fun `Cancel leaves the picked files on screen`() {
+        installTestWorkManager(app, workDataOf(ConcatWorker.KEY_OUTPUT_PATH to "/dev/null"))
+        val pick = ParkedPickDispatcher()
+        val viewModel = JoinViewModel(app, pickDispatcher = pick)
+        val actions = joinActions(viewModel, onPickInputs = {}, onSave = {})
+        viewModel.onInputsPicked(TWO_INPUTS)
+        pick.runAll()
+        val before = viewModel.state.value
+
+        actions.onCancel()
+
+        assertEquals(before, viewModel.state.value)
+    }
+
+    @Test
+    fun `Join starts the job rather than cancelling or resetting it`() {
+        // The third of the join screen's interchangeable trio, and the one whose transposition is
+        // worst: a Join button bound to cancel does nothing at all, which reads as a dead button.
+        installTestWorkManager(app, workDataOf(ConcatWorker.KEY_OUTPUT_PATH to "/dev/null"))
+        val pick = ParkedPickDispatcher()
+        val viewModel = JoinViewModel(app, pickDispatcher = pick)
+        val actions = joinActions(viewModel, onPickInputs = {}, onSave = {})
+        viewModel.onInputsPicked(TWO_INPUTS)
+        pick.runAll()
+
+        actions.onJoin()
+
+        assertTrue(
+            "Join must leave Ready for a running state, not sit still and not go Idle: " +
+                "${viewModel.state.value}",
+            viewModel.state.value is JoinState.Joining || viewModel.state.value is JoinState.Joined,
+        )
+    }
+
+    private companion object {
+        val INPUT_URI: Uri = Uri.parse("content://test/holiday.mov")
+        val TWO_INPUTS = listOf(
+            Uri.parse("content://test/a.mp4"),
+            Uri.parse("content://test/b.mp4"),
+        )
+    }
+}

app/src/test/java/org/libremediaconverter/convert/SettingsEditsTest.kt

@@ -0,0 +1,197 @@
+package org.libremediaconverter.convert
+
+import android.app.Application
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Assert.assertNull
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.AudioCodec
+import org.libremediaconverter.model.Container
+import org.libremediaconverter.model.EnginePreference
+import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.model.QualityTier
+import org.libremediaconverter.model.VideoCodec
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+/**
+ * The seven one-line edits the settings sheet makes, and what each one leaves alone.
+ *
+ * ## Why these needed a file of their own
+ *
+ * `setPreset` was covered. The six beside it — `setContainer`, `setVideoCodec`, `setAudioCodec`,
+ * `applySuggestion`, `setQuality`, `setEnginePreference` — and `cancel()` had **no coverage at
+ * all**, which is the tell: they are reachable from the JVM suite by exactly the route `setPreset`
+ * already takes, and nothing had asked.
+ *
+ * ## What is actually being asserted
+ *
+ * Not "the setter sets something". Each of these copies into a nested `OutputSpec`, so the failure
+ * worth catching is **a setter that writes the right value into the wrong field, or that rebuilds
+ * the spec and silently discards the other two**. So every test here asserts the field it changed
+ * *and* that the rest of the spec survived — a `setContainer` implemented as
+ * `it.copy(spec = OutputFormat.MP4_H265.spec.copy(container = container))` would pass a test that
+ * only checked the container.
+ *
+ * `ConverterScreenContentTest` cannot cover this: it builds `ConverterActions` itself and never
+ * touches the ViewModel. That the *screen* calls these is #156's, and neither implies the other.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class SettingsEditsTest {
+
+    private lateinit var app: Application
+    private lateinit var viewModel: ConversionViewModel
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        installTestWorkManager(app, Data.EMPTY)
+        viewModel = ConversionViewModel(app)
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    @Test
+    fun `choosing a preset replaces the whole spec`() {
+        viewModel.setPreset(OutputFormat.WEBM_VP9)
+
+        assertEquals(OutputFormat.WEBM_VP9.spec, viewModel.settings.value.spec)
+    }
+
+    @Test
+    fun `changing the container leaves both codecs alone`() {
+        // Moved off the default spec first, and that is load-bearing rather than tidiness. The
+        // default IS `OutputFormat.MP4_H265.spec`, so a `setContainer` that rebuilt the spec from
+        // that preset instead of from the current one produced an identical answer and the
+        // mutation went green. Editing the codecs away from the default first is what makes
+        // "the other two survived" an assertion rather than a coincidence.
+        viewModel.setPreset(OutputFormat.WEBM_VP9)
+        val before = viewModel.settings.value.spec
+
+        viewModel.setContainer(Container.MKV)
+
+        val after = viewModel.settings.value.spec
+        assertEquals(Container.MKV, after.container)
+        assertEquals("the video codec is not the container's to change", before.videoCodec, after.videoCodec)
+        assertEquals("the audio codec is not the container's to change", before.audioCodec, after.audioCodec)
+    }
+
+    @Test
+    fun `changing the video codec leaves the container and the audio codec alone`() {
+        // The transposition this guards against is real: setVideoCodec and setAudioCodec take
+        // different enum types, but a copy(...) naming the wrong field compiles wherever the types
+        // happen to line up, and the picker would silently set the other one.
+        val before = viewModel.settings.value.spec
+
+        viewModel.setVideoCodec(VideoCodec.VP9)
+
+        val after = viewModel.settings.value.spec
+        assertEquals(VideoCodec.VP9, after.videoCodec)
+        assertEquals(before.container, after.container)
+        assertEquals(before.audioCodec, after.audioCodec)
+    }
+
+    @Test
+    fun `changing the audio codec leaves the container and the video codec alone`() {
+        val before = viewModel.settings.value.spec
+
+        viewModel.setAudioCodec(AudioCodec.OPUS)
+
+        val after = viewModel.settings.value.spec
+        assertEquals(AudioCodec.OPUS, after.audioCodec)
+        assertEquals(before.container, after.container)
+        assertEquals(before.videoCodec, after.videoCodec)
+    }
+
+    @Test
+    fun `applying a suggestion replaces the spec without disturbing quality or engine`() {
+        // A suggestion comes from ContainerCapabilities when the current spec is invalid, so it is
+        // a whole spec by construction. What it must not do is reset the two settings beside it.
+        viewModel.setQuality(QualityTier.BEST)
+        viewModel.setEnginePreference(EnginePreference.FORCE_SOFTWARE)
+
+        viewModel.applySuggestion(OutputFormat.MKV_H264.spec)
+
+        val settings = viewModel.settings.value
+        assertEquals(OutputFormat.MKV_H264.spec, settings.spec)
+        assertEquals(QualityTier.BEST, settings.quality)
+        assertEquals(EnginePreference.FORCE_SOFTWARE, settings.enginePreference)
+    }
+
+    @Test
+    fun `changing the quality leaves the spec and the engine preference alone`() {
+        // Both neighbours are moved off their defaults first. Asserting against AUTO -- which is
+        // what `ConversionSettings` starts with -- let a `setQuality` that also reset the engine
+        // preference to AUTO pass, because the reset and the survival looked identical.
+        viewModel.setPreset(OutputFormat.WEBM_VP9)
+        viewModel.setEnginePreference(EnginePreference.FORCE_SOFTWARE)
+        val before = viewModel.settings.value.spec
+
+        viewModel.setQuality(QualityTier.BEST)
+
+        val settings = viewModel.settings.value
+        assertEquals(QualityTier.BEST, settings.quality)
+        assertEquals(before, settings.spec)
+        assertEquals(
+            "quality is not the engine preference's to change",
+            EnginePreference.FORCE_SOFTWARE,
+            settings.enginePreference,
+        )
+    }
+
+    @Test
+    fun `changing the engine preference leaves the spec and the quality alone`() {
+        // Off the defaults for the same reason as the test above: QualityTier.FAST is the starting
+        // value, so asserting it here would have been satisfied by a reset as readily as by a
+        // survival.
+        viewModel.setPreset(OutputFormat.WEBM_VP9)
+        viewModel.setQuality(QualityTier.BEST)
+        val before = viewModel.settings.value.spec
+
+        viewModel.setEnginePreference(EnginePreference.FORCE_SOFTWARE)
+
+        val settings = viewModel.settings.value
+        assertEquals(EnginePreference.FORCE_SOFTWARE, settings.enginePreference)
+        assertEquals(before, settings.spec)
+        assertEquals(
+            "the engine preference is not the quality's to change",
+            QualityTier.BEST,
+            settings.quality,
+        )
+    }
+
+    @Test
+    fun `editing past every preset leaves no matching preset`() {
+        // `matchingPreset` is what the settings sheet reads to decide whether to show a preset as
+        // selected or to say "Custom". Editing one field of a preset must drop it out of the list
+        // rather than leaving the old one highlighted.
+        viewModel.setPreset(OutputFormat.MP4_H265)
+        assertEquals(OutputFormat.MP4_H265, viewModel.settings.value.matchingPreset)
+
+        viewModel.setAudioCodec(AudioCodec.FLAC)
+
+        assertNull(
+            "an edited spec is no longer any preset, and the sheet says Custom",
+            viewModel.settings.value.matchingPreset,
+        )
+        assertNotEquals(OutputFormat.MP4_H265.spec, viewModel.settings.value.spec)
+    }
+
+    @Test
+    fun `cancelling with no active job does nothing rather than throwing`() {
+        // `activeWorkId?.let(...)` -- the null side. A user can reach Cancel through a state that
+        // has already finished, and taking the app down for it would be worse than doing nothing.
+        viewModel.cancel()
+
+        assertEquals(ConversionState.Idle, viewModel.state.value)
+    }
+}

app/src/test/java/org/libremediaconverter/convert/StaleLauncherResultTest.kt

@@ -0,0 +1,147 @@
+package org.libremediaconverter.convert
+
+import android.app.Application
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.WorkManager
+import androidx.work.workDataOf
+import kotlinx.coroutines.Dispatchers
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.join.JoinState
+import org.libremediaconverter.join.JoinViewModel
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.work.ConcatWorker
+import org.libremediaconverter.work.ConversionWorker
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+/**
+ * An answer that arrives after the screen has moved on does nothing.
+ *
+ * Four refusal arms, cold before this file:
+ *
+ * ```
+ * convert/ConversionViewModel.kt:513   currentInput() ?: return
+ * convert/ConversionViewModel.kt:600   pendingSave() ?: return
+ * join/JoinViewModel.kt:316            (as? Ready)?.inputs ?: return
+ * join/JoinViewModel.kt:390            pendingSave() ?: return
+ * ```
+ *
+ * They are not merely defensive. `ConverterScreen.kt:91` wires `convert()` to the
+ * **POST_NOTIFICATIONS result**, and `:83` wires `save()` to the CreateDocument result — so both
+ * are entered by a system callback rather than by a tap, and a result redelivered after process
+ * death arrives at a brand-new ViewModel sitting on `Idle`.
+ *
+ * ## The production change that came with this
+ *
+ * `currentInput()` used to answer for `Converting`, `Waiting` and `Converted` as well as `Ready`.
+ * Those arms were unreachable by tapping Convert but reachable through that permission callback,
+ * and reaching one enqueued a **second** job over a live one — `activeWorkId` overwritten, the
+ * first job still running with an orphaned notification and nothing holding its id.
+ *
+ * #202 decided to narrow rather than to test it as it stood, because a test written against the old
+ * shape would have frozen the double-enqueue as intended behaviour. `JoinViewModel.join()` has been
+ * `(_state.value as? JoinState.Ready)?.inputs ?: return` all along; the two screens are the same
+ * shape and only one was over-general.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class StaleLauncherResultTest {
+
+    private lateinit var app: Application
+    private lateinit var workManager: WorkManager
+    private lateinit var staged: java.io.File
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        val publisher = RecordingPublisher(app)
+        ConversionDependencies.publisher = { publisher }
+        ConversionDependencies.probe = { _, _ -> InputProbe() }
+        // A real staged file, because a SUCCEEDED job with no output path maps to Failed rather
+        // than Converted -- and Converted is the state this file's second case has to reach.
+        staged = publisher.createStagingFile("holiday.mp4").apply { writeBytes(ByteArray(4096)) }
+        installTestWorkManager(
+            app,
+            workDataOf(
+                ConversionWorker.KEY_OUTPUT_PATH to staged.absolutePath,
+                ConversionWorker.KEY_SUGGESTED_NAME to "holiday.mp4",
+                ConversionWorker.KEY_MIME_TYPE to "video/mp4",
+            ),
+        )
+        workManager = WorkManager.getInstance(app)
+    }
+
+    @After
+    fun tearDown() = ConversionDependencies.reset()
+
+    @Test
+    fun `a permission answer arriving on an empty screen enqueues nothing`() {
+        val viewModel = ConversionViewModel(app, Dispatchers.Unconfined)
+        awaitState(viewModel.state, "Idle") { it is ConversionState.Idle }
+
+        viewModel.convert()
+
+        assertEquals(ConversionState.Idle, viewModel.state.value)
+        assertEquals("nothing may be enqueued for a file that is not there", 0, conversionJobs())
+    }
+
+    /**
+     * The narrowing itself: a permission answer that arrives while a conversion is already running
+     * must not start a second one.
+     *
+     * Reached by converting once — the synchronous test WorkManager finishes it inline, so the
+     * screen is `Converted`, which is one of the three arms `currentInput()` used to answer for.
+     * Calling `convert()` again from there is precisely what the permission callback can do.
+     */
+    @Test
+    fun `a permission answer arriving after the job finished does not start a second one`() {
+        val viewModel = ConversionViewModel(app, Dispatchers.Unconfined)
+        viewModel.onInputPicked(Uri.parse("content://test/holiday.mkv"))
+        awaitState(viewModel.state, "Ready") { it is ConversionState.Ready }
+        viewModel.convert()
+        val converted = awaitState(viewModel.state, "Converted") { it is ConversionState.Converted }
+        assertEquals("the fixture needs exactly one job to start with", 1, conversionJobs())
+
+        viewModel.convert()
+
+        assertEquals("a second job must not be enqueued over the first", 1, conversionJobs())
+        assertEquals("and the screen must not move", converted, viewModel.state.value)
+    }
+
+    @Test
+    fun `a save answer arriving on an empty screen does nothing`() {
+        val viewModel = ConversionViewModel(app, Dispatchers.Unconfined)
+        awaitState(viewModel.state, "Idle") { it is ConversionState.Idle }
+
+        viewModel.save(DESTINATION)
+
+        assertEquals(ConversionState.Idle, viewModel.state.value)
+    }
+
+    @Test
+    fun `a join answer arriving on an empty screen enqueues nothing`() {
+        val viewModel = JoinViewModel(app, Dispatchers.Unconfined)
+        awaitState(viewModel.state, "Idle") { it is JoinState.Idle }
+
+        viewModel.join()
+        viewModel.save(DESTINATION)
+
+        assertEquals(JoinState.Idle, viewModel.state.value)
+        assertEquals(0, joinJobs())
+    }
+
+    private fun conversionJobs() = jobsTagged(ConversionWorker::class.java.name)
+
+    private fun joinJobs() = jobsTagged(ConcatWorker::class.java.name)
+
+    private fun jobsTagged(tag: String) = workManager.getWorkInfosByTag(tag).get().size
+
+    private companion object {
+        val DESTINATION: Uri = Uri.parse("content://test/destination.mp4")
+    }
+}

app/src/test/java/org/libremediaconverter/convert/UnreadableJoinInputTest.kt

@@ -0,0 +1,70 @@
+package org.libremediaconverter.convert
+
+import android.net.Uri
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.ConcatPlanner
+import org.libremediaconverter.model.ConcatStrategy
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+/**
+ * A clip in a join that nothing could read, from the probe all the way to the strategy.
+ *
+ * Both halves of this are covered already, and separately: `MediaProbeTrackWalkTest` pins what
+ * `concatInputFrom` makes of a track list, and `ConcatPlannerTest`'s
+ * `an unknown codec is not treated as a match` pins what the planner does with a hand-built
+ * `ConcatInput(video = null)`. **Nothing spanned the two**, and the span is the load-bearing part:
+ * the planner's safety rests on the probe really producing that shape, and the hand-built fixture
+ * would go on passing if it stopped.
+ *
+ * Measured rather than asserted: mutating `concatInputFrom`'s initial `video` to a non-null
+ * placeholder leaves `ConcatPlannerTest` green and turns this red.
+ *
+ * ## The asymmetry this protects
+ *
+ * `ConcatPlanner` guards its video check against a null codec (`ConcatStrategy.kt:51`) and its
+ * audio check not at all (`:54`). **That is correct, not an oversight.** `MediaProbe.shortName`
+ * returns a non-null `String`, so in `concatInputFrom` a null `audioCodec` means the track is
+ * *absent* — and two clips with no audio genuinely do match. A null `videoCodec` carries both
+ * meanings, absent or unreadable, which is why only that one is guarded.
+ *
+ * So the audio check is safe *because* the video guard fires first on a clip nothing could read.
+ * Nothing wrote that coupling down and nothing held it.
+ *
+ * ## What this deliberately does not cover
+ *
+ * `probeForConcat`'s `catch` arm (`MediaProbe.kt:300-302`). It is **not reachable on the JVM**:
+ * Robolectric's `MediaExtractor` never throws from `setDataSource`, measured across an
+ * unregistered `content://` authority, a missing `file://`, a file of garbage bytes and an `http://`
+ * URL — all four returned normally with `trackCount = 0`. So the failure arrives here as an empty
+ * track list rather than as an exception, which reaches the same `ConcatInput(null, null, 0, 0, 0)`
+ * by the other road. The catch stays device-only, and this file does not pretend otherwise.
+ */
+@RunWith(RobolectricTestRunner::class)
+class UnreadableJoinInputTest {
+
+    @Test
+    fun `a clip nothing could read probes as unknown, and an unknown clip is re-encoded`() {
+        val unreadable = MediaProbe.probeForConcat(RuntimeEnvironment.getApplication(), UNREADABLE)
+
+        assertNull("an unreadable clip proves nothing about its video codec", unreadable.videoCodec)
+        assertNull("nor about its audio codec", unreadable.audioCodec)
+        assertEquals("nor about its dimensions", 0, unreadable.width)
+        assertEquals(0, unreadable.height)
+        assertEquals(0, unreadable.frameRate)
+
+        assertEquals(
+            "a clip nothing could read is not evidence of a match with anything",
+            ConcatStrategy.REENCODE,
+            ConcatPlanner.plan(listOf(unreadable, unreadable)),
+        )
+    }
+
+    private companion object {
+        /** `content://` so the probe takes the SAF branch a real pick takes. Nothing answers it. */
+        val UNREADABLE: Uri = Uri.parse("content://test/vanished.mp4")
+    }
+}

app/src/test/java/org/libremediaconverter/ffmpeg/FFmpegCommandBuilderTest.kt

@@ -166,6 +166,54 @@ class FFmpegCommandBuilderTest {
         assertPair(cmd(OutputFormat.OPUS), "-c:a", "libopus")
     }
 
+    /**
+     * The arm most conversions actually take, and the only one in `audioArgs` with no test.
+     *
+     * `flac wav and opus select the right encoders` above covers the three named arms; MP3 has its
+     * own. AAC arrives through the `else`, so nothing named it and nothing pinned either half of
+     * what it emits -- neither `aac` nor `192k` appeared anywhere in this file. Both are shipped
+     * defaults: MP4 and M4A are the formats the picker offers first, so this is the audio
+     * every ordinary conversion gets.
+     *
+     * The bitrate is asserted as well as the encoder because it is the half a refactor is likelier
+     * to lose. An `-b:a` that quietly changed would not fail anything, would not look wrong in a
+     * command line, and would show up only as files that sound different from the ones the app
+     * produced last month.
+     */
+    @Test
+    fun `aac is the default encoder, at the bitrate the app ships`() {
+        assertPair(cmd(OutputFormat.MP4_H264), "-c:a", "aac")
+        assertPair(cmd(OutputFormat.MP4_H264), "-b:a", "192k")
+        // Through the `else` rather than through a named arm, so an AAC branch added above it later
+        // has to keep answering the same way.
+        assertPair(cmd(OutputFormat.M4A_AAC), "-c:a", "aac")
+        assertPair(cmd(OutputFormat.M4A_AAC), "-b:a", "192k")
+    }
+
+    /**
+     * Turning audio off, which the Advanced picker offers and nothing had ever built a command for.
+     *
+     * `audioArgs`' `Drop` arm was `ci == 0`. The suite's only `-an` assertion is in
+     * `gif generates a palette to avoid banding and drops audio`, and that one comes from the image
+     * path (`FFmpegCommandBuilder.kt:79`/`:90`), which emits `-an` directly and never reaches
+     * `audioArgs`. Two sites, one string, one tested.
+     *
+     * It is a live path rather than defensive code: `AdvancedPicker` renders all of
+     * `AudioCodec.entries` including `NONE`, `ContainerCapabilities.validate` permits audio-off
+     * whenever the input has video, and MKV routes the job to FFmpeg.
+     *
+     * Both halves are asserted. `-an` alone would still pass if the arm fell through to the `else`
+     * and emitted an AAC encoder beside it -- a file that is silent because the flag won, carrying
+     * an encoder nobody asked for.
+     */
+    @Test
+    fun `turning audio off drops the track instead of encoding one`() {
+        val args = cmd(OutputSpec(Container.MKV, VideoCodec.H264, AudioCodec.NONE))
+
+        assertTrue("audio turned off must emit -an, got $args", args.contains("-an"))
+        assertFalse("a dropped track must not also carry an encoder, got $args", args.contains("-c:a"))
+    }
+
     @Test
     fun `audio only formats never carry a video encoder`() {
         listOf(OutputFormat.MP3, OutputFormat.FLAC, OutputFormat.WAV, OutputFormat.OPUS)

app/src/test/java/org/libremediaconverter/ffmpeg/SessionOutcomeTest.kt

@@ -0,0 +1,127 @@
+package org.libremediaconverter.ffmpeg
+
+import com.arthenica.ffmpegkit.ReturnCode
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * What a finished FFmpegKit session means, for both engines at once.
+ *
+ * `FFmpegEngine` and `ConcatEngine` each carried their own copy of this `when`, and the copies had
+ * drifted: one preferred the fail stack trace and fell back to the log tail, the other only ever
+ * read the log tail. Neither was tested, because both live inside a callback handed to `FFmpegKit`,
+ * which does not run on the JVM — so nothing could see that the two disagreed.
+ *
+ * **JVM-safe, verified rather than assumed.** `javap` over the committed AAR's runtime jar shows
+ * `ReturnCode(int)` as a plain public constructor with `SUCCESS`/`CANCEL` int constants and pure
+ * static `isSuccess`/`isCancel`; its `<clinit>` is constant initialisation and loads no native
+ * library.
+ *
+ * The unification is #203's decision, so the tests pin it as one: a join failure now carries the
+ * stack trace a conversion failure always did, while the two prefixes stay distinct.
+ */
+class SessionOutcomeTest {
+
+    @Test
+    fun `a return code of zero is success`() {
+        assertEquals(SessionOutcome.Success, outcome(ReturnCode(ReturnCode.SUCCESS)))
+    }
+
+    /**
+     * Cancellation is a separate outcome from failure, and the distinction is the point: the engines
+     * resume the continuation *cancelled* rather than exceptionally, so a user who pressed Cancel
+     * does not get an error card.
+     */
+    @Test
+    fun `a return code of 255 is a cancellation, not a failure`() {
+        assertEquals(SessionOutcome.Cancelled, outcome(ReturnCode(ReturnCode.CANCEL)))
+    }
+
+    @Test
+    fun `any other return code fails, and the sentence carries the number`() {
+        val failed = outcome(ReturnCode(1), stackTrace = "boom") as SessionOutcome.Failed
+
+        assertTrue("the code belongs in the message, got: ${failed.message}", failed.message.contains("(1)"))
+    }
+
+    /**
+     * The half that was different between the two engines before #203, now the same in both.
+     */
+    @Test
+    fun `the stack trace is preferred over the log tail`() {
+        val failed = outcome(ReturnCode(1), stackTrace = "the real cause", logTail = "…noise…")
+            as SessionOutcome.Failed
+
+        assertTrue(failed.message.contains("the real cause"))
+        assertTrue("the log tail must not be appended as well", !failed.message.contains("noise"))
+    }
+
+    @Test
+    fun `a blank stack trace falls back to the log tail`() {
+        val blank = outcome(ReturnCode(1), stackTrace = "   ", logTail = "the last few lines") as SessionOutcome.Failed
+        val absent = outcome(ReturnCode(1), stackTrace = null, logTail = "the last few lines") as SessionOutcome.Failed
+
+        assertTrue(blank.message.contains("the last few lines"))
+        assertTrue("a null stack trace is a blank one", absent.message.contains("the last few lines"))
+    }
+
+    /**
+     * Both sources empty still has to produce a sentence. A message ending in a dangling colon is
+     * thin, but it is what the user gets when FFmpeg said nothing at all, and it must not be an
+     * exception on the way to the screen.
+     */
+    @Test
+    fun `a failure with nothing to say still names the code`() {
+        val failed = outcome(ReturnCode(1), stackTrace = null, logTail = null) as SessionOutcome.Failed
+
+        assertEquals("FFmpeg failed (1): ", failed.message)
+    }
+
+    /**
+     * `getReturnCode()` is nullable and a session killed before it reported anything has none.
+     * Neither success nor cancellation, so it fails — and the sentence says so rather than throwing.
+     */
+    @Test
+    fun `a session with no return code at all fails`() {
+        val failed = outcome(null, logTail = "whatever was logged") as SessionOutcome.Failed
+
+        assertTrue("got: ${failed.message}", failed.message.startsWith("FFmpeg failed (null): "))
+    }
+
+    /**
+     * Unifying the *strategy* must not unify the *sentence*: the two engines describe different
+     * jobs, and a join that reports "FFmpeg failed" is a worse message than the one it replaced.
+     */
+    @Test
+    fun `each engine keeps its own prefix`() {
+        val join = sessionOutcome(ReturnCode(1), "Joining", { "cause" }, { null }) as SessionOutcome.Failed
+
+        assertTrue(join.message.startsWith("Joining failed (1): "))
+    }
+
+    /**
+     * Neither message source is read unless the outcome is a failure.
+     *
+     * They are calls onto a native session, and reading them on the happy path is work every
+     * successful conversion would do for nothing — which the shape this replaced did not, since it
+     * read them inside the `else` branch. That is why the parameters are lambdas, and this is what
+     * would notice if they stopped being.
+     */
+    @Test
+    fun `a session that succeeded reads neither the stack trace nor the log`() {
+        var reads = 0
+        fun counted(): String? {
+            reads++
+            return null
+        }
+
+        sessionOutcome(ReturnCode(ReturnCode.SUCCESS), "FFmpeg", ::counted, ::counted)
+        sessionOutcome(ReturnCode(ReturnCode.CANCEL), "FFmpeg", ::counted, ::counted)
+
+        assertEquals("neither source may be touched unless the session failed", 0, reads)
+    }
+
+    private fun outcome(rc: ReturnCode?, stackTrace: String? = null, logTail: String? = null) =
+        sessionOutcome(rc, "FFmpeg", { stackTrace }, { logTail })
+}

app/src/test/java/org/libremediaconverter/join/JoinStateMappingTest.kt

@@ -0,0 +1,200 @@
+package org.libremediaconverter.join
+
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import androidx.work.WorkInfo
+import androidx.work.workDataOf
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.InputFile
+import org.libremediaconverter.model.ConcatStrategy
+import org.libremediaconverter.work.ConcatWorker
+import org.robolectric.RobolectricTestRunner
+
+/**
+ * Every answer [joinStateFrom] can give, chosen rather than stumbled into.
+ *
+ * The join-side twin of `ConversionStateMappingTest`, and the argument is the same one: the mapping
+ * ran on every test that drove a real `ConcatWorker`, but a real worker only ever reaches a terminal
+ * state with well-formed output, so five arms had never been *chosen* by anything.
+ *
+ * ## The one that is not just coverage
+ *
+ * `an unknown strategy name is read as a re-encode rather than thrown` covers a real defect this
+ * seam exposed. The line it replaces was:
+ *
+ * ```kotlin
+ * .getString(ConcatWorker.KEY_STRATEGY)?.let(ConcatStrategy::valueOf) ?: ConcatStrategy.REENCODE
+ * ```
+ *
+ * `valueOf` throws on a name this build does not define, and this runs inside a `viewModelScope`
+ * collect with no handler — so it does not become a `Failed` state, it takes the process down.
+ * `ConcatWorker.kt` had already made this exact change for `KEY_FORMAT` and written down why; the
+ * matching read on this side had not been changed with it.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class JoinStateMappingTest {
+
+    @Test
+    fun `a running join is joining`() {
+        assertEquals(JoinState.Joining(INPUTS), map(WorkInfo.State.RUNNING))
+    }
+
+    @Test
+    fun `a blocked join looks like one that is starting`() {
+        // Folded into the RUNNING arm deliberately: a job waiting on a prerequisite is nothing the
+        // user can act on, and a separate word for it would be noise.
+        assertEquals(JoinState.Joining(INPUTS), map(WorkInfo.State.BLOCKED))
+    }
+
+    @Test
+    fun `an enqueued join that has already run is waiting to retry`() {
+        assertEquals(JoinState.Waiting(INPUTS), map(WorkInfo.State.ENQUEUED, runAttemptCount = 1))
+    }
+
+    @Test
+    fun `an enqueued join that has never run is simply starting`() {
+        // The other side. Without it, a mapping that ignored runAttemptCount passes the test above.
+        assertEquals(JoinState.Joining(INPUTS), map(WorkInfo.State.ENQUEUED, runAttemptCount = 0))
+    }
+
+    @Test
+    fun `a success that named no file is a failure, not an empty success`() {
+        assertEquals(
+            JoinState.Failed(JOINED_WITHOUT_A_FILE_MESSAGE),
+            map(WorkInfo.State.SUCCEEDED, data = Data.EMPTY),
+        )
+    }
+
+    @Test
+    fun `a success carries the strategy the worker actually used`() {
+        // Not cosmetic: the join screen tells the user whether their files were stream-copied or
+        // re-encoded, which is the difference between lossless and lossy.
+        val joined = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(
+                ConcatWorker.KEY_OUTPUT_PATH to "/cache/conversions/joined.mp4",
+                ConcatWorker.KEY_STRATEGY to ConcatStrategy.STREAM_COPY.name,
+            ),
+        ) as JoinState.Joined
+
+        assertEquals(ConcatStrategy.STREAM_COPY, joined.strategy)
+    }
+
+    @Test
+    fun `an unknown strategy name is read as a re-encode rather than thrown`() {
+        // The defect. A build that added a third strategy leaves finished joins in the queue naming
+        // it, and WorkManager keeps those about a week -- the premise WorkerEnumFallbackTest and
+        // JobTags are both written on. With `valueOf` this throws IllegalArgumentException inside a
+        // viewModelScope collect that has no handler, so it is not a Failed state, it is a crash.
+        //
+        // REENCODE rather than STREAM_COPY because it is the conservative answer: describing an
+        // unknown join as lossless would be a claim the app cannot support.
+        val joined = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(
+                ConcatWorker.KEY_OUTPUT_PATH to "/cache/conversions/joined.mp4",
+                ConcatWorker.KEY_STRATEGY to "SMART_CONCAT_V2",
+            ),
+        ) as JoinState.Joined
+
+        assertEquals(ConcatStrategy.REENCODE, joined.strategy)
+    }
+
+    @Test
+    fun `a success with no strategy at all falls back the same way`() {
+        val joined = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(ConcatWorker.KEY_OUTPUT_PATH to "/cache/conversions/joined.mp4"),
+        ) as JoinState.Joined
+
+        assertEquals(ConcatStrategy.REENCODE, joined.strategy)
+    }
+
+    @Test
+    fun `a success from older work falls back to the format such a job really used`() {
+        val joined = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(ConcatWorker.KEY_OUTPUT_PATH to "/cache/conversions/joined.mp4"),
+        ) as JoinState.Joined
+
+        assertEquals(ConcatWorker.outputNameFor(ConcatWorker.DEFAULT_FORMAT), joined.suggestedName)
+        assertEquals(ConcatWorker.DEFAULT_FORMAT.mimeType, joined.mimeType)
+    }
+
+    @Test
+    fun `a blank name or type falls back the same way a missing one does`() {
+        val joined = map(
+            WorkInfo.State.SUCCEEDED,
+            data = workDataOf(
+                ConcatWorker.KEY_OUTPUT_PATH to "/cache/conversions/joined.mp4",
+                ConcatWorker.KEY_SUGGESTED_NAME to "",
+                ConcatWorker.KEY_MIME_TYPE to "  ",
+            ),
+        ) as JoinState.Joined
+
+        assertEquals(ConcatWorker.outputNameFor(ConcatWorker.DEFAULT_FORMAT), joined.suggestedName)
+        assertEquals(ConcatWorker.DEFAULT_FORMAT.mimeType, joined.mimeType)
+    }
+
+    @Test
+    fun `a failure carries the reason the worker gave`() {
+        assertEquals(
+            JoinState.Failed("Not enough free space to join these files."),
+            map(
+                WorkInfo.State.FAILED,
+                data = workDataOf(
+                    ConcatWorker.KEY_ERROR to "Not enough free space to join these files.",
+                ),
+            ),
+        )
+    }
+
+    @Test
+    fun `a failure with nothing said still says something`() {
+        assertEquals(
+            JoinState.Failed(ConcatWorker.GENERIC_FAILURE_MESSAGE),
+            map(WorkInfo.State.FAILED, data = Data.EMPTY),
+        )
+    }
+
+    @Test
+    fun `a failure whose message is blank falls back like a missing one`() {
+        assertEquals(
+            JoinState.Failed(ConcatWorker.GENERIC_FAILURE_MESSAGE),
+            map(WorkInfo.State.FAILED, data = workDataOf(ConcatWorker.KEY_ERROR to " ")),
+        )
+    }
+
+    @Test
+    fun `a cancellation lands wherever the caller said it should`() {
+        // A join started here goes back to Ready with the picked files; one picked up by reattach
+        // goes to Idle, because those URIs belong to a process that no longer exists.
+        assertEquals(
+            JoinState.Ready(INPUTS),
+            map(WorkInfo.State.CANCELLED, cancelled = JoinState.Ready(INPUTS)),
+        )
+        assertEquals(JoinState.Idle, map(WorkInfo.State.CANCELLED, cancelled = JoinState.Idle))
+    }
+
+    private fun map(
+        state: WorkInfo.State,
+        runAttemptCount: Int = 0,
+        data: Data = Data.EMPTY,
+        cancelled: JoinState = JoinState.Ready(INPUTS),
+    ): JoinState = joinStateFrom(
+        JoinUpdate(state = state, runAttemptCount = runAttemptCount, outputData = data),
+        inputs = INPUTS,
+        cancelled = cancelled,
+    )
+
+    private companion object {
+        val INPUTS = listOf(
+            InputFile(Uri.parse("content://test/a.mp4"), "a.mp4", 1024L),
+            InputFile(Uri.parse("content://test/b.mp4"), "b.mp4", 2048L),
+        )
+    }
+}

app/src/test/java/org/libremediaconverter/join/SharedFailureMessagesTest.kt

@@ -0,0 +1,102 @@
+package org.libremediaconverter.join
+
+import android.app.Application
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.ListenableWorker
+import androidx.work.testing.TestListenableWorkerBuilder
+import androidx.work.workDataOf
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.ConversionDependencies
+import org.libremediaconverter.convert.RecordingPublisher
+import org.libremediaconverter.convert.installTestWorkManager
+import org.libremediaconverter.work.ConcatWorker
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+/**
+ * The two layers that refuse a short join, refusing it with one sentence.
+ *
+ * ## Why this is not "assert a constant equals itself"
+ *
+ * `ConcatWorker` and `JoinViewModel` both reject a join of fewer than two files, and before #158
+ * each carried **its own copy of the literal**. Only the worker's was pinned — by `RefusedJobTest`,
+ * added in #139 — so the wording on the screen could drift away from the wording in the job with no
+ * test saying anything, for one message the user sees from one condition.
+ *
+ * Sharing a constant makes them agree by construction. What it does *not* do is prove that both
+ * layers still reach it: a refactor that stops `JoinViewModel` refusing at all, or that gives it a
+ * different message, passes any test that only reads `TOO_FEW_INPUTS_MESSAGE`. So each layer is
+ * driven for real here — the ViewModel through `onInputsPicked`, the worker through `doWork` — and
+ * the assertion is that the two answers are **the same string**, taken from two running layers
+ * rather than from one declaration.
+ *
+ * That is the shape `CLAUDE.md` asks for: revert the sharing and this goes red, because the two
+ * sites drift the moment they are allowed to.
+ *
+ * ## Scope
+ *
+ * The arity guard's own behaviour on the ViewModel side — that it refuses one file, that it accepts
+ * two, that it claims ownership first — is #155's, and this deliberately does not duplicate it.
+ * This file is about the *agreement between layers*, which is what #158 changed.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class SharedFailureMessagesTest {
+
+    private lateinit var app: Application
+    private lateinit var viewModel: JoinViewModel
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        ConversionDependencies.publisher = { RecordingPublisher(app) }
+        installTestWorkManager(app, workDataOf(ConcatWorker.KEY_OUTPUT_PATH to "/dev/null"))
+        viewModel = JoinViewModel(app)
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    @Test
+    fun `both layers refuse a one-file join with the same sentence`() {
+        // The ViewModel, refusing before anything is enqueued.
+        viewModel.onInputsPicked(listOf(ONE_FILE))
+        val fromScreen = (viewModel.state.value as JoinState.Failed).message
+
+        // The worker, refusing a job that reached the queue anyway -- which it can, because
+        // ConcatWorker.request(...) takes a List<Uri> and checks nothing about its length.
+        val result = runBlocking { worker(ONE_FILE).doWork() }
+        val fromJob = (result as ListenableWorker.Result.Failure)
+            .outputData.getString(ConcatWorker.KEY_ERROR)
+
+        assertEquals(
+            "the screen and the job must say the same thing about the same refusal",
+            fromScreen,
+            fromJob,
+        )
+        // And that the shared sentence is the one either layer would have written on its own,
+        // rather than both having drifted together to something else.
+        assertEquals(ConcatWorker.TOO_FEW_INPUTS_MESSAGE, fromScreen)
+    }
+
+    private fun worker(vararg inputs: Uri): ConcatWorker = TestListenableWorkerBuilder<ConcatWorker>(
+        context = app,
+        inputData = workDataOf(
+            ConcatWorker.KEY_INPUT_URIS to inputs.map(Uri::toString).toTypedArray(),
+            ConcatWorker.KEY_TOTAL_BYTES to 1024L,
+        ),
+        runAttemptCount = 0,
+    ).build()
+
+    private companion object {
+        val ONE_FILE: Uri = Uri.parse("content://test/holiday.mp4")
+    }
+}

app/src/test/java/org/libremediaconverter/model/ContainerCapabilitiesTest.kt

@@ -11,6 +11,11 @@ import org.junit.Test
  * `OutputFormat` used to be twelve hand-picked triples, and its KDoc defended that on the grounds
  * that a closed set was what made routing decidable. Opening it up moves that burden here, so this
  * is where decidability now has to be proven.
+ *
+ * That includes what a refusal offers instead. `Validation.Invalid` promises every suggestion is
+ * itself valid and names this class as the proof, so a branch that assembles its own suggestion
+ * list rather than going through `suggestions()` is only checked here if some row happens to reach
+ * it — which is how a dead-end chip survived two widenings of that table.
  */
 class ContainerCapabilitiesTest {
 
@@ -35,6 +40,29 @@ class ContainerCapabilitiesTest {
         container = Container.MP3,
     )
 
+    /**
+     * An audio-only input carrying a codec MP4 has no place for at all.
+     *
+     * Vorbis lives in Ogg and Matroska; MP4 carries AAC, MP3, Opus and FLAC. That gap is what turns
+     * a suggestion which merely drops the video track into a second refusal.
+     */
+    private val vorbisSource = InputProbe(
+        videoCodec = null,
+        audioCodec = "vorbis",
+        hasVideo = false,
+        kind = InputKind.AUDIO_ONLY,
+        container = Container.OGG,
+    )
+
+    /** The same shape, for the other codec MP4 refuses. One case is a coincidence; two is the rule. */
+    private val pcmSource = InputProbe(
+        videoCodec = null,
+        audioCodec = "pcm_s16le",
+        hasVideo = false,
+        kind = InputKind.AUDIO_ONLY,
+        container = Container.WAV,
+    )
+
     // --- copy and encode are different questions ----------------------------
 
     /**
@@ -97,7 +125,17 @@ class ContainerCapabilitiesTest {
         }
     }
 
-    /** A suggestion that is itself invalid is worse than no suggestion. */
+    /**
+     * A suggestion that is itself invalid is worse than no suggestion.
+     *
+     * Only a branch that assembles its own suggestion list can break that promise: [suggestions]
+     * ends by filtering on `validate(...).isValid`, so everything routed through it is valid by
+     * construction. Those branches are what this table has to cover — the image output, and copy
+     * the video from a file that has none, which built its list by hand and came back refused for
+     * a Vorbis or PCM source into MP4 and an MP3 into WebM. The Advanced picker showed a one-tap
+     * fix that led straight to a second error, through two widenings of this table that never
+     * reached the branch.
+     */
     @Test
     fun `every suggestion is itself valid`() {
         val cases = listOf(
@@ -109,15 +147,31 @@ class ContainerCapabilitiesTest {
             OutputSpec(Container.MP4, VideoCodec.COPY, AudioCodec.NONE) to mp3Source,
             OutputSpec(Container.MP4, VideoCodec.NONE, AudioCodec.NONE) to mp3Source,
             OutputSpec(Container.MP4, VideoCodec.COPY, AudioCodec.AAC) to mp3Source,
+            // Copy-the-video-from-a-file-with-no-video, the last branch that built its offer by
+            // hand. It escaped the five rows above because `spec.copy(videoCodec = NONE)` is valid
+            // exactly when the audio axis happens to be fine — true for the AAC and MP3 sources
+            // used there, false for any audio the target container cannot carry.
+            OutputSpec(Container.MP4, VideoCodec.COPY, AudioCodec.COPY) to vorbisSource,
+            OutputSpec(Container.MP4, VideoCodec.COPY, AudioCodec.COPY) to pcmSource,
+            OutputSpec(Container.WEBM, VideoCodec.COPY, AudioCodec.COPY) to mp3Source,
+            // The same branch with audio the container *can* hold, which is the half that already
+            // worked and must keep working: the repair here is a copy, so the offer is the very
+            // spec the caller handed to `suggestions`. It survives only because the exclusion is
+            // against what the user asked for rather than against the repair.
+            OutputSpec(Container.MP4, VideoCodec.COPY, AudioCodec.COPY) to mp3Source,
+            // The one branch that still builds its list by hand, so that it is asserted rather
+            // than merely reasoned about: an image container takes `None + None` and nothing else,
+            // which makes its single offer valid by construction.
+            OutputSpec(Container.GIF, VideoCodec.H264, AudioCodec.AAC) to h264Source,
         )
 
         cases.forEach { (spec, probe) ->
             val invalid = ContainerCapabilities.validate(spec, probe) as? Validation.Invalid
                 ?: throw AssertionError("expected $spec to be rejected")
-            assertTrue("no alternatives offered for $spec", invalid.suggestions.isNotEmpty())
+            assertTrue("no alternatives offered for $spec on $probe", invalid.suggestions.isNotEmpty())
             invalid.suggestions.forEach { suggestion ->
                 assertTrue(
-                    "suggested $suggestion for $spec is itself invalid",
+                    "suggested $suggestion for $spec on $probe is itself invalid",
                     ContainerCapabilities.validate(suggestion, probe).isValid,
                 )
             }
@@ -304,4 +358,213 @@ class ContainerCapabilitiesTest {
                 assertEquals(emptyList<VideoCodec>(), ContainerCapabilities.encodableVideo(container))
             }
     }
+
+    // --- the audio axis -----------------------------------------------------
+    //
+    // Every rule below has a video twin already tested above. The two halves of `validate` were
+    // written together and only one of them was ever checked, so these are deliberately shaped like
+    // their twins rather than as a fresh idea about what to assert.
+
+    @Test
+    fun `an unidentifiable source audio codec cannot be copied`() {
+        // The audio twin of `an unidentifiable source codec cannot be copied`. Never guess: a copy
+        // of an unidentified codec is how you ship a file that does not play.
+        val unknownAudio = InputProbe(videoCodec = "h264", audioCodec = null, container = Container.MP4)
+        val spec = OutputSpec(Container.MP4, VideoCodec.H264, AudioCodec.COPY)
+
+        val invalid = ContainerCapabilities.validate(spec, unknownAudio) as? Validation.Invalid
+            ?: throw AssertionError("copying an unidentified audio codec must be refused")
+
+        assertTrue(invalid.message, invalid.message.contains("could not be identified"))
+        assertEverySuggestionValid(invalid, unknownAudio)
+    }
+
+    @Test
+    fun `copying an audio codec the container cannot hold is refused`() {
+        // MP4 carries AAC, MP3, Opus and FLAC. Vorbis lives in Ogg and Matroska, so a stream copy
+        // out of a Vorbis source into MP4 has nowhere to put the track.
+        val vorbisAudio = InputProbe(videoCodec = "h264", audioCodec = "vorbis", container = Container.MKV)
+        val spec = OutputSpec(Container.MP4, VideoCodec.H264, AudioCodec.COPY)
+
+        val invalid = ContainerCapabilities.validate(spec, vorbisAudio) as? Validation.Invalid
+            ?: throw AssertionError("Vorbis copied into MP4 must be refused")
+
+        assertEquals("MP4 cannot hold Vorbis audio.", invalid.message)
+        assertEverySuggestionValid(invalid, vorbisAudio)
+    }
+
+    @Test
+    fun `an audio codec the container cannot hold is refused on the encode path too`() {
+        // WAV carries PCM and nothing else. The twin is `H265 in AVI is refused`.
+        val spec = OutputSpec(Container.WAV, VideoCodec.NONE, AudioCodec.AAC)
+
+        val invalid = ContainerCapabilities.validate(spec, mp3Source) as? Validation.Invalid
+            ?: throw AssertionError("AAC in WAV must be refused")
+
+        assertEquals("WAV cannot hold AAC audio.", invalid.message)
+        assertEverySuggestionValid(invalid, mp3Source)
+    }
+
+    @Test
+    fun `an audio codec this app cannot encode is refused, and copying is offered instead`() {
+        // Matroska carries Vorbis; nothing here encodes it. The refusal has to say so *and* say
+        // what would work, which is the audio twin of `copying is offered as the fix when the codec
+        // is right but unencodable`.
+        val spec = OutputSpec(Container.MKV, VideoCodec.H264, AudioCodec.VORBIS)
+
+        val invalid = ContainerCapabilities.validate(spec, h264Source) as? Validation.Invalid
+            ?: throw AssertionError("encoding Vorbis must be refused")
+
+        assertEquals(
+            "This app cannot encode Vorbis audio. It can still be copied from a Vorbis source.",
+            invalid.message,
+        )
+        assertEverySuggestionValid(invalid, h264Source)
+    }
+
+    @Test
+    fun `copying a video codec the container cannot hold is refused`() {
+        // Not the audio axis, but the one video refusal with no test: AVI predates H.265, so a
+        // stream copy out of an HEVC source into AVI has nowhere to put the track. `H265 in AVI is
+        // refused` covers the matrix; this covers what validate() does with it.
+        val h265Source = InputProbe(videoCodec = "hevc", audioCodec = "mp3", container = Container.MP4)
+        val spec = OutputSpec(Container.AVI, VideoCodec.COPY, AudioCodec.MP3)
+
+        val invalid = ContainerCapabilities.validate(spec, h265Source) as? Validation.Invalid
+            ?: throw AssertionError("H.265 copied into AVI must be refused")
+
+        assertEquals("AVI cannot hold H.265 video.", invalid.message)
+        assertEverySuggestionValid(invalid, h265Source)
+    }
+
+    @Test
+    fun `no audio track is accepted by every container in both modes`() {
+        // The audio twin of VideoCodec.NONE -> true. A container that refused "no audio" would make
+        // every video-only output invalid.
+        Container.entries.forEach { container ->
+            listOf(CodecMode.COPY, CodecMode.ENCODE).forEach { mode ->
+                assertTrue(
+                    "$container should accept no audio track ($mode)",
+                    ContainerCapabilities.accepts(container, AudioCodec.NONE, mode),
+                )
+            }
+        }
+    }
+
+    /**
+     * The video twin of `no audio track is accepted by every container in both modes`.
+     *
+     * Dead in production today, and deliberately so: every caller guards `NONE` before asking the
+     * matrix, so nothing reaches this arm through the app. **The asymmetry is the argument, not the
+     * reachability** -- its audio counterpart at the top of the same `when` has had a dedicated
+     * test since #136, and one of a matched pair being covered is how a later reader concludes the
+     * other was considered and exempted. It was not; it was simply missed.
+     *
+     * Not the same shape as the two `COPY -> error(...)` arms, which `docs/coverage-read-findings.md`
+     * records as a named exemption (F4). Those are guards that must not be provokable. This is a
+     * documented answer -- "no video track fits anywhere" -- and an answer is a thing to pin.
+     */
+    @Test
+    fun `no video track is accepted by every container in both modes`() {
+        Container.entries.forEach { container ->
+            listOf(CodecMode.COPY, CodecMode.ENCODE).forEach { mode ->
+                assertTrue(
+                    "$container should accept no video track ($mode)",
+                    ContainerCapabilities.accepts(container, VideoCodec.NONE, mode),
+                )
+            }
+        }
+    }
+
+    /**
+     * A suggestion that keeps the codec the user asked for, rather than falling back to the
+     * container's first encodable one.
+     *
+     * `repairVideo`'s third arm -- "the request is not a copy, and this container can encode it" --
+     * is the one that preserves intent, and it was the only arm of the four nothing reached. The
+     * property test above executes `repairVideo` on every case it walks and lands elsewhere each
+     * time: an explicit COPY that works, a source the container can carry untouched, or no video
+     * track at all.
+     *
+     * The route is indirect because it is the only one the app has. VP9 into WebM is a perfectly
+     * good video request; what makes it invalid is the *audio* -- WebM carries Opus and Vorbis, not
+     * AAC. So `validateAudio` refuses, `suggestions` looks for a container that can hold what was
+     * asked for, and MP4 can encode VP9. The suggestion has to come back carrying VP9: swapping to
+     * the container's first encodable codec would discard the choice the user made.
+     */
+    @Test
+    fun `a repaired suggestion keeps the video codec the user chose`() {
+        val invalid = ContainerCapabilities.validate(
+            OutputSpec(Container.WEBM, VideoCodec.VP9, AudioCodec.AAC),
+            h264Source,
+        )
+
+        assertTrue("WebM cannot hold AAC, so this spec is invalid", invalid is Validation.Invalid)
+        val suggestions = (invalid as Validation.Invalid).suggestions
+        assertTrue(
+            "expected a suggestion that still encodes VP9, got $suggestions",
+            suggestions.any { it.videoCodec == VideoCodec.VP9 },
+        )
+        assertEverySuggestionValid(invalid, h264Source)
+    }
+
+    /**
+     * The fallback in `firstContainerHolding`: when the input's own container cannot hold the
+     * codec the user asked for, any container that can will do.
+     *
+     * The preferred half -- "the container the input already uses" -- is what every other case
+     * reaches, because they all start from a file whose own container carries the codec in
+     * question. The elvis after it had never run.
+     *
+     * AVI is the input that makes it run: AVI predates H.265 and has no mapping for it, so asking
+     * an AVI for H.265 is refused, and the container the input already uses cannot be part of the
+     * answer. Without the fallback the only candidates left are AVI itself and the container
+     * holding the *source* codec -- also AVI -- so the refusal still offers something, but what it
+     * offers is H.264: the app quietly declines the codec the user asked for instead of moving them
+     * to a container that supports it.
+     *
+     * That is why this asserts the codec survives rather than that the list is non-empty. A
+     * non-empty assertion passes with the fallback deleted -- measured, not assumed.
+     */
+    @Test
+    fun `an input whose container cannot hold the requested codec is moved, not downgraded`() {
+        val aviSource = InputProbe(videoCodec = "h264", audioCodec = "aac", container = Container.AVI)
+
+        val invalid = ContainerCapabilities.validate(
+            OutputSpec(Container.AVI, VideoCodec.H265, AudioCodec.AAC),
+            aviSource,
+        )
+
+        assertTrue("AVI has no mapping for H.265", invalid is Validation.Invalid)
+        val suggestions = (invalid as Validation.Invalid).suggestions
+        assertTrue(
+            "expected a container that can actually hold H.265, got $suggestions",
+            suggestions.any { it.videoCodec == VideoCodec.H265 },
+        )
+        assertEverySuggestionValid(invalid, aviSource)
+    }
+
+    @Test
+    fun `resolving audio COPY before asking the matrix is required`() {
+        // The audio twin of `resolving COPY before asking the matrix is required`, and the reason is
+        // identical: silently answering "false" would refuse a perfectly good remux.
+        runCatching { ContainerCapabilities.accepts(Container.MP4, AudioCodec.COPY, CodecMode.COPY) }
+            .onSuccess { throw AssertionError("expected audio COPY to be rejected by the matrix") }
+    }
+
+    /**
+     * Every alternative a refusal offers has to be one the same input could actually take.
+     *
+     * `Validation.Invalid` promises exactly this and names this class as the proof. The global
+     * property test walks the presets; these paths reach `suggestions()` through `validateAudio`,
+     * which no preset does.
+     */
+    private fun assertEverySuggestionValid(invalid: Validation.Invalid, probe: InputProbe) {
+        invalid.suggestions.forEach {
+            assertTrue(
+                "suggestion $it is itself invalid, so the chip leads to a second error",
+                ContainerCapabilities.validate(it, probe).isValid,
+            )
+        }
+    }
 }

app/src/test/java/org/libremediaconverter/ui/TagTableUniquenessTest.kt

@@ -28,6 +28,7 @@ class TagTableUniquenessTest {
     fun `every tag constant has its own value`() {
         val tags = tagsIn(
             TestTags::class.java,
+            TestTags.Shell::class.java,
             TestTags.Converter::class.java,
             TestTags.Join::class.java,
         )

app/src/test/java/org/libremediaconverter/ui/theme/ThemeFollowsSystemTest.kt

@@ -0,0 +1,89 @@
+package org.libremediaconverter.ui.theme
+
+import androidx.compose.material3.ColorScheme
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.ui.test.junit4.v2.createComposeRule
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+/**
+ * The theme called the way the app calls it: with no arguments at all.
+ *
+ * [ThemeColorSchemeTest] resolves every branch of the `when` and always passes `darkTheme`
+ * explicitly, so the `$default` bridge is never entered and **`isSystemInDarkTheme()` is never
+ * called**. `MainActivity.kt:79` is its only default-argument caller and does not execute on the
+ * JVM, which left the app's actual call shape the one nothing exercised —
+ * `LibreMediaConverterTheme` reported `mi=21, mb=6, cb=12` at method level.
+ *
+ * ## Not #68
+ *
+ * #68 is about the two **unreachable** arms, `DarkColorScheme` and `LightColorScheme`, which cannot
+ * run because `dynamicColor` is always `true` and nothing can flip it. That is an open product
+ * decision. This is the reachable half — whether the default follows the system — and closing it
+ * does not close that.
+ *
+ * ## Why the assertion compares schemes rather than reading a number
+ *
+ * A luminance threshold would be a guess about the device palette. What is asserted instead is that
+ * the no-argument call resolves to **the same scheme** an explicit `darkTheme` of the matching
+ * value does, and a different one from its opposite. That holds whatever palette the platform
+ * hands back, and it is exactly the claim: the default reads the system rather than picking a side.
+ *
+ * Both schemes are resolved in one composition because `setContent` may be called once per test.
+ */
+@RunWith(RobolectricTestRunner::class)
+class ThemeFollowsSystemTest {
+
+    @get:Rule
+    val composeRule = createComposeRule()
+
+    @Test
+    @Config(qualifiers = "+night")
+    fun `with no arguments the theme follows a system in dark mode`() {
+        val resolved = resolve()
+
+        assertEquals("the default must resolve what darkTheme = true does", resolved.dark, resolved.bare)
+        assertNotEquals(resolved.light, resolved.bare)
+    }
+
+    @Test
+    @Config(qualifiers = "+notnight")
+    fun `with no arguments the theme follows a system in light mode`() {
+        val resolved = resolve()
+
+        assertEquals("the default must resolve what darkTheme = false does", resolved.light, resolved.bare)
+        assertNotEquals(resolved.dark, resolved.bare)
+    }
+
+    /**
+     * The three colours are read together as one value, because any single one could coincide
+     * between the two schemes on some palette while the schemes themselves differ. Background is
+     * what dark mode is chiefly about; primary and surface are along to make a coincidence
+     * implausible rather than merely unlikely.
+     */
+    private data class Fingerprint(val background: Long, val primary: Long, val surface: Long)
+
+    private fun ColorScheme.fingerprint() =
+        Fingerprint(background.value.toLong(), primary.value.toLong(), surface.value.toLong())
+
+    private class Resolved(val bare: Fingerprint, val dark: Fingerprint, val light: Fingerprint)
+
+    private fun resolve(): Resolved {
+        lateinit var bare: Fingerprint
+        lateinit var dark: Fingerprint
+        lateinit var light: Fingerprint
+        composeRule.setContent {
+            // No arguments — the call MainActivity makes, and the one nothing exercised.
+            LibreMediaConverterTheme { bare = MaterialTheme.colorScheme.fingerprint() }
+            LibreMediaConverterTheme(darkTheme = true) { dark = MaterialTheme.colorScheme.fingerprint() }
+            LibreMediaConverterTheme(darkTheme = false) { light = MaterialTheme.colorScheme.fingerprint() }
+        }
+        composeRule.waitForIdle()
+        return Resolved(bare, dark, light)
+    }
+}

app/src/test/java/org/libremediaconverter/work/ConcatFailureTest.kt

@@ -0,0 +1,237 @@
+package org.libremediaconverter.work
+
+import android.app.Application
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import androidx.work.ListenableWorker
+import androidx.work.testing.TestListenableWorkerBuilder
+import androidx.work.workDataOf
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.ConcatJoiner
+import org.libremediaconverter.convert.ConversionDependencies
+import org.libremediaconverter.convert.StagingNames
+import org.libremediaconverter.convert.installTestWorkManager
+import org.libremediaconverter.ffmpeg.ConcatEngine
+import org.libremediaconverter.model.ConcatStrategy
+import org.libremediaconverter.model.OutputFormat
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.io.File
+import java.util.UUID
+
+/**
+ * What a join does when the engine fails partway.
+ *
+ * Everything past `ConcatWorker`'s `setForeground` was untested on **every** source set, and the
+ * repo had already measured the cost: `PerJobStagingTest`'s KDoc records that reverting
+ * `ConcatWorker` to a constant staging name left all 257 tests green, because nothing in the JVM
+ * suite can get past a `ConcatEngine` constructed in place. `RefusedJobTest` says the same from the
+ * other side -- "the next thing past the count guard is `ConcatEngine`, which is native".
+ * `ConcatEngineTest` on a device tests the engine directly, bypassing the worker, and
+ * `ConcatWorkerTest` covers only the too-few-inputs guard and the happy path.
+ *
+ * `ConversionDependencies.concat` is the seam that closes it, added here to sit beside the
+ * `.hardware` and `.software` that `ConversionWorker` has had all along -- the asymmetry between the
+ * two workers was the whole reason one of them had a tested failure path and the other did not.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class ConcatFailureTest {
+
+    private lateinit var app: Application
+    private lateinit var publisher: AlwaysRoomPublisher
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        publisher = AlwaysRoomPublisher(app)
+        ConversionDependencies.publisher = { publisher }
+        installTestWorkManager(app, Data.EMPTY)
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    @Test
+    fun `a join whose engine fails reports the engine's own reason`() {
+        ConversionDependencies.concat = { FailingJoiner { error(DEMUXER_MESSAGE) } }
+
+        val result = runBlocking { joinWorker().doWork() }
+
+        assertEquals(
+            ListenableWorker.Result.failure(workDataOf(ConcatWorker.KEY_ERROR to DEMUXER_MESSAGE)),
+            result,
+        )
+    }
+
+    /**
+     * A failure carrying no message at all, which Kotlin and Java both allow and FFmpegKit's
+     * wrappers can produce.
+     *
+     * Without the fallback the user is shown an empty error, and `JoinViewModel` cannot tell that
+     * from a job that reported nothing -- the two would be one blank screen with different causes.
+     */
+    @Test
+    fun `a failure with no message of its own still says something`() {
+        ConversionDependencies.concat = { FailingJoiner { throw RuntimeException() } }
+
+        val result = runBlocking { joinWorker().doWork() }
+
+        assertEquals(
+            ListenableWorker.Result.failure(
+                workDataOf(ConcatWorker.KEY_ERROR to ConcatWorker.GENERIC_FAILURE_MESSAGE),
+            ),
+            result,
+        )
+    }
+
+    /**
+     * The staged file is deleted on the way out.
+     *
+     * The joiner writes before it fails, exactly as `PartialThenFailingTranscoder` does on the
+     * conversion side: a stub that only threw would let a missing `staged.delete()` pass. What it
+     * costs to lose is a full-size partial per failed join, sitting in cache until the sweep is old
+     * enough to be sure nobody is coming back for it.
+     *
+     * Asserted against the file the joiner was actually handed rather than by scanning the staging
+     * directory for a name. The first draft did scan, for a `"join-"` prefix that
+     * `StagingNames.forJob` does not produce -- it names files `<jobId>.<ext>` -- so the assertion
+     * was trivially true and the mutation walked straight through it.
+     */
+    @Test
+    fun `a failed join leaves nothing behind in staging`() {
+        val joiner = FailingJoiner { error(DEMUXER_MESSAGE) }
+        ConversionDependencies.concat = { joiner }
+
+        runBlocking { joinWorker().doWork() }
+
+        val staged = requireNotNull(joiner.lastOutput) { "the joiner never ran, so this proves nothing" }
+        assertEquals(
+            "the fixture has to write before it fails, or the delete is unobservable",
+            PARTIAL_BYTES,
+            joiner.bytesWritten,
+        )
+        assertFalse("a failed join must not leave its partial behind: $staged", staged.exists())
+    }
+
+    /**
+     * The success path, and the staging name #159's fixture and `PerJobStagingTest` both care about.
+     *
+     * Worth its place rather than a happy-path formality: `PerJobStagingTest`'s KDoc records that
+     * **reverting `ConcatWorker` to a constant staging name left all 257 tests green**, because
+     * nothing could reach the line that names the file. This is the test that was missing when that
+     * was written -- the join's output `Data` had never been read by anything on the JVM.
+     *
+     * The staged path is asserted to carry the job id, not a constant: two joins of the same format
+     * sharing one name is the defect, and `ConcatEngine`'s list file collided harder still.
+     */
+    @Test
+    fun `a join that works reports its own staged file, strategy and name`() {
+        val joiner = SucceedingJoiner()
+        ConversionDependencies.concat = { joiner }
+
+        val result = runBlocking { joinWorker().doWork() }
+
+        assertTrue("got $result", result is ListenableWorker.Result.Success)
+        val data = (result as ListenableWorker.Result.Success).outputData
+        assertEquals(
+            "the staged file has to be this job's, not a name every join shares",
+            File(publisherStagingDir(), StagingNames.forJob(JOB_ID, OutputFormat.MP4_H264.extension)).absolutePath,
+            data.getString(ConcatWorker.KEY_OUTPUT_PATH),
+        )
+        assertEquals(ConcatStrategy.STREAM_COPY.name, data.getString(ConcatWorker.KEY_STRATEGY))
+        assertEquals(OutputFormat.MP4_H264.mimeType, data.getString(ConcatWorker.KEY_MIME_TYPE))
+        assertTrue(
+            "the save dialog needs a name with the right extension, got ${data.getString(
+                ConcatWorker.KEY_SUGGESTED_NAME,
+            )}",
+            data.getString(ConcatWorker.KEY_SUGGESTED_NAME).orEmpty().endsWith(".${OutputFormat.MP4_H264.extension}"),
+        )
+    }
+
+    /**
+     * The arm beside the count guard: no URI array at all.
+     *
+     * Covered today only by `UnopenableUriTest` on a device, although it runs before staging and
+     * before any native code. It is the exact sibling of `RefusedJobTest`'s ConversionWorker twin,
+     * and it belongs on the JVM with it -- a device test for a branch that needs no device is a
+     * slower test that reports later.
+     */
+    @Test
+    fun `a join with no input array at all is refused with a message`() {
+        val result = runBlocking {
+            TestListenableWorkerBuilder<ConcatWorker>(
+                context = app,
+                inputData = workDataOf(ConcatWorker.KEY_FORMAT to OutputFormat.MP4_H264.name),
+                runAttemptCount = 0,
+            ).setId(JOB_ID).build().doWork()
+        }
+
+        assertEquals(
+            ListenableWorker.Result.failure(workDataOf(ConcatWorker.KEY_ERROR to ConcatWorker.NO_INPUTS_MESSAGE)),
+            result,
+        )
+    }
+
+    private fun publisherStagingDir(): File? = publisher.createStagingFile("probe").parentFile
+
+    private fun joinWorker(): ConcatWorker = TestListenableWorkerBuilder<ConcatWorker>(
+        context = app,
+        inputData = workDataOf(
+            ConcatWorker.KEY_INPUT_URIS to arrayOf(FIRST.toString(), SECOND.toString()),
+            ConcatWorker.KEY_TOTAL_BYTES to TOTAL_BYTES,
+            ConcatWorker.KEY_FORMAT to OutputFormat.MP4_H264.name,
+        ),
+        runAttemptCount = 0,
+    ).setId(JOB_ID).build()
+
+    private companion object {
+        val FIRST: Uri = Uri.parse("file:///tmp/one.mp4")
+        val SECOND: Uri = Uri.parse("file:///tmp/two.mp4")
+        const val TOTAL_BYTES = 2048L
+        const val DEMUXER_MESSAGE = "the demuxer rejected the input list"
+        const val PARTIAL_BYTES = 2048
+        val JOB_ID: UUID = UUID.fromString("00000000-0000-4000-8000-00000000000b")
+    }
+}
+
+/** A joiner that writes something and then fails, so a missing `staged.delete()` cannot pass. */
+private class FailingJoiner(private val failure: () -> Nothing) : ConcatJoiner {
+
+    /** The handle the worker created, kept so a test can ask whether it survived the failure. */
+    var lastOutput: File? = null
+    var bytesWritten = 0
+
+    override suspend fun join(inputs: List<Uri>, output: File, format: OutputFormat): ConcatEngine.Result {
+        lastOutput = output
+        output.writeBytes(ByteArray(PARTIAL_BYTES))
+        bytesWritten = PARTIAL_BYTES
+        failure()
+    }
+
+    private companion object {
+        const val PARTIAL_BYTES = 2048
+    }
+}
+
+/** The joiner that finishes, so the success path and the output `Data` can be read on the JVM. */
+private class SucceedingJoiner : ConcatJoiner {
+    override suspend fun join(inputs: List<Uri>, output: File, format: OutputFormat): ConcatEngine.Result {
+        output.writeBytes(ByteArray(OUTPUT_BYTES))
+        return ConcatEngine.Result(ConcatStrategy.STREAM_COPY, output)
+    }
+
+    private companion object {
+        const val OUTPUT_BYTES = 4096
+    }
+}

app/src/test/java/org/libremediaconverter/work/DeniedForegroundStartTest.kt

@@ -27,9 +27,6 @@ import org.robolectric.RobolectricTestRunner
 import org.robolectric.RuntimeEnvironment
 import java.io.File
 import java.util.UUID
-import java.util.concurrent.ExecutionException
-import java.util.concurrent.Executor
-import java.util.concurrent.TimeUnit
 
 /**
  * That a refused foreground-service start does not end the job.
@@ -125,6 +122,40 @@ class DeniedForegroundStartTest {
         )
     }
 
+    @Test
+    fun `a join denied past the attempt bound fails with a message the user can act on`() {
+        // The join twin of the conversion case above. ConcatWorker reaches the same FailureOutcome
+        // through its own `when`, and that arm was the only one of its three with no test -- so a
+        // join that gave up silently, or gave up with an empty Data, would have looked identical to
+        // one that retried.
+        val worker = concatWorker(runAttemptCount = FailureOutcome.MAX_FOREGROUND_START_ATTEMPTS)
+
+        val result = runBlocking { worker.doWork() }
+
+        assertEquals(
+            ListenableWorker.Result.failure(
+                workDataOf(ConcatWorker.KEY_ERROR to FailureOutcome.FOREGROUND_DENIED_MESSAGE),
+            ),
+            result,
+        )
+    }
+
+    @Test
+    fun `a join that gives up collects the partial it had already staged`() {
+        // The delete lives on ConcatWorker's `catch (e: Throwable)` path, which every give-up goes
+        // through. Written first so a missing delete cannot pass by asking whether a file nobody
+        // wrote is absent.
+        concatStagedFile().writeBytes(ByteArray(PARTIAL_BYTES))
+
+        runBlocking { concatWorker(runAttemptCount = FailureOutcome.MAX_FOREGROUND_START_ATTEMPTS).doWork() }
+
+        assertEquals(
+            "a join that gave up must not orphan what it staged",
+            emptyList<String>(),
+            stagedNames(),
+        )
+    }
+
     private fun conversionWorker(runAttemptCount: Int = 0): ConversionWorker =
         TestListenableWorkerBuilder<ConversionWorker>(
             context = app,
@@ -141,18 +172,22 @@ class DeniedForegroundStartTest {
             .setForegroundUpdater(DenyingForegroundUpdater)
             .build()
 
-    private fun concatWorker(): ConcatWorker = TestListenableWorkerBuilder<ConcatWorker>(
+    private fun concatWorker(runAttemptCount: Int = 0): ConcatWorker = TestListenableWorkerBuilder<ConcatWorker>(
         context = app,
         inputData = workDataOf(
             ConcatWorker.KEY_INPUT_URIS to arrayOf(INPUT.toString(), "content://test/second.mp4"),
             ConcatWorker.KEY_TOTAL_BYTES to INPUT_BYTES,
-            ConcatWorker.KEY_FORMAT to OutputFormat.MP4_H264.name,
+            ConcatWorker.KEY_FORMAT to CONCAT_FORMAT.name,
         ),
-        runAttemptCount = 0,
+        runAttemptCount = runAttemptCount,
     ).setId(CONCAT_ID)
         .setForegroundUpdater(DenyingForegroundUpdater)
         .build()
 
+    /** The staging path the join will compute, asked for rather than spelled out here. */
+    private fun concatStagedFile(): File =
+        publisher.createStagingFile(StagingNames.forJob(CONCAT_ID, CONCAT_FORMAT.extension))
+
     /** The staging path the worker will compute, asked for rather than spelled out here. */
     private fun stagedFile(): File = publisher.createStagingFile(StagingNames.forJob(CONVERSION_ID, SPEC.extension))
 
@@ -164,6 +199,7 @@ class DeniedForegroundStartTest {
         const val INPUT_BYTES = 1024L
         const val PARTIAL_BYTES = 2048
         val SPEC = OutputFormat.MP4_H265.spec
+        val CONCAT_FORMAT = OutputFormat.MP4_H264
         val CONVERSION_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000001")
         val CONCAT_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000002")
     }
@@ -182,18 +218,3 @@ private object DenyingForegroundUpdater : ForegroundUpdater {
         ),
     )
 }
-
-/**
- * An already-failed future, written out rather than pulled from a futures library.
- *
- * `await()` takes the `isDone` fast path and unwraps the `ExecutionException`, which is what puts
- * the platform's own exception in front of the worker's catch rather than a wrapper.
- */
-private class FailedFuture(private val failure: Throwable) : ListenableFuture<Void> {
-    override fun addListener(listener: Runnable, executor: Executor): Unit = executor.execute(listener)
-    override fun cancel(mayInterruptIfRunning: Boolean): Boolean = false
-    override fun isCancelled(): Boolean = false
-    override fun isDone(): Boolean = true
-    override fun get(): Void = throw ExecutionException(failure)
-    override fun get(timeout: Long, unit: TimeUnit): Void = throw ExecutionException(failure)
-}

app/src/test/java/org/libremediaconverter/work/ForegroundTypeRegimeTest.kt

@@ -0,0 +1,88 @@
+package org.libremediaconverter.work
+
+import android.content.pm.ServiceInfo
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+/**
+ * [ConversionForegroundType.current] answers differently on each of the three API regimes, and
+ * until this file only one of them was ever executed.
+ *
+ * `app/src/test/resources/robolectric.properties` pins the whole JVM suite to `sdk=36`, so every
+ * Robolectric test that reaches a `ForegroundInfo` takes the `mediaProcessing` arm and no other.
+ * The 33 and 34 arms were cold: 3 lines and 3 of 4 branches, measured on `main` at `d354f64`.
+ *
+ * **The instrumented test is not a substitute, and the reason is specific.**
+ * `ConversionWorkerTest.foregroundTypeMatchesTheRunningApiLevel` asserts against whichever API the
+ * leg happens to be — one arm per leg, never the other two — and the legs that would cover 33 and
+ * 34 are the ones issue #122 wedges. `docs/coverage-read-findings.md` records an API 33 run that
+ * reported `received: 60` and `failed: unknown`: the regime *was* exercised, and that leg could
+ * not have said so if it had broken. Four `@Config` classes here pin all three arms
+ * deterministically, in the same `./gradlew` invocation as everything else.
+ *
+ * `minSdk` is 33, so none of these is dead code — each is a device someone is running the app on.
+ *
+ * **SDK 35 is in the list for the boundary, not for the answer.** It shares its answer with 36,
+ * which would make it look redundant. It is not: relaxing `>= VANILLA_ICE_CREAM` to `>` is invisible
+ * at every level except exactly 35, so without this class that mutation survives the suite.
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [33])
+class ForegroundTypeApi33Test {
+
+    /**
+     * Zero rather than a named constant because there is no constant to name: API 33 does not
+     * require a type, and `mediaProcessing` does not exist here to pass. `ForegroundInfo` reads 0
+     * as "no type at all", which is what this regime wants.
+     */
+    @Test
+    fun `api 33 asks for no foreground service type`() {
+        assertEquals(0, ConversionForegroundType.current())
+    }
+}
+
+/**
+ * API 34 makes a type mandatory and still has no `mediaProcessing`, so `dataSync` is the only
+ * sensible fit. See [ForegroundTypeApi33Test] for why this file exists.
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [34])
+class ForegroundTypeApi34Test {
+
+    @Test
+    fun `api 34 falls back to dataSync, the only type that fits`() {
+        assertEquals(ServiceInfo.FOREGROUND_SERVICE_TYPE_DATA_SYNC, ConversionForegroundType.current())
+    }
+}
+
+/**
+ * The first level with `mediaProcessing`, and therefore the one that tells `>=` from `>`.
+ * See [ForegroundTypeApi33Test].
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [35])
+class ForegroundTypeApi35Test {
+
+    @Test
+    fun `api 35 is the first level that takes mediaProcessing`() {
+        assertEquals(ServiceInfo.FOREGROUND_SERVICE_TYPE_MEDIA_PROCESSING, ConversionForegroundType.current())
+    }
+}
+
+/**
+ * The level the rest of the suite runs at, asserted here rather than assumed — it is the one arm
+ * that was already covered, and leaving it out would make this file look like it is about the old
+ * levels rather than about all three regimes. See [ForegroundTypeApi33Test].
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [36])
+class ForegroundTypeApi36Test {
+
+    @Test
+    fun `api 36 keeps mediaProcessing`() {
+        assertEquals(ServiceInfo.FOREGROUND_SERVICE_TYPE_MEDIA_PROCESSING, ConversionForegroundType.current())
+    }
+}

app/src/test/java/org/libremediaconverter/work/HardwareFallbackTest.kt

@@ -0,0 +1,226 @@
+package org.libremediaconverter.work
+
+import android.app.Application
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import androidx.work.ListenableWorker
+import androidx.work.testing.TestListenableWorkerBuilder
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertThrows
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.ConversionDependencies
+import org.libremediaconverter.convert.HardwareTranscoder
+import org.libremediaconverter.convert.SoftwareTranscoder
+import org.libremediaconverter.convert.installTestWorkManager
+import org.libremediaconverter.model.Container
+import org.libremediaconverter.model.ConversionRequest
+import org.libremediaconverter.model.DeviceCodecs
+import org.libremediaconverter.model.EnginePreference
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.model.OutputFormat
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.io.File
+import java.util.UUID
+
+/**
+ * What happens when the hardware engine does not finish the job.
+ *
+ * `runMedia3OrFallBack` was eleven lines at 0% on the JVM and `isCancellation` had never been
+ * called by any unit test at all. Its own KDoc calls the fallback the protection against vendor
+ * hardware encoders that "cannot be tested for correctness", so it is the branch most likely to
+ * matter on a device nobody here owns — and it was reachable the whole time through
+ * `ConversionDependencies.hardware`, which no unit test had ever used.
+ *
+ * The sharp one is cancellation. `runMedia3OrFallBack` catches `Throwable`, so without the
+ * `isCancellation` re-throw a user cancelling a hardware transcode would have the app quietly
+ * start a *second* conversion in software — the one thing cancelling is supposed to prevent.
+ *
+ * `ForcedFailureTest` covers the failure half on a device. It does not cover the cancellation half,
+ * and this host cannot run it either way.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class HardwareFallbackTest {
+
+    private lateinit var app: Application
+    private lateinit var hardware: RecordingHardwareTranscoder
+    private lateinit var software: RecordingSoftwareTranscoder
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        hardware = RecordingHardwareTranscoder()
+        software = RecordingSoftwareTranscoder()
+        ConversionDependencies.publisher = { AlwaysRoomPublisher(app) }
+        ConversionDependencies.hardware = { hardware }
+        ConversionDependencies.software = { software }
+        // A probe with real codecs, not the default: `InputProbe()` reports UNPARSEABLE, which
+        // PERMISSIVE.canDecode refuses, and the router would send every job here straight to
+        // FFmpeg without any of these tests mentioning why.
+        ConversionDependencies.probe = { _, _ -> H264_SOURCE }
+        ConversionDependencies.deviceCodecs = { DeviceCodecs.PERMISSIVE }
+        installTestWorkManager(app, Data.EMPTY)
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    @Test
+    fun `a hardware failure runs the job again in software, on a clean staging file`() {
+        hardware.failWith = { error("the vendor encoder produced nothing usable") }
+
+        val result = runBlocking { worker().doWork() }
+
+        assertTrue("the job should still succeed, got $result", result is ListenableWorker.Result.Success)
+        assertEquals("the hardware engine gets exactly one attempt", 1, hardware.attempts)
+        assertEquals("and the job then goes to software", 1, software.attempts)
+        // The `staged.delete()` between the two, asserted where it is observable: FFmpeg must not
+        // find a half-written hardware output sitting at the path it is about to write.
+        assertFalse(
+            "the partial hardware output must be gone before FFmpeg starts",
+            software.outputExistedOnEntry,
+        )
+        assertEquals("the hardware engine is closed either way", 1, hardware.closes)
+    }
+
+    @Test
+    fun `a cancelled hardware transcode is not quietly retried in software`() {
+        hardware.failWith = { throw CancellationException("the user pressed Cancel") }
+
+        assertThrows(CancellationException::class.java) { runBlocking { worker().doWork() } }
+
+        assertEquals("the hardware engine ran", 1, hardware.attempts)
+        assertEquals(
+            "cancelling must not start a second conversion -- that is the whole point of cancelling",
+            0,
+            software.attempts,
+        )
+        assertEquals("and the engine is still closed on the way out", 1, hardware.closes)
+    }
+
+    @Test
+    fun `a hardware transcode that works never reaches the software engine`() {
+        val result = runBlocking { worker().doWork() }
+
+        assertTrue("got $result", result is ListenableWorker.Result.Success)
+        assertEquals(1, hardware.attempts)
+        assertEquals("the fallback is a fallback, not a second pass", 0, software.attempts)
+        assertEquals(1, hardware.closes)
+    }
+
+    /**
+     * #169: the display-name fallback, which reaches further than the notification title.
+     *
+     * `inputData.getString(KEY_DISPLAY_NAME) ?: "input"` had never taken its right-hand side. The
+     * value is not only the foreground notification's title: it feeds `outputNameFor`, so it is
+     * also the filename offered in the user's save dialog. A job enqueued by an older build, or
+     * built by hand, carries no such key.
+     */
+    @Test
+    fun `a job that names no input file still suggests an output name`() {
+        val result = runBlocking { worker(displayName = null).doWork() }
+
+        assertTrue("got $result", result is ListenableWorker.Result.Success)
+        val suggested = (result as ListenableWorker.Result.Success)
+            .outputData.getString(ConversionWorker.KEY_SUGGESTED_NAME)
+        assertTrue(
+            "expected a name built from the fallback, got $suggested",
+            suggested.orEmpty().startsWith("input"),
+        )
+    }
+
+    private fun worker(displayName: String? = DISPLAY_NAME): ConversionWorker {
+        val spec = OutputFormat.MP4_H265.spec
+        val entries = buildMap<String, Any> {
+            put(ConversionWorker.KEY_INPUT_URI, INPUT.toString())
+            displayName?.let { put(ConversionWorker.KEY_DISPLAY_NAME, it) }
+            put(ConversionWorker.KEY_SIZE_BYTES, INPUT_BYTES)
+            put(ConversionWorker.KEY_CONTAINER, spec.container.name)
+            put(ConversionWorker.KEY_VIDEO_CODEC, spec.videoCodec.name)
+            put(ConversionWorker.KEY_AUDIO_CODEC, spec.audioCodec.name)
+            // AUTO rather than FORCE_SOFTWARE, which is what every other worker test uses and is
+            // exactly why this path had no coverage: forcing software never enters the function.
+            put(ConversionWorker.KEY_ENGINE_PREFERENCE, EnginePreference.AUTO.name)
+        }
+        return TestListenableWorkerBuilder<ConversionWorker>(
+            context = app,
+            inputData = Data.Builder().putAll(entries).build(),
+            runAttemptCount = 0,
+        ).setId(JOB_ID).build()
+    }
+
+    private companion object {
+        val INPUT: Uri = Uri.parse("file:///tmp/holiday.mp4")
+        const val DISPLAY_NAME = "holiday.mp4"
+        const val INPUT_BYTES = 1024L
+        val JOB_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000009")
+        val H264_SOURCE = InputProbe(
+            videoCodec = "h264",
+            audioCodec = "aac",
+            container = Container.MP4,
+            durationMs = 1_000,
+        )
+    }
+}
+
+/**
+ * A hardware engine that writes something before it fails, and remembers being closed.
+ *
+ * Writing first is the point, exactly as it is for `PartialThenFailingTranscoder`: an engine that
+ * only threw would let a missing `staged.delete()` pass unnoticed.
+ */
+@UnstableApi
+private class RecordingHardwareTranscoder : HardwareTranscoder {
+
+    var attempts = 0
+    var closes = 0
+    var failWith: (() -> Unit)? = null
+
+    override suspend fun transcode(input: Uri, output: File, request: ConversionRequest, onProgress: (Int) -> Unit) {
+        attempts++
+        output.writeBytes(ByteArray(PARTIAL_BYTES))
+        failWith?.invoke()
+    }
+
+    override fun close() {
+        closes++
+    }
+
+    private companion object {
+        const val PARTIAL_BYTES = 2048
+    }
+}
+
+/** The software engine, recording whether the hardware attempt's leftovers were cleared first. */
+private class RecordingSoftwareTranscoder : SoftwareTranscoder {
+
+    var attempts = 0
+    var outputExistedOnEntry = false
+
+    override suspend fun run(
+        request: ConversionRequest,
+        inputPath: String,
+        output: File,
+        durationMs: Long,
+        onProgress: (Int) -> Unit,
+    ) {
+        attempts++
+        outputExistedOnEntry = output.exists()
+        output.writeBytes(ByteArray(OUTPUT_BYTES))
+    }
+
+    private companion object {
+        const val OUTPUT_BYTES = 512
+    }
+}

app/src/test/java/org/libremediaconverter/work/JobSnapshotsTest.kt

@@ -16,6 +16,7 @@ import androidx.work.testing.WorkManagerTestInitHelper
 import androidx.work.workDataOf
 import kotlinx.coroutines.runBlocking
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
 import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Test
@@ -125,6 +126,39 @@ class JobSnapshotsTest {
         assertEquals(newer.absolutePath, Reattachment.choose(snapshots)?.job?.outputPath)
     }
 
+    /**
+     * A job in the tag query that never recorded an output path at all.
+     *
+     * Distinct from the three cases above, which all *have* a path and differ in what it names. A
+     * job still running, or one that finished without writing its result key, carries no path at
+     * all -- and `getWorkInfosByTagFlow` returns it alongside the finished ones, because the tag is
+     * the worker class and every attempt ever enqueued carries it.
+     *
+     * The guard is the `?.` in `path?.let(::File)`. Without it the null goes straight into a `File`
+     * constructor. What this pins is the consequence rather than the null check: such a job must
+     * not be offered as a result, so `Reattachment.choose` has to walk past it to the job that
+     * really produced a file. Choosing it would put a Converted screen in front of the user with a
+     * Save button that has nothing to save.
+     */
+    @Test
+    fun `a job that recorded no output path is not offered as a result`() {
+        val real = stagedFile("real.mp4", bytes = 4096)
+        finishedWithOutput(real)
+        finishedWithNoOutput()
+
+        val snapshots = snapshots()
+
+        assertEquals("both jobs carry the tag, so both come back", 2, snapshots.size)
+        val silent = snapshots.single { it.outputPath == null }
+        assertFalse("no path means no output, not an empty one", silent.outputExists)
+        assertEquals("and no time either, for the same reason", 0L, silent.outputModifiedAt)
+        assertEquals(
+            "the reattachment has to walk past it to the job that really produced a file",
+            real.absolutePath,
+            Reattachment.choose(snapshots)?.job?.outputPath,
+        )
+    }
+
     private fun snapshots(): List<JobSnapshot> = runBlocking {
         workManager.jobSnapshots(
             tag = ConversionWorker::class.java.name,
@@ -152,6 +186,11 @@ class JobSnapshotsTest {
         ).result.get()
     }
 
+    /** A job that carries the tag and no result key -- still running, or finished without one. */
+    private fun finishedWithNoOutput() {
+        workManager.enqueue(OneTimeWorkRequestBuilder<ConversionWorker>().build()).result.get()
+    }
+
     private companion object {
         /** Two fixed moments a day apart, so the ordering is stated rather than raced for. */
         const val OLDER_MS = 1_700_000_000_000L

app/src/test/java/org/libremediaconverter/work/NotificationProgressTextTest.kt

@@ -0,0 +1,89 @@
+package org.libremediaconverter.work
+
+import android.app.Notification
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.installTestWorkManager
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.util.UUID
+
+/**
+ * The two things a progress notification can say, and that they are not the same thing.
+ *
+ * An assertion gap rather than a coverage one, and the distinction is the reason this file exists.
+ * JaCoCo is green on `build`'s `if (indeterminate)`, because `ProgressNotificationTest` drives it
+ * through a real worker -- but that test reads only the notification id and
+ * `Notification.EXTRA_PROGRESS`. **Nothing had ever read the text.** Swapping the two branches, or
+ * collapsing them into one string, passed the entire suite.
+ *
+ * What it costs to get wrong is small and constant: a conversion that has been running for four
+ * minutes still saying "Preparing", or one that has not started reporting yet claiming 0%. Neither
+ * is a crash, and neither would be found by anything else here -- which is exactly the kind of
+ * thing that survives for a long time.
+ *
+ * Nothing else in the suite constructs [ConversionNotifications] directly.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class NotificationProgressTextTest {
+
+    /**
+     * `build` reaches `WorkManager.getInstance` for the Cancel action's PendingIntent, so the
+     * notification cannot be built at all without one. That coupling is why nothing had ever
+     * constructed this class directly and read what it produced.
+     */
+    @Before
+    fun setUp() {
+        installTestWorkManager(RuntimeEnvironment.getApplication(), Data.EMPTY)
+    }
+
+    @Test
+    fun `an indeterminate notification says something different from a measured one`() {
+        val context = RuntimeEnvironment.getApplication()
+        val notifications = ConversionNotifications(context)
+
+        val preparing = notifications.build(JOB_ID, TITLE, percent = 0, indeterminate = true).text()
+        val measured = notifications.build(JOB_ID, TITLE, percent = 42, indeterminate = false).text()
+
+        assertNotEquals(
+            "the two states have to read differently, or the text says nothing at all",
+            preparing,
+            measured,
+        )
+        assertTrue(
+            "a measured notification has to carry its percentage, got \"$measured\"",
+            measured.contains("42"),
+        )
+        assertTrue(
+            "an indeterminate one must not invent one, got \"$preparing\"",
+            !preparing.contains("42") && !preparing.contains("0"),
+        )
+    }
+
+    /**
+     * The title is the caller's, not the builder's -- it is the file the user picked, and it is what
+     * tells two simultaneous conversions apart in the shade.
+     */
+    @Test
+    fun `the notification is titled with the file it is converting`() {
+        val context = RuntimeEnvironment.getApplication()
+
+        val built = ConversionNotifications(context).build(JOB_ID, TITLE, percent = 10)
+
+        assertEquals(TITLE, built.extras.getString(Notification.EXTRA_TITLE))
+    }
+
+    private fun Notification.text(): String = extras.getString(Notification.EXTRA_TEXT).orEmpty()
+
+    private companion object {
+        const val TITLE = "holiday.mp4"
+        val JOB_ID: UUID = UUID.fromString("00000000-0000-4000-8000-00000000000a")
+    }
+}

app/src/test/java/org/libremediaconverter/work/ProgressNotificationTest.kt

@@ -21,8 +21,10 @@ import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.libremediaconverter.convert.ConversionDependencies
+import org.libremediaconverter.convert.HardwareTranscoder
 import org.libremediaconverter.convert.SoftwareTranscoder
 import org.libremediaconverter.convert.installTestWorkManager
+import org.libremediaconverter.model.Container
 import org.libremediaconverter.model.ConversionRequest
 import org.libremediaconverter.model.DeviceCodecs
 import org.libremediaconverter.model.EnginePreference
@@ -129,6 +131,40 @@ class ProgressNotificationTest {
         )
     }
 
+    /**
+     * The same plumbing on the engine most conversions actually use, which had none.
+     *
+     * `ConversionWorker.kt:208-210` is a second `onProgress` lambda at a second call site — the one
+     * handed to `engine.transcode` — and it reported `ci == 0`. Every test above drives the FFmpeg
+     * path; `HardwareFallbackTest` reaches `runMedia3OrFallBack` but its recording transcoder
+     * records the call and never invokes the callback it was given. So the two engines' progress
+     * wiring was one tested and one not, and the untested one is the default: `ConversionRouter`
+     * sends everything it can to Media3.
+     *
+     * `AUTO` with a real H.264 probe, because `FORCE_SOFTWARE` is precisely what keeps the other
+     * tests out of this branch. The probe and the permissive codec profile are what let the router
+     * choose Media3 at all — `InputProbe()` reports `UNPARSEABLE`, which routes straight to FFmpeg.
+     *
+     * Asserted on the *percentage*, not merely on an update having happened: `publishProgress`
+     * takes a display name and a percent, and replacing the percent with a constant compiles.
+     */
+    @Test
+    fun `progress from the hardware engine reaches WorkManager the same way FFmpeg's does`() {
+        ConversionDependencies.probe = { _, _ -> H264_SOURCE }
+        val reporting = ReportingHardwareTranscoder { onProgress -> onProgress(PERCENT) }
+        ConversionDependencies.hardware = { reporting }
+
+        runBlocking { workerReporting(EnginePreference.AUTO) { }.doWork() }
+
+        assertEquals("the job must have gone to the hardware engine", 1, reporting.attempts)
+        val progressUpdates = updater.infos.drop(1)
+        assertEquals("one throttled progress update expected", 1, progressUpdates.size)
+        assertEquals(
+            PERCENT,
+            progressUpdates.single().notification.extras.getInt(Notification.EXTRA_PROGRESS),
+        )
+    }
+
     /**
      * A worker routed to the software engine, whose engine is [report] and a written output.
      *
@@ -137,7 +173,10 @@ class ProgressNotificationTest {
      * bridge, which is native. [report] is handed the worker's own progress callback, and runs with
      * the worker as its receiver so a test can stop it mid-transcode.
      */
-    private fun workerReporting(report: ConversionWorker.((Int) -> Unit) -> Unit): ConversionWorker {
+    private fun workerReporting(
+        enginePreference: EnginePreference = EnginePreference.FORCE_SOFTWARE,
+        report: ConversionWorker.((Int) -> Unit) -> Unit,
+    ): ConversionWorker {
         val worker = TestListenableWorkerBuilder<ConversionWorker>(
             context = app,
             inputData = workDataOf(
@@ -147,7 +186,7 @@ class ProgressNotificationTest {
                 ConversionWorker.KEY_CONTAINER to SPEC.container.name,
                 ConversionWorker.KEY_VIDEO_CODEC to SPEC.videoCodec.name,
                 ConversionWorker.KEY_AUDIO_CODEC to SPEC.audioCodec.name,
-                ConversionWorker.KEY_ENGINE_PREFERENCE to EnginePreference.FORCE_SOFTWARE.name,
+                ConversionWorker.KEY_ENGINE_PREFERENCE to enginePreference.name,
             ),
             runAttemptCount = 0,
         ).setId(JOB_ID)
@@ -171,6 +210,17 @@ class ProgressNotificationTest {
         const val TICKS = 50
         val SPEC = OutputFormat.MP4_H265.spec
         val JOB_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000021")
+
+        /**
+         * A probe the router can actually route. `InputProbe()` reports `UNPARSEABLE`, which
+         * `PERMISSIVE.canDecode` refuses, so every job would reach FFmpeg with no test saying why.
+         */
+        val H264_SOURCE = InputProbe(
+            videoCodec = "h264",
+            audioCodec = "aac",
+            container = Container.MP4,
+            durationMs = 1_000,
+        )
     }
 }
 
@@ -211,3 +261,22 @@ private class ReportingTranscoder(private val report: ((Int) -> Unit) -> Unit) :
         const val OUTPUT_BYTES = 512
     }
 }
+
+/** A hardware engine that reports whatever [report] wants reported, then writes an output. */
+@UnstableApi
+private class ReportingHardwareTranscoder(private val report: ((Int) -> Unit) -> Unit) : HardwareTranscoder {
+
+    var attempts = 0
+
+    override suspend fun transcode(input: Uri, output: File, request: ConversionRequest, onProgress: (Int) -> Unit) {
+        attempts++
+        report(onProgress)
+        output.writeBytes(ByteArray(OUTPUT_BYTES))
+    }
+
+    override fun close() = Unit
+
+    private companion object {
+        const val OUTPUT_BYTES = 16
+    }
+}

app/src/test/java/org/libremediaconverter/work/RefusedJobTest.kt

@@ -0,0 +1,276 @@
+package org.libremediaconverter.work
+
+import android.app.Application
+import android.net.Uri
+import androidx.media3.common.util.UnstableApi
+import androidx.work.Data
+import androidx.work.ListenableWorker
+import androidx.work.testing.TestListenableWorkerBuilder
+import androidx.work.workDataOf
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.ConversionDependencies
+import org.libremediaconverter.convert.OutputPublisher
+import org.libremediaconverter.convert.SoftwareTranscoder
+import org.libremediaconverter.convert.installTestWorkManager
+import org.libremediaconverter.model.AudioCodec
+import org.libremediaconverter.model.ContainerCapabilities
+import org.libremediaconverter.model.ConversionRequest
+import org.libremediaconverter.model.DeviceCodecs
+import org.libremediaconverter.model.EnginePreference
+import org.libremediaconverter.model.InputProbe
+import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.model.OutputSpec
+import org.libremediaconverter.model.Validation
+import org.libremediaconverter.model.VideoCodec
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import java.io.File
+import java.util.UUID
+
+/**
+ * Jobs the worker refuses before it converts anything, and what it says about them.
+ *
+ * Two exits, both cold before this file, and both reachable for the same underlying reason: **a job
+ * does not have to come from the picker.** WorkManager keeps queued and finished work for about a
+ * week, so a downgrade or a rollback hands this build a job enqueued by another one — the premise
+ * `WorkerEnumFallbackTest` and `JobTags` are both written on — and `ConversionWorker.request(...)`
+ * is callable directly.
+ *
+ * What makes these worth their own file rather than another case in an existing one is that both
+ * are about the *message*. A refusal that fails with empty output `Data` renders the UI's generic
+ * "Conversion failed." with nothing else to say, which is the defect shape `DeniedForegroundStartTest`
+ * records from the device pass. Asserting the verdict alone would pass against exactly that.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class RefusedJobTest {
+
+    private lateinit var app: Application
+    private lateinit var publisher: OutputPublisher
+    private lateinit var engine: RefusingTranscoder
+
+    @Before
+    fun setUp() {
+        app = RuntimeEnvironment.getApplication()
+        publisher = AlwaysRoomPublisher(app)
+        engine = RefusingTranscoder()
+        ConversionDependencies.publisher = { publisher }
+        ConversionDependencies.software = { engine }
+        // Neither test is about probing or about this machine's codecs; both would otherwise decide
+        // the outcome for reasons no assertion mentions. See WorkerCancellationTest's setUp.
+        ConversionDependencies.probe = { _, _ -> InputProbe() }
+        ConversionDependencies.deviceCodecs = { DeviceCodecs.PERMISSIVE }
+        installTestWorkManager(app, Data.EMPTY)
+    }
+
+    @After
+    fun tearDown() {
+        ConversionDependencies.reset()
+    }
+
+    @Test
+    fun `a job with no input URI fails with a message rather than a bare failure`() {
+        val result = runBlocking { workerWithout(ConversionWorker.KEY_INPUT_URI).doWork() }
+
+        // `Failure.equals` compares output data, so this pins the message and the verdict together.
+        assertEquals(
+            ListenableWorker.Result.failure(workDataOf(ConversionWorker.KEY_ERROR to "No input file.")),
+            result,
+        )
+    }
+
+    @Test
+    fun `a job with no input URI stages nothing`() {
+        // The URI read is the first thing doWork does -- above the space check, above the staging
+        // name, above the try. A refusal there must not have reserved anything.
+        runBlocking { workerWithout(ConversionWorker.KEY_INPUT_URI).doWork() }
+
+        assertEquals("a job refused for having no input must not stage a file", emptyList<String>(), stagedNames())
+    }
+
+    @Test
+    fun `a spec the picker would never have allowed is refused with the reason`() {
+        // WAV carries PCM and nothing else. The picker cannot produce this combination today, which
+        // is exactly why the worker checks: the job can arrive from a queue written before the
+        // settings changed, or from a direct request(...) call.
+        val expected = ContainerCapabilities.validate(REFUSED_SPEC, InputProbe()) as? Validation.Invalid
+            ?: throw AssertionError("the fixture spec is supposed to be invalid; ContainerCapabilities disagrees")
+
+        val result = runBlocking { worker(REFUSED_SPEC).doWork() }
+
+        assertEquals(
+            ListenableWorker.Result.failure(workDataOf(ConversionWorker.KEY_ERROR to expected.message)),
+            result,
+        )
+    }
+
+    @Test
+    fun `a refused spec never reaches an engine`() {
+        // The half that says it failed *before* converting rather than during. Without this, a
+        // worker that ran the job and then reported the validation message would pass the test
+        // above -- and would have spent the user's battery on a file it was going to refuse.
+        runBlocking { worker(REFUSED_SPEC).doWork() }
+
+        assertTrue("a refused spec must be refused before any engine runs", engine.invocations.isEmpty())
+    }
+
+    @Test
+    fun `a valid spec is not refused`() {
+        // The control. Every assertion above is about a refusal, so without this they would all
+        // still pass against a worker that refused everything.
+        val result = runBlocking { worker(OutputFormat.MP4_H265.spec).doWork() }
+
+        assertEquals(ListenableWorker.Result.success(), stripOutput(result))
+        assertEquals(listOf(OutputFormat.MP4_H265.spec), engine.invocations)
+    }
+
+    // --- the same refusal, on the join side ----------------------------------
+
+    @Test
+    fun `a join of a single file is refused with a message rather than joined`() {
+        // The arm beside it -- a job with no URI array at all -- is covered on the device by
+        // `UnopenableUriTest.aJoinWithNoInputArrayFailsWithAMessage`. This one was covered by
+        // nothing in either source set, which a coverage report cannot say because it cannot see
+        // androidTest: the two arms are adjacent lines and only one of them had a test.
+        //
+        // Reachable for the reason this file's header gives, plus one of its own: `request(...)`
+        // takes a `List<Uri>` and checks nothing about its length, so a single-item join is a
+        // well-formed call, not a corrupted queue entry.
+        val result = runBlocking { joinWorker(INPUT).doWork() }
+
+        assertEquals(
+            ListenableWorker.Result.failure(
+                workDataOf(ConcatWorker.KEY_ERROR to ConcatWorker.TOO_FEW_INPUTS_MESSAGE),
+            ),
+            result,
+        )
+    }
+
+    @Test
+    fun `a join of two files is not refused for its count`() {
+        // The control, and the half that makes the test above bite on the boundary rather than on
+        // the message: without it, `uris.size < 3` passes everything here.
+        //
+        // It refuses the space instead of letting the job run, because the next thing past the
+        // count guard is `ConcatEngine`, which is native -- `NamingPublisher`'s KDoc records that
+        // no JVM test gets past it. A refusal with the *space* message is proof that execution
+        // reached line 57, which is proof it got past line 42, and it costs no engine to say so.
+        val noRoom = NamingPublisher(app).apply { refuseSpace = true }
+        ConversionDependencies.publisher = { noRoom }
+
+        val result = runBlocking { joinWorker(INPUT, SECOND_INPUT).doWork() }
+
+        assertEquals(
+            ListenableWorker.Result.failure(
+                workDataOf(ConcatWorker.KEY_ERROR to "Not enough free space to join these files."),
+            ),
+            result,
+        )
+    }
+
+    /** [ListenableWorker.Result.Success] compares its output data, which these tests do not pin. */
+    private fun stripOutput(result: ListenableWorker.Result): ListenableWorker.Result =
+        if (result is ListenableWorker.Result.Success) ListenableWorker.Result.success() else result
+
+    private fun worker(spec: OutputSpec): ConversionWorker = build(
+        workDataOf(
+            ConversionWorker.KEY_INPUT_URI to INPUT.toString(),
+            ConversionWorker.KEY_DISPLAY_NAME to DISPLAY_NAME,
+            ConversionWorker.KEY_SIZE_BYTES to INPUT_BYTES,
+            ConversionWorker.KEY_CONTAINER to spec.container.name,
+            ConversionWorker.KEY_VIDEO_CODEC to spec.videoCodec.name,
+            ConversionWorker.KEY_AUDIO_CODEC to spec.audioCodec.name,
+            ConversionWorker.KEY_ENGINE_PREFERENCE to EnginePreference.FORCE_SOFTWARE.name,
+        ),
+    )
+
+    /**
+     * The ordinary input `Data`, less one key.
+     *
+     * Built by removal rather than by spelling out a shorter map, so the test cannot drift into
+     * omitting something else as well and passing for a reason it does not name.
+     */
+    private fun workerWithout(key: String): ConversionWorker {
+        val full = OutputFormat.MP4_H265.spec
+        val entries = mapOf(
+            ConversionWorker.KEY_INPUT_URI to INPUT.toString(),
+            ConversionWorker.KEY_DISPLAY_NAME to DISPLAY_NAME,
+            ConversionWorker.KEY_SIZE_BYTES to INPUT_BYTES,
+            ConversionWorker.KEY_CONTAINER to full.container.name,
+            ConversionWorker.KEY_VIDEO_CODEC to full.videoCodec.name,
+            ConversionWorker.KEY_AUDIO_CODEC to full.audioCodec.name,
+            ConversionWorker.KEY_ENGINE_PREFERENCE to EnginePreference.FORCE_SOFTWARE.name,
+        ) - key
+        return build(Data.Builder().putAll(entries).build())
+    }
+
+    private fun build(data: Data): ConversionWorker =
+        TestListenableWorkerBuilder<ConversionWorker>(context = app, inputData = data, runAttemptCount = 0)
+            .setId(JOB_ID)
+            .build()
+
+    /**
+     * A join job carrying [inputs], a declared total, and a format.
+     *
+     * The total is declared so `hasRoomFor` takes its `hasSpaceFor` branch: the other branch is
+     * `hasSpaceForUnknownSize`, which `NamingPublisher` does not override and which would measure
+     * this machine's real disk.
+     */
+    private fun joinWorker(vararg inputs: Uri): ConcatWorker = TestListenableWorkerBuilder<ConcatWorker>(
+        context = app,
+        inputData = workDataOf(
+            ConcatWorker.KEY_INPUT_URIS to inputs.map(Uri::toString).toTypedArray(),
+            ConcatWorker.KEY_TOTAL_BYTES to INPUT_BYTES * inputs.size,
+            ConcatWorker.KEY_FORMAT to OutputFormat.MP4_H264.name,
+        ),
+        runAttemptCount = 0,
+    ).setId(JOB_ID).build()
+
+    private fun stagedNames(): List<String> =
+        publisher.createStagingFile("anything").parentFile?.listFiles().orEmpty().map { it.name }.sorted()
+
+    private companion object {
+        val INPUT: Uri = Uri.parse("file:///tmp/holiday.mp4")
+        const val DISPLAY_NAME = "holiday.mp4"
+        const val INPUT_BYTES = 1024L
+
+        /** A join needs two, and "two" is the boundary the count guard is about. */
+        val SECOND_INPUT: Uri = Uri.parse("file:///tmp/holiday-2.mp4")
+
+        /** WAV carries PCM and nothing else, so AAC in WAV has nowhere to go. */
+        val REFUSED_SPEC = OutputSpec(
+            org.libremediaconverter.model.Container.WAV,
+            VideoCodec.NONE,
+            AudioCodec.AAC,
+        )
+        val JOB_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000005")
+    }
+}
+
+/** An engine that records what it was asked for and writes an output, so a success is a success. */
+private class RefusingTranscoder : SoftwareTranscoder {
+
+    /** Every spec that actually reached an engine. Empty is the assertion for a refused job. */
+    val invocations = mutableListOf<OutputSpec>()
+
+    override suspend fun run(
+        request: ConversionRequest,
+        inputPath: String,
+        output: File,
+        durationMs: Long,
+        onProgress: (Int) -> Unit,
+    ) {
+        invocations += request.spec
+        output.writeBytes(ByteArray(OUTPUT_BYTES))
+    }
+
+    private companion object {
+        const val OUTPUT_BYTES = 512
+    }
+}

app/src/test/java/org/libremediaconverter/work/WorkerCancellationTest.kt

@@ -1,12 +1,16 @@
 package org.libremediaconverter.work
 
 import android.app.Application
+import android.content.Context
 import android.net.Uri
 import androidx.media3.common.util.UnstableApi
 import androidx.work.Data
+import androidx.work.ForegroundInfo
+import androidx.work.ForegroundUpdater
 import androidx.work.ListenableWorker
 import androidx.work.testing.TestListenableWorkerBuilder
 import androidx.work.workDataOf
+import com.google.common.util.concurrent.ListenableFuture
 import kotlinx.coroutines.CancellationException
 import kotlinx.coroutines.runBlocking
 import org.junit.After
@@ -18,6 +22,7 @@ import org.junit.runner.RunWith
 import org.libremediaconverter.convert.ConversionDependencies
 import org.libremediaconverter.convert.OutputPublisher
 import org.libremediaconverter.convert.SoftwareTranscoder
+import org.libremediaconverter.convert.StagingNames
 import org.libremediaconverter.convert.installTestWorkManager
 import org.libremediaconverter.model.ConversionRequest
 import org.libremediaconverter.model.DeviceCodecs
@@ -108,6 +113,54 @@ class WorkerCancellationTest {
         assertEquals("a failed attempt must not leave its partial behind", emptyList<String>(), stagedNames())
     }
 
+    @Test
+    fun `a cancelled join propagates instead of being turned into a Result`() {
+        val thrown = runCatching { runBlocking { concatWorker().doWork() } }.exceptionOrNull()
+
+        assertTrue(
+            "cancellation must leave doWork as cancellation, not as a Result; got $thrown",
+            thrown is CancellationException,
+        )
+    }
+
+    @Test
+    fun `a cancelled join still deletes the partial it had already staged`() {
+        // Written first, so a missing delete cannot pass by asking whether a file nobody wrote is
+        // absent -- the same reason PartialThenFailingTranscoder writes before it throws.
+        concatStagedFile().writeBytes(ByteArray(PARTIAL_STAGED_BYTES))
+
+        runCatching { runBlocking { concatWorker().doWork() } }
+
+        assertEquals("a cancelled join must not leave its partial behind", emptyList<String>(), stagedNames())
+    }
+
+    /**
+     * A join whose foreground start is cancelled rather than denied.
+     *
+     * The conversion twin cancels *inside the engine*, which is the honest shape there because
+     * `ConversionDependencies` has a seam for it. `ConcatWorker` calls `ConcatEngine` directly and
+     * has no such seam -- it is native, and nothing here gets past it -- so the cancellation is
+     * injected at the only other point inside the `try`: `setForeground`. That is not a contrivance.
+     * A job cancelled while WorkManager is promoting it to the foreground is precisely when the
+     * window is open, and what is being tested is the `catch` arm, which cannot tell where in the
+     * `try` the cancellation came from.
+     */
+    private fun concatWorker(): ConcatWorker = TestListenableWorkerBuilder<ConcatWorker>(
+        context = app,
+        inputData = workDataOf(
+            ConcatWorker.KEY_INPUT_URIS to arrayOf(INPUT.toString(), "file:///tmp/second.mp4"),
+            ConcatWorker.KEY_TOTAL_BYTES to INPUT_BYTES,
+            ConcatWorker.KEY_FORMAT to CONCAT_FORMAT.name,
+        ),
+        runAttemptCount = 0,
+    ).setId(CONCAT_ID)
+        .setForegroundUpdater(CancellingForegroundUpdater)
+        .build()
+
+    /** The staging path the join will compute, asked for rather than spelled out here. */
+    private fun concatStagedFile(): File =
+        publisher.createStagingFile(StagingNames.forJob(CONCAT_ID, CONCAT_FORMAT.extension))
+
     /**
      * A worker routed to the software engine, which is [failure] and nothing else.
      *
@@ -142,7 +195,10 @@ class WorkerCancellationTest {
         const val DISPLAY_NAME = "holiday.mp4"
         const val INPUT_BYTES = 1024L
         val SPEC = OutputFormat.MP4_H265.spec
+        val CONCAT_FORMAT = OutputFormat.MP4_H264
+        const val PARTIAL_STAGED_BYTES = 2048
         val JOB_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000003")
+        val CONCAT_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000004")
     }
 }
 
@@ -167,3 +223,19 @@ private class PartialThenFailingTranscoder(private val failure: () -> Nothing) :
         const val PARTIAL_BYTES = 2048
     }
 }
+
+/**
+ * Stands in for a job cancelled while WorkManager is promoting it to the foreground.
+ *
+ * The mechanism `DeniedForegroundStartTest` documents, carrying a different exception:
+ * `WorkForegroundUpdater` propagates whatever the future failed with, and
+ * `ListenableFuture.await()` unwraps the `ExecutionException`, so the worker meets a bare
+ * `CancellationException` exactly where a real cancellation would put one.
+ */
+private object CancellingForegroundUpdater : ForegroundUpdater {
+    override fun setForegroundAsync(
+        context: Context,
+        id: UUID,
+        foregroundInfo: ForegroundInfo,
+    ): ListenableFuture<Void> = FailedFuture(CancellationException("cancelled while going foreground"))
+}

app/src/test/java/org/libremediaconverter/work/WorkerEnumFallbackTest.kt

@@ -22,6 +22,7 @@ import org.libremediaconverter.model.DeviceCodecs
 import org.libremediaconverter.model.EnginePreference
 import org.libremediaconverter.model.InputProbe
 import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.model.OutputSpec
 import org.libremediaconverter.model.QualityTier
 import org.robolectric.RobolectricTestRunner
 import org.robolectric.RuntimeEnvironment
@@ -109,6 +110,50 @@ class WorkerEnumFallbackTest {
         )
     }
 
+    @Test
+    fun `a container this build does not define falls back to the default spec`() {
+        assertFallsBackToDefault(container = "HOLOTAPE")
+    }
+
+    @Test
+    fun `a video codec this build does not define falls back to the default spec`() {
+        assertFallsBackToDefault(video = "H267")
+    }
+
+    @Test
+    fun `an audio codec this build does not define falls back to the default spec`() {
+        assertFallsBackToDefault(audio = "SUPER_AAC")
+    }
+
+    /**
+     * Drives a job whose spec is [NOT_THE_FALLBACK] on every axis but the one named, and asserts the
+     * whole spec came back as [DEFAULT_SPEC].
+     *
+     * **The baseline is the point.** `readSpec` returns the *entire* fallback spec the moment any
+     * one axis fails to resolve, so a test starting from `MP4_H265` -- which is itself the fallback
+     * -- could not tell a worker that read the spec correctly from one that gave up on it. Starting
+     * from MKV/H.264 makes the difference visible on two axes at once.
+     *
+     * Asserting the spec that *ran*, rather than only that a `Result` came back, is the other half:
+     * the defect these three are written for threw out of `doWork` entirely, so "a Result at all"
+     * would pass against a fallback to something arbitrary.
+     */
+    private fun assertFallsBackToDefault(
+        container: String = NOT_THE_FALLBACK.container.name,
+        video: String = NOT_THE_FALLBACK.videoCodec.name,
+        audio: String = NOT_THE_FALLBACK.audioCodec.name,
+    ) {
+        val transcoder = RequestRecordingTranscoder()
+        ConversionDependencies.software = { transcoder }
+
+        val result = runBlocking {
+            conversionWorker(container = container, video = video, audio = audio).doWork()
+        }
+
+        assertEquals(ListenableWorker.Result.success(), stripOutput(result))
+        assertEquals(listOf(DEFAULT_SPEC), transcoder.specs)
+    }
+
     /** [ListenableWorker.Result.Success] compares its output data, which these tests do not pin. */
     private fun stripOutput(result: ListenableWorker.Result): ListenableWorker.Result =
         if (result is ListenableWorker.Result.Success) ListenableWorker.Result.success() else result
@@ -116,15 +161,18 @@ class WorkerEnumFallbackTest {
     private fun conversionWorker(
         quality: String = QualityTier.FAST.name,
         preference: String = EnginePreference.FORCE_SOFTWARE.name,
+        container: String = SPEC.container.name,
+        video: String = SPEC.videoCodec.name,
+        audio: String = SPEC.audioCodec.name,
     ): ConversionWorker = TestListenableWorkerBuilder<ConversionWorker>(
         context = app,
         inputData = workDataOf(
             ConversionWorker.KEY_INPUT_URI to INPUT.toString(),
             ConversionWorker.KEY_DISPLAY_NAME to DISPLAY_NAME,
             ConversionWorker.KEY_SIZE_BYTES to INPUT_BYTES,
-            ConversionWorker.KEY_CONTAINER to SPEC.container.name,
-            ConversionWorker.KEY_VIDEO_CODEC to SPEC.videoCodec.name,
-            ConversionWorker.KEY_AUDIO_CODEC to SPEC.audioCodec.name,
+            ConversionWorker.KEY_CONTAINER to container,
+            ConversionWorker.KEY_VIDEO_CODEC to video,
+            ConversionWorker.KEY_AUDIO_CODEC to audio,
             ConversionWorker.KEY_QUALITY to quality,
             ConversionWorker.KEY_ENGINE_PREFERENCE to preference,
         ),
@@ -146,6 +194,12 @@ class WorkerEnumFallbackTest {
         const val DISPLAY_NAME = "holiday.mp4"
         const val INPUT_BYTES = 1024L
         val SPEC = OutputFormat.MP4_H265.spec
+
+        /** What `readSpec` returns when any axis fails to resolve. */
+        val DEFAULT_SPEC = OutputFormat.MP4_H265.spec
+
+        /** A spec that differs from [DEFAULT_SPEC] on container *and* video codec. See the helper. */
+        val NOT_THE_FALLBACK = OutputFormat.MKV_H264.spec
         val CONVERSION_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000021")
         val CONCAT_ID: UUID = UUID.fromString("00000000-0000-4000-8000-000000000022")
     }
@@ -156,6 +210,9 @@ private class RequestRecordingTranscoder : SoftwareTranscoder {
 
     val qualities = mutableListOf<QualityTier>()
 
+    /** The spec each run was asked for. Which one ran is what the three readSpec tests assert. */
+    val specs = mutableListOf<OutputSpec>()
+
     override suspend fun run(
         request: ConversionRequest,
         inputPath: String,
@@ -164,6 +221,7 @@ private class RequestRecordingTranscoder : SoftwareTranscoder {
         onProgress: (Int) -> Unit,
     ) {
         qualities += request.quality
+        specs += request.spec
         output.writeBytes(ByteArray(OUTPUT_BYTES))
     }
 

app/src/test/java/org/libremediaconverter/work/WorkerStubs.kt

@@ -1,10 +1,14 @@
 package org.libremediaconverter.work
 
 import android.content.Context
+import com.google.common.util.concurrent.ListenableFuture
 import org.libremediaconverter.convert.OutputPublisher
 import org.libremediaconverter.convert.SoftwareTranscoder
 import org.libremediaconverter.model.ConversionRequest
 import java.io.File
+import java.util.concurrent.ExecutionException
+import java.util.concurrent.Executor
+import java.util.concurrent.TimeUnit
 
 /**
  * Scaffolding more than one worker test needs.
@@ -68,3 +72,25 @@ object WritingTranscoder : SoftwareTranscoder {
 
     private const val OUTPUT_BYTES = 512
 }
+
+/**
+ * An already-failed future, written out rather than pulled from a futures library.
+ *
+ * `await()` takes the `isDone` fast path and unwraps the `ExecutionException`, which is what puts
+ * the original exception in front of the worker's `catch` rather than a wrapper. That is the whole
+ * mechanism behind driving a `ForegroundUpdater` to fail: `WorkForegroundUpdater` propagates
+ * whatever the future failed with rather than swallowing it, so `setForeground()` throws exactly
+ * what is handed here.
+ *
+ * Shared because two tests inject two different failures through it -- a denied foreground start
+ * and a cancellation -- and Kotlin will not take two file-private top-level classes of one name in
+ * one package.
+ */
+internal class FailedFuture(private val failure: Throwable) : ListenableFuture<Void> {
+    override fun addListener(listener: Runnable, executor: Executor): Unit = executor.execute(listener)
+    override fun cancel(mayInterruptIfRunning: Boolean): Boolean = false
+    override fun isCancelled(): Boolean = false
+    override fun isDone(): Boolean = true
+    override fun get(): Void = throw ExecutionException(failure)
+    override fun get(timeout: Long, unit: TimeUnit): Void = throw ExecutionException(failure)
+}

app/src/test/resources/robolectric.properties

@@ -10,3 +10,9 @@
 # Set here rather than in a @Config on each class so a later Robolectric test does not have
 # to rediscover it. Remove it once Robolectric ships an android-all jar for 37.
 sdk=36
+
+# Every test gets TestLibreMediaConverterApp, whose only difference from the real one is that the
+# startup sweep runs inline rather than on Dispatchers.IO. Set suite-wide because the race it fixes
+# (#159) is suite-wide: any class that builds an Application leaves a sweep of the shared staging
+# directory in flight for whatever runs next. TestLibreMediaConverterApp explains the choice.
+application=org.libremediaconverter.TestLibreMediaConverterApp

docs/coverage-read-findings.md

@@ -0,0 +1,543 @@
+# Coverage-read findings
+
+**Status:** ten findings, none fixed, none urgent. F1-F4 came from the 2026-08-26 read; F5 was added
+on 2026-08-27 while decomposing #132; **F6-F10 were added on 2026-09-02 from the wave-4 read**. Every
+entry here is a *code* observation — something a test would document rather than repair. The test
+gaps found in the same reads are tickets, not entries here; see [Not covered here](#not-covered-here).
+**Scope:** what a JaCoCo read turned up that writing a test would not fix. This is a survey, not a
+work order. Acting on any entry is a separate decision and would be its own commit.
+**Last verified:** `main` at `54ca2dd`, 2026-09-02. Coverage measured that day with
+`./gradlew :app:jacocoTestReport`: **92.8% line (2183/2352), 81.3% branch (1091/1342)**, against
+**584 JVM tests in 87 classes**, matching what `CLAUDE.md` quotes.
+
+The wave-4 read that produced F6-F10 also produced twelve test tickets, **#192-#203**, plus **#204**
+for four candidates whose cost was not obviously worth paying. The split between them is the same one
+this document has always drawn: a ticket is where a test goes, an entry here is where a test would not
+help.
+
+## Why this document is separate from `defect-audit.md`
+
+`defect-audit.md` is the record of the 2026-08-22 defect sweep: sixteen entries, each a thing that
+is *wrong at runtime*. Nothing here is wrong at runtime today. These are arms that cannot be
+reached, accessors nobody calls, and two KDocs that contradict the code beside them — the category
+`defect-audit.md` calls **latent**, plus several that are not defects at all and are recorded so the
+next coverage read does not re-file them.
+
+They are here rather than in that document because folding them in would inflate a sixteen-entry
+audit whose status metadata has already gone stale once, and because they share a provenance:
+every one fell out of reading a coverage report, and every one is the kind of thing a coverage
+report is *good* at surfacing and a test is bad at fixing. F5 is the clearest case — it was filed
+as a test gap first, and only stopped being one when someone went looking for its callers.
+
+Entry ids are `F1`–`F10` so they cannot be confused with `defect-audit.md`'s `D1`–`D16`.
+
+## How to read the confidence labels
+
+Same vocabulary as `defect-audit.md`, deliberately, so the two read alike:
+
+- **Confirmed by inspection** — the control flow is fully readable and the finding follows from it.
+- **Latent** — not reachable through today's UI, but wrong, and one change away from being live.
+- **No action** — recorded because it looks like a finding and is not.
+
+Nothing below was observed on a device, and nothing below needs to be: every entry is a claim about
+what the code says, checkable by reading it.
+
+---
+
+## F1 — `FFmpegCommandBuilder` emits a Vorbis encoder that `ContainerCapabilities` says does not exist
+
+**Severity: low · Latent · the more interesting reading is a missing feature, not dead code**
+
+```
+app/src/main/java/org/libremediaconverter/ffmpeg/FFmpegCommandBuilder.kt:188
+app/src/main/java/org/libremediaconverter/model/ContainerCapabilities.kt:84-91
+```
+
+`FFmpegCommandBuilder.audioArgs` carries a live Vorbis arm:
+
+```kotlin
+AudioCodec.VORBIS -> listOf("-c:a", "libvorbis", "-q:a", "5")
+```
+
+`ContainerCapabilities` states, immediately above the set that governs it, that no such thing
+exists:
+
+> `/** Vorbis is absent for the same reason: nothing here emits a Vorbis encoder. */`
+> `private val ENCODABLE_AUDIO = setOf(AAC, OPUS, MP3, FLAC, PCM)`
+
+One of those two is wrong. The comment is the one that is wrong as written — something here does
+emit a Vorbis encoder, twelve lines of `FFmpegCommandBuilder`.
+
+### Why the arm is unreachable today
+
+Traced, not assumed:
+
+| step | where | effect |
+|---|---|---|
+| `validate` runs before routing | `ConversionWorker.kt:123` | a spec is checked on every job, however it was enqueued |
+| `validateAudio` refuses non-encodable | `ContainerCapabilities.kt:246-251` | `VORBIS !in ENCODABLE_AUDIO` → `Invalid("This app cannot encode Vorbis audio.")` |
+| the only spec→plan encode path | `CopyPlanner.kt:104` | `AudioPlan.Encode(requested)` — but `requested` cannot be Vorbis by the row above |
+| the fallback encode path | `CopyPlanner.kt:112-115` | draws from `encodableAudio(container)`, itself filtered by `ENCODABLE_AUDIO` |
+
+So `AudioPlan.Encode(VORBIS)` is not constructible through the app, and line 188 is dead.
+
+### The reading that matters more
+
+`CARRIES_AUDIO` lists Vorbis for WebM (`ContainerCapabilities.kt:62`) and OGG (`:67`). Because
+`encodableAudio` filters through `ENCODABLE_AUDIO`, the picker offers **Opus and nothing else** for
+WebM, and Opus/FLAC for OGG. FFmpeg on this device can encode Vorbis — the command is written and
+correct — and the app declines to offer it.
+
+So the honest framing is not "delete a dead arm". It is: **is `ENCODABLE_AUDIO`'s omission of
+Vorbis a deliberate product call, or an accident that has been costing WebM/OGG users a format the
+app already supports?** Nothing in the repo records that decision.
+
+### The precedent for whichever way it goes
+
+`Media3Engine.audioMimeTypeFor` has the *same* Vorbis arm, and handles it exactly right
+(`Media3Engine.kt:221-233`): the KDoc names it dead, says why the arm stays anyway ("deleting a
+right answer out of unreachable code buys nothing"), and points at `Media3EngineMimeTypesTest`,
+which asserts which three of six codecs actually arrive — so the set moving fails a test rather
+than surprising someone.
+
+`FFmpegCommandBuilder`'s arm has none of that. Whatever is decided, the fix is to make the two
+files agree and to say so in one place.
+
+### What a fix has to decide
+
+1. Whether Vorbis belongs in `ENCODABLE_AUDIO`. If yes, this is a feature and needs an e2e test
+   that produces a playable Vorbis file; if no, go to 2.
+2. Correct the `ContainerCapabilities.kt:84` comment, which is false as written, and give the
+   `FFmpegCommandBuilder` arm the treatment `Media3Engine.kt:221-233` already models.
+
+---
+
+## F2 — `ConversionRequest.hardwareEncodeAvailable` is written, read by nothing, and its KDoc describes behaviour that was removed
+
+**Severity: low · Confirmed by inspection**
+
+```
+app/src/main/java/org/libremediaconverter/model/OutputFormat.kt:211-219
+app/src/main/java/org/libremediaconverter/work/ConversionWorker.kt:117
+```
+
+The property is set on every request:
+
+```kotlin
+hardwareEncodeAvailable = devices.canEncode(spec.videoCodec),
+```
+
+`grep -rn 'hardwareEncodeAvailable' app/src/main` returns **that line and nothing else**. No
+production code reads it. Its getter is one of three uncovered methods in `OutputFormat.kt`, which
+is what surfaced it.
+
+Its KDoc (`OutputFormat.kt:211-218`) explains at length what it is for:
+
+> Knowing this lets the Fast tier choose a genuinely fast software preset instead of a mislabelled
+> slow one.
+
+`FFmpegCommandBuilder` no longer does that, and its own test says so —
+`FFmpegCommandBuilderTest.kt:132`, `the encoder choice no longer depends on hardware availability`:
+
+> Once FFmpeg stopped selecting MediaCodec encoders, this flag only affects whether the router sends
+> the job to Media3 at all — not what FFmpeg does.
+
+That second clause is also not true. `ConversionRouter` decides hardware encodability by calling
+`device.canEncode(videoEncode)` itself (`ConversionRouter.kt:153`); it never reads
+`request.hardwareEncodeAvailable`. The flag is computed from the same source the router
+independently consults, carried through the request, and dropped.
+
+This is the shape of open issue **#68** — a KDoc promising a switch that does not exist.
+
+**Not harmful.** It costs one `canEncode` call per job and a field on a data class. It is recorded
+because the KDoc actively misleads: a reader changing the Fast-tier preset logic would look here
+first, and this is not where that decision lives.
+
+### What a fix has to decide
+
+Whether to delete the property (and the constructor parameter, and the four
+`FFmpegCommandBuilderTest` call sites that pass it) or to keep it and rewrite the KDoc to say it is
+vestigial. Deleting is cleaner; the test at `:132` is worth keeping either way, since it pins the
+"FFmpeg does not select MediaCodec encoders" rule that the deletion would otherwise erase.
+
+---
+
+## F3 — `ConversionRequest.videoCodec` and `.audioCodec` have no callers anywhere
+
+**Severity: low · Confirmed by inspection**
+
+```
+app/src/main/java/org/libremediaconverter/model/OutputFormat.kt:222-223
+```
+
+```kotlin
+val container: Container get() = spec.container      // used: FFmpegConcatCommand.kt:42, :80
+val videoCodec: VideoCodec get() = spec.videoCodec   // no callers
+val audioCodec: AudioCodec get() = spec.audioCodec   // no callers
+```
+
+Three delegating accessors on `ConversionRequest`; the first is used twice, the other two are used
+nowhere in `main`, `test` or `androidTest`. Everything that wants those values reads
+`request.spec.videoCodec` or takes the `OutputSpec` directly.
+
+**This is not a test gap and must not be filed as one.** A test asserting
+`request.videoCodec == request.spec.videoCodec` is vacuous by construction — it restates the
+implementation and would pass against any delegation, right or wrong. That is precisely the failure
+mode `CLAUDE.md` records from the mutation review (9 of 46 mutations vacuous, five over completely
+unguarded paths).
+
+The two accessors are either convenience worth keeping for symmetry with `container`, or two lines
+to delete. Deleting them costs nothing and removes two uncovered methods that will otherwise be
+re-found by every future coverage read.
+
+---
+
+## F4 — Two guards are reachable only by direct call, and that is correct
+
+**Severity: n/a · No action**
+
+```
+app/src/main/java/org/libremediaconverter/ffmpeg/FFmpegCommandBuilder.kt:167-168
+app/src/main/java/org/libremediaconverter/model/ConversionRouter.kt:175-176
+```
+
+```kotlin
+VideoCodec.COPY, VideoCodec.NONE -> error("encodeVideo called for $codec, which is not an encode")
+```
+
+```kotlin
+if (plan.video == VideoPlan.Copy && video == null) return false
+if (plan.audio == AudioPlan.Copy && audio == null) return false
+```
+
+Both sit in private functions (`encodeVideo`, `media3CanMux`), and both are unreachable because a
+caller upstream already excluded the case — which each says in its own comment. `ConversionRouter`'s
+is labelled "the second line of defence"; `CopyPlanner` is the first.
+
+**Recorded so the next coverage read does not treat them as gaps.** A second line of defence that
+can be provoked is not a second line of defence. Making these reachable from a test would mean
+widening the functions to `internal`, which buys a test that asserts an `error()` fires when called
+in a way production cannot call it. This is the same judgement issue **#88** reached about
+`getForegroundInfo` and closed on: naming the exemption rather than covering it.
+
+Neither should change unless the upstream guard does. If `CopyPlanner` ever stops resolving `COPY`
+before the builder sees it, `FFmpegCommandBuilder.kt:167` becomes live and wants a test that day.
+
+---
+
+## F5 — `ConversionNotifications.areEnabled()` is never called
+
+**Severity: low · Confirmed by inspection · found while decomposing the test-gap ticket**
+
+```
+app/src/main/java/org/libremediaconverter/work/ConversionNotifications.kt:60-62
+```
+
+```kotlin
+fun areEnabled(): Boolean = context.getSystemService(NotificationManager::class.java)
+    .areNotificationsEnabled()
+    .also { if (!it) Log.i(TAG, "Notifications disabled; progress will not be visible.") }
+```
+
+`grep -rn 'areEnabled' app/src` returns **that declaration and nothing else**. `ConversionNotifications`
+is constructed in both workers (`ConversionWorker.kt:55`, `ConcatWorker.kt:35`) and only `build()` is
+ever called on it.
+
+**This entry exists because it was very nearly filed as a test gap.** Its three lines are cold on the
+JVM, it has a KDoc explaining real user-visible stakes — a foreground service without
+`POST_NOTIFICATIONS` shows only in the Task Manager, so progress silently vanishes — and Robolectric
+can flip that permission in one line. Everything about it reads like a cheap, worthwhile test.
+
+It is not, because **the behaviour the KDoc describes does not happen**. Nothing consults
+`areEnabled()`, so nothing warns, degrades, or logs when notifications are off. A test would assert
+that a function nobody calls returns what the platform told it — green, vacuous, and actively
+misleading, since it would imply the app handles the disabled-notification case. That is the failure
+mode `CLAUDE.md` records from the mutation review, reached from the opposite direction: not a test
+that fails to bite, but a test with nothing to bite.
+
+### What a fix has to decide
+
+Whether the app should act on this at all. The KDoc argues it should — a conversion whose progress is
+invisible is a real complaint, and `ConversionViewModel` or the worker's foreground start is where a
+check would go. If yes, that is a **feature** with a test; if no, delete the method and the KDoc's
+claim with it. What must not happen is a test that makes the current state look handled.
+
+Related: **#16** is open on an adjacent gap — a user who *can* unblock a foreground-denied retry has
+no way to make it happen now.
+
+---
+
+## F6 — Four more arms that cannot be reached, and one KDoc among them that is false
+
+**Severity: low · Confirmed by inspection · F4's family, found in the wave-4 read**
+
+```
+app/src/main/java/org/libremediaconverter/model/ConversionRouter.kt:178-179
+app/src/main/java/org/libremediaconverter/model/ConversionRouter.kt:221
+app/src/main/java/org/libremediaconverter/model/ContainerCapabilities.kt:297
+app/src/main/java/org/libremediaconverter/model/ContainerCapabilities.kt:324, :340
+```
+
+Four sites that a coverage report flags and that no test can reach. Each is recorded with the
+upstream guard that makes it unreachable, because that guard is what would have to change first.
+
+- **`ConversionRouter:178-179`** — the missed branch is `orEmpty()`'s absent-key arm on
+  `MEDIA3_MUXABLE_VIDEO[plan.container]`. `MEDIA3_CONTAINERS` is `setOf(MP4)` and `route()` returns at
+  `:104` for anything else, so `media3CanMux` only ever sees MP4, which both maps key. Same function
+  as F4's second pair, one line below it.
+- **`ConversionRouter:221`** — `DeviceCodecs.PERMISSIVE.canDecode` returning **false** for
+  `InputProbe.UNPARSEABLE`. `PERMISSIVE` has no production caller at all (tests only), and the
+  router's one `canDecode` call at `:128` is already preceded by `:117` returning FFMPEG for
+  `UNPARSEABLE`. **Its KDoc at `:214-217` is false as written:**
+
+  > That exception matters: a device double that claims it can decode an unparseable file would let
+  > the router send a doomed job to Media3.
+
+  It would not — `:117` already caught it. This is F2's shape: a comment that describes a hazard the
+  code upstream has removed. Correcting it is a one-line change and should not be bundled with
+  anything.
+- **`ContainerCapabilities:297`** — `if (container == GIF || container == IMAGE_SEQUENCE) return null`
+  in `repair`. `repair`'s only caller is `suggestions` (`:281`); `validate` returns at `:121` for
+  `isImageOutput` (which is exactly GIF ∥ IMAGE_SEQUENCE) before `suggestions` is reached, and
+  `firstContainerHolding` filters on `CARRIES_VIDEO`, which is empty for both.
+- **`ContainerCapabilities:324` and `:340`** — the `else ->` arms themselves are exercised; what is
+  missed is the elvis tail, `firstOrNull() ?: VideoCodec.NONE` / `?: AudioCodec.NONE`. Reaching it
+  needs a container with no encodable codec on that axis. Audio-only containers return early at
+  `:307`, and the only containers with an empty audio set are GIF and IMAGE_SEQUENCE, excluded at
+  `:297` above.
+
+**Recorded so the next read does not re-file them.** F4's rule applies unchanged: a second line of
+defence that can be provoked is not a second line of defence, and widening a private function to make
+one reachable buys a test that asserts a fallback fires when called in a way production cannot call
+it.
+
+---
+
+## F7 — `probeWithExtractor`'s catch is unreachable for the same measured reason `probeForConcat`'s is
+
+**Severity: n/a · No action · completes a measurement already on record**
+
+```
+app/src/main/java/org/libremediaconverter/convert/MediaProbe.kt:180-182
+```
+
+```kotlin
+} catch (e: Exception) {
+    Log.i(TAG, "Platform extractor could not read $uri.", e)
+    null
+}
+```
+
+`CLAUDE.md` records the measurement for the *other* extractor site: Robolectric's `MediaExtractor`
+never throws from `setDataSource`, checked across an unregistered `content://` authority, a missing
+`file://`, a file of garbage bytes and an `http://` URL — all four returned with `trackCount = 0`.
+
+`probeWithExtractor` calls the same overload, three lines apart in the same file, and the measurement
+covers it identically. It was simply not written down for this site, so a future read would re-derive
+it. It stays device-only, alongside `probeForConcat`'s.
+
+**Two neighbouring line counts are artifacts of this, not separate gaps.** `MediaProbe:184` and
+`:331` each report 27 missed instructions and are the `finally` block's synthetic exception-path copy
+— JaCoCo duplicates a `finally` per exit path, and the exceptional one is unreachable for the reason
+above. Do not read them as a third and fourth site.
+
+---
+
+## F8 — Three more dead members, and six unused defaults
+
+**Severity: low · Confirmed by inspection · F3's family**
+
+```
+app/src/main/java/org/libremediaconverter/model/CopyPlanner.kt:28          ConversionPlan.hasVideo
+app/src/main/java/org/libremediaconverter/codec/AndroidDeviceCodecs.kt:39  hardwareEncoders()
+app/src/main/java/org/libremediaconverter/ffmpeg/ConcatEngine.kt:30        Result.output
+app/src/main/java/org/libremediaconverter/convert/Transcoders.kt:28, :29, :40, :61
+app/src/main/java/org/libremediaconverter/work/Reattachment.kt:28, :30
+```
+
+- **`ConversionPlan.hasVideo`** — zero callers in `main`, `test` or `androidTest`. Every `hasVideo`
+  hit in the tree is `InputProbe.hasVideo`, `OutputSpec.hasVideo` or `Container.extensionFor(hasVideo)`,
+  which are different properties on different types. A test asserting
+  `plan.hasVideo == (plan.video != VideoPlan.Drop)` is vacuous by construction.
+- **`AndroidDeviceCodecs.hardwareEncoders()`** — its only caller is `RealMediaBenchmark`, in
+  `androidTest`. Production reads capabilities through `DeviceCodecs`, never the raw set.
+- **`ConcatEngine.Result.output`** — `ConcatWorker` reads `result.strategy` and uses the `staged`
+  file it passed in, never `.output`.
+- **`Transcoders.kt`'s default arguments** — `request` and `onProgress` on
+  `HardwareTranscoder.transcode` (`:28`, `:29`), `onProgress` on `SoftwareTranscoder.run` (`:40`),
+  and `format` on `ConcatJoiner.join` (`:61`). All three production call sites
+  (`ConversionWorker.kt:208`, `:234`, `ConcatWorker.kt:79`) pass every argument, so the synthesised
+  `$default` bridges and `$DefaultImpls` copies are never entered. The
+  `request: ConversionRequest = ConversionRequest(OutputFormat.MP4_H265.spec)` default is the one
+  worth a second look: nothing anywhere omits it, so an interface silently promises H.265 to a
+  caller that does not exist.
+- **`JobSnapshot`'s `outputModifiedAt` and `tags` defaults** — `JobSnapshots.kt:32-42` passes all
+  seven fields, so the synthesised `$default` constructor (20 missed instructions at
+  `Reattachment.kt:14`) is never entered.
+
+**Not a test gap, for F3's reason.** Delete them, or keep them and know they are unused; either is a
+decision, and a test restating the compiler is not.
+
+---
+
+## F9 — Both workers' `getForegroundInfo` overrides are dead, and this is why
+
+**Severity: n/a · No action · sharpens #88 rather than reopening it**
+
+```
+app/src/main/java/org/libremediaconverter/work/ConversionWorker.kt:342-346
+app/src/main/java/org/libremediaconverter/work/ConcatWorker.kt:132-136
+```
+
+**#88 already closed on these**, after reading both and finding no decision worth a seam — the
+correct call, and it stands. What #88 did not name is the reason they are cold in the first place,
+which is stronger than "the JVM cannot reach them":
+
+WorkManager calls `getForegroundInfoAsync()` **only for expedited work**. `ConversionWorker`'s own
+KDoc says expedited is deliberately not used, and `grep -rn 'setExpedited\|OutOfQuotaPolicy' app/src`
+returns nothing. So both overrides are dead in production today, not merely untested — a test would
+assert the shape of something nothing invokes.
+
+They are still correct to keep: `ForegroundInfo` is required by the `CoroutineWorker` contract and
+`setForeground` is called explicitly elsewhere. **What would reopen this** is the same trigger #88
+named — a `getForegroundInfo` that starts branching — plus one more: the day anything calls
+`setExpedited`.
+
+---
+
+## F10 — Three arms that are reachable, uncovered, and cannot be made to bite
+
+**Severity: n/a · No action · the shape a coverage number cannot distinguish**
+
+```
+app/src/main/java/org/libremediaconverter/convert/ConversionViewModel.kt:550, :553
+app/src/main/java/org/libremediaconverter/join/JoinViewModel.kt:349, :352, :278
+```
+
+F4 and F6 hold arms that cannot be *reached*. These can — and a test written against them would still
+pass under the mutation that ought to redden it, which is the harder case to spot and the more
+expensive one to discover halfway through writing the test.
+
+- **`observer?.cancel()`'s non-null arm** (`ConversionViewModel:550`, `JoinViewModel:349`). Reachable
+  by calling `convert()` twice. But `ScreenOwnership`'s token is what actually blocks the superseded
+  write — the ViewModel's own KDoc at `reset()` says the cancel is "a request honoured at the next
+  suspension point" and "the claim is what actually stops that write". Delete `observer?.cancel()`
+  and the suite stays green, correctly.
+- **`if (info == null) return@collect`** (`ConversionViewModel:553`, `JoinViewModel:352`). Reachable
+  through `pruneWork()`. But when the null arrives the state is already terminal, so removing the
+  guard crashes the collector and **leaves the state unchanged** — a state assertion is green under
+  the mutation. The only observable is an escaped coroutine exception, which the ViewModel's own KDoc
+  documents as unreliable on the JVM: kotlinx-coroutines-test's process-wide collector hands it to
+  whichever `runTest` starts next.
+- **`JoinViewModel:278`'s `Ambiguous` arm.** Looks like the twin of `ReattachGuardsTest`'s "a result
+  two jobs both claim", and is not. An `Ambiguous` requires a shared `outputPath`, so it can only be a
+  *finished* job — which maps to `Joined`, a state that reads nothing from `inputs`. **The Convert-side
+  twin does bite**, because `displayNameOf(tags)` reaches the file card; the asymmetry is the point.
+
+**Recorded because each of these was picked up as a candidate and put down again.** The wave-4 read
+lost time to all three before the mutation test was run in the head rather than the editor, which is
+the cheaper order.
+
+---
+
+## Summary
+
+| ID | Finding | Severity | Evidence | Action |
+|---|---|---|---|---|
+| F1 | `FFmpegCommandBuilder` emits a Vorbis encoder `ContainerCapabilities` says does not exist | low | confirmed by inspection; unreachability traced through four call sites | **decide**: feature or dead arm — the comment is false either way |
+| F2 | `hardwareEncodeAvailable` written, never read; KDoc describes removed behaviour | low | confirmed by inspection; `FFmpegCommandBuilderTest:132` corroborates | **decide**: delete or mark vestigial |
+| F3 | `ConversionRequest.videoCodec` / `.audioCodec` have no callers | low | confirmed by inspection | delete, or keep for symmetry — **not** a test gap |
+| F4 | Two private guards reachable only by direct call | n/a | confirmed by inspection | **no action** — named exemption, per #88 |
+| F5 | `ConversionNotifications.areEnabled()` is never called | low | confirmed by inspection; grep returns the declaration only | **decide**: act on it or delete it — **not** a test gap |
+| F6 | Four more unreachable arms; `ConversionRouter:214-217`'s KDoc is false | low | confirmed by inspection; each traced to its upstream guard | **no action**, except the one-line KDoc fix |
+| F7 | `probeWithExtractor`'s catch is unreachable, as `probeForConcat`'s is | n/a | measured across four URI shapes (recorded in `CLAUDE.md`) | **no action** — device-only, now written down for both sites |
+| F8 | Three more dead members and six unused defaults | low | confirmed by inspection; grep per member | delete or keep knowingly — **not** a test gap |
+| F9 | Both `getForegroundInfo` overrides are dead: expedited work is never used | n/a | confirmed by inspection; `grep setExpedited` returns nothing | **no action** — sharpens #88's close |
+| F10 | Three reachable arms where no mutation bites | n/a | confirmed by inspection; each mutation traced to its masking guard | **no action** — recorded to stop the next read re-picking them |
+
+Order, if these are acted on: **F1 and F5 first, separately.** They are the two with a possible
+user-visible answer — a format the app can produce and does not offer, and a warning the app
+documents and does not give — and either answer changes what the tidying should look like. F2, F3 and
+F8 are tidying and belong in one commit with each other, not with F1 or F5. F6's KDoc correction is a
+third kind: one line, no decision, and it should not wait on the tidying. F4, F7, F9 and F10 are
+finished by being written down.
+
+**Six of the ten are now "no action" or "not a test gap", and that is the useful shape.** By wave 4
+the report's remaining red is mostly this: arms nothing can reach, members nothing calls, and arms a
+test can reach but not pin. A coverage number cannot tell any of them from a real gap, which is why
+this document exists and why it grows faster than the percentage moves.
+
+**F1 and F5 share a shape worth naming:** both are places where a comment describes behaviour the
+code does not have, and in both the tempting fix (delete the dead arm, test the dead method) would
+freeze the wrong answer in place. The decision comes first.
+
+## Not covered here
+
+**The test gaps from the same read.** Seven JVM-side gaps (**#132**) and three seam questions
+(**#133**) came out of this coverage read and are tracked there, because they are work rather than
+observations. This document holds only what a test would not fix. #133 also records why
+`AndroidDeviceCodecs.probe()` was considered and left out **through `ShadowMediaCodecList`**, so that
+spike is not run a third time.
+
+**Updated 2026-09-02:** #194 proposes reaching the same code through a *pure seam* instead, which is a
+different mechanism and one #133 did not evaluate — the builder objection it turns on (no
+`setIsAlias`, no `setCanonicalName`) does not apply to a function taking its own entry type. #133's
+close stands for the shadow; it is not a close on the seam. #194 also carries the reason the seam is
+worth cutting at all, which is not coverage: the `runCatching` fallback logs "assuming permissive" and
+returns empty sets, which makes `canEncode` and `canDecode` answer *no* for everything.
+
+**`ConversionForegroundType.current()`**, which looked like the sharpest gap in the read and is not.
+Its API 33 and 34 arms are cold on the JVM, but issue **#88** already established that the class is
+covered by `ConversionWorkerTest.foregroundTypeMatchesTheRunningApiLevel` across the CI matrix, and
+that its 0% is the `testDebugUnitTest`-only measurement boundary.
+
+The premise worth re-checking was whether the 33/34 legs still complete, given #122's wedge.
+**They mostly do, and #122 is not resolved** — this entry said "they do" on first writing, from a
+single green run, and the PR carrying this very document proved that wrong:
+
+| run | API 33 leg | shape |
+|---|---|---|
+| `32933262839` (#127) | success, 7m16s | `expected 60, received 60, failed 0, completed cleanly: yes` |
+| `33033036857` (PR #131, docs-only) | **failure, 23m08s** | `expected 60, received 60, failed unknown, wedged: yes — gradle killed after 1200s` |
+
+Five of the last six completed API 33 legs passed in about seven minutes, so the wedge is
+intermittent rather than systematic. **What it costs is the verdict, not the execution**: `received:
+60` on the wedged run means all sixty tests still reported, so the API 33 regime *was* exercised —
+but `failed:` reads `unknown`, so that leg could not have told anyone if it had broken.
+
+That is why this stays a note and not a ticket, and also why it is not simply deleted: #88's
+reasoning holds, but the leg it rests on cannot be relied on to report a failure. A
+`@Config(sdk = 33)` / `@Config(sdk = 34)` JVM test would pin all three arms deterministically in one
+run for about three lines. Small, and worth doing the next time this file is opened — but it is
+insurance against a flaky leg, not the uncovered behaviour it first looked like.
+
+**The Compose screens' branch coverage.** `ConverterScreenKt` reports 110 of 200 branches missed and
+`JoinScreenKt` 60 of 82, which looks alarming and is not a signal: the Compose compiler synthesises
+`$changed`/`$dirty` recomposition-skip tests that JaCoCo counts as branches. The line figures are
+the real ones — **34 of 383** and **20 of 143** missed — and the screens are among the
+better-covered files in the repo, which is what #52, #57 and #61 were for. **Do not chase the
+branch number here.** If a future read wants a screen metric, use lines.
+
+**Updated 2026-09-02: the same codegen inflates the *instruction* count, which wave 3's filter did
+not allow for.** Wave 3 selected candidates on `mi > 0` — at least one missed instruction — which was
+right to prefer over a bare branch count and is still wrong on these files. `JoinScreen.kt:222` reads
+`mi=10` and looks uncovered; it also reads `ci=38`, and `JoinStateAffordancesTest` already clicks that
+Save button and asserts `save:joined.mp4`. Every `onClick` lambda body flagged this way turned out to
+be covered at method level, the missed instructions being the recomposition-skip path again.
+
+Use `ci == 0` — the line never executed, which is JaCoCo's own missed-line definition — and pair it
+with a method-level `ci > 0 && mb > 0` pass for covered methods with cold arms. Neither filter alone
+is enough: `ConversionViewModel.cancel()` misses no line at all, yet its non-null arm had never been
+entered in 584 tests (#192). `CLAUDE.md`'s coverage entry carries the same correction.
+
+**Also codegen, also not gaps**, recorded once so they are not re-derived: the synthetic
+`NoWhenBranchMatchedException` closing an exhaustive `when` (`ConverterScreen:399`, `:686`,
+`JoinScreen:278`, `MainActivity:160`); the inner `is Idle -> Unit` arms at `ConverterScreen:253-254`
+and `JoinScreen:158-159`, which are structurally unreachable because the outer `when` already routed
+`Idle`; and the closing brace of a `launch` block whose `collect` never terminates
+(`ConversionViewModel:578`, `JoinViewModel:371`).
+
+**Anything requiring a device.** `MediaProbe`'s FFprobe half (`MediaProbe.kt:151, 156-158, 173-188`)
+and `FFmpegEngine` in full report 0% on the JVM and are covered by `androidTest`. JaCoCo measures
+`testDebugUnitTest` only; their zeroes are a boundary, as #84, #85, #86 and #88 each recorded
+before this.

docs/e2e-read-findings.md

@@ -0,0 +1,327 @@
+# E2E-read findings
+
+**Status:** six findings, none fixed, none urgent — **plus one confirmed vacuous test, which is a
+ticket rather than an entry here** (see [Not covered here](#not-covered-here)). `E1`–`E6` came from
+the 2026-09-05 read of the instrumented suite. Every entry here is a *test-suite* observation —
+something a new test would not fix, because the test already exists and the problem is what it
+claims rather than what it runs.
+**Scope:** what reading all 60 instrumented tests turned up that writing a 61st would not fix.
+**Last verified:** `main` at `4b02294`, 2026-09-05. **60 `@Test` methods in 12 classes**, three
+carrying `@FailsOnEmulatorApi37`, gating API 37 leg 57.
+
+## Why this document exists, and why it is separate from the other two
+
+`docs/coverage-read-findings.md` (`F1`–`F10`) came from reading a **JaCoCo report**, and JaCoCo
+measures `testDebugUnitTest` only. So four waves of coverage work have been shaped by a number that
+**cannot see `app/src/androidTest` at all**. The instrumented suite has never had the equivalent
+read: nothing has asked what those 60 tests actually pin, only that they are green.
+
+That is the gap this read is in. It is a **triage, not a test push** — the same shape as wave 4's
+read, which "moved no number at all, and that is its result".
+
+`docs/defect-audit.md` (`D1`–`D16`) is the record of things *wrong at runtime*. Nothing here is
+wrong at runtime. These are tests whose names, KDoc or reputation overstate what they execute.
+
+Entry ids are `E1`–`E6` so they cannot be confused with `F1`–`F10` or `D1`–`D16`.
+
+## How to read the confidence labels
+
+Same vocabulary as the other two documents, deliberately:
+
+- **Confirmed by inspection** — the control flow is fully readable and the finding follows from it.
+- **Confirmed by measurement** — observed in a CI artifact, with the run id recorded.
+- **No action** — recorded because it looks like a finding and is not.
+
+## The method, and the one filter that found everything
+
+A coverage number is useless here by construction, so the read used a different question, applied
+to every one of the 60 tests:
+
+> **If the behaviour this test is named for stopped working, would it go red?**
+
+Three answers, and only the third is a gap:
+
+- **yes** — the test bites. Most of the suite.
+- **no, and that is deliberate and written down** — `RealMediaBenchmark` asserts nothing on purpose
+  (E2); `transcodesH264ToH265AndReportsProgress` declines to assert progress for a stated reason
+  (E3). These are entries here, not tickets.
+- **no, and nothing says so** — the gap. One test, and it is the most important one in the suite.
+
+**The reusable part is the second filter**, because "does it assert something?" would have cleared
+the vacuous test — it asserts two things. What it does not do is *reach the code it names*:
+
+> **Does the test's own premise hold on the machine that runs it?**
+
+`HardwareFallbackTest` asserts `SUCCEEDED` and a non-empty output, and both are true of a
+conversion that never went near the path it exists to prove (**#223**). See
+[Not covered here](#not-covered-here); it is filed rather than recorded here because a test fixes it.
+
+---
+
+## E1 — `RemuxTest`'s class KDoc argues for engine assertions three of its tests do not make, and they are right not to
+
+**Severity: low · Confirmed by inspection · the KDoc is what is wrong, not the tests**
+
+```
+app/src/androidTest/java/org/libremediaconverter/convert/RemuxTest.kt:31-42
+```
+
+The class KDoc is headed **"Why these assert the engine, not just the file"** and makes a specific
+argument:
+
+> A remux routed to FFmpeg produces a perfectly correct file — `-c copy` moves the same samples
+> into the same container. So an output-only assertion passes whether the hardware transmux path
+> ran or never executed at all […] which makes "silently always FFmpeg" the most likely way for
+> this feature to regress.
+
+Five of its seven tests run a conversion. **Three assert no engine at all:**
+
+| test | output container | asserts engine? |
+|---|---|---|
+| `mkvToMp4RemuxesOnHardware` | MP4 | **yes** — `MEDIA3` |
+| `mp4ToMkvRemuxesOnFFmpeg` | MKV | **yes** — `FFMPEG` |
+| `webmToMkvKeepsVp9WithoutReencoding` | MKV | no |
+| `audioOnlySourceRemuxesIntoMka` | MKV (`.mka`) | no |
+| `mp4ToMpegTsAndAviProduceTheirOwnContainers` | MPEG-TS, then AVI | **TS only**; the AVI half does not |
+
+### Why this is not a gap
+
+`ConversionRouter.MEDIA3_CONTAINERS = setOf(Container.MP4)` (`ConversionRouter.kt:37`), and every
+one of the three produces MKV or AVI. **They can only ever be FFmpeg**, so the regression the KDoc
+names — "silently always FFmpeg" — is not a thing that can happen to them. The two tests where the
+hardware path is genuinely at risk are exactly the two that assert it.
+
+An engine assertion on the other three would be near-tautological given today's router. It would
+catch one thing: somebody adding MKV or AVI to `MEDIA3_CONTAINERS` without a muxer to match — which
+is what `Media3MuxersTest` is for, on the JVM, where it does not need a device.
+
+### Why it is recorded rather than dropped
+
+**This was the strongest-looking candidate of the whole read and it dissolved on tracing**, which
+is the same shape as `F5` in the coverage document (filed as a test gap, and only stopped being one
+when someone went looking for its callers). Recorded so the next read does not re-file it.
+
+**The fix is one line of KDoc**, not three tests: the class asserts the engine *where the engine is
+in doubt*, which is a better rule than the one it currently states.
+
+---
+
+## E2 — three of the 60 instrumented tests assert nothing, and two of them never run
+
+**Severity: n/a · No action — deliberate, documented, and load-bearing as documentation**
+
+```
+app/src/androidTest/java/org/libremediaconverter/bench/RealMediaBenchmark.kt:25-53
+```
+
+`reportDeviceEncoderCapabilities` logs and asserts nothing. `hardwareVersusSoftwareOnRealVideo` and
+`av1InputRoutesAccordingToDeviceDecodeSupport` are `assumeTrue`-guarded on media that is **not
+committed** and must be staged by hand into the app's internal `filesDir`, so they skip in every
+automated run — they are the "2 skipped" every green leg reports, and `docs/local-emulator.md:305`
+says so.
+
+The class KDoc is unambiguous: *"This is a benchmark, not part of the automated suite […] Not a
+correctness test — the assertions are deliberately loose."*
+
+**No action.** Recorded for one reason: **the suite's headline number is 60, and three of those 60
+are not tests.** Any future statement of the form "60 instrumented tests cover X" is off by three,
+and two of the three have never executed on CI at all.
+
+**It is the opposite of E-nothing, though** — `reportDeviceEncoderCapabilities` runs on every leg
+and logs `BENCH can-encode:`, and **that log line is what confirmed the vacuous test this read
+found** (**#223**). An assertion-free test that prints the machine's capabilities turned out to be
+the only oracle in the suite. See [Not covered here](#not-covered-here).
+
+---
+
+## E3 — `transcodesH264ToH265AndReportsProgress` does not assert that progress was reported
+
+**Severity: low · No action on the test; the name is the inaccurate part**
+
+```
+app/src/androidTest/java/org/libremediaconverter/convert/Media3EngineTest.kt:73, :90-93
+```
+
+```kotlin
+// Deliberately NOT asserting that progress fired. Polling is on a 250 ms tick,
+// and a 3 s 320x240 clip can finish inside one tick on fast hardware, which
+// would make the assertion fail intermittently for no real defect.
+seen.forEach { assertTrue("progress out of range: $it", it in 0..100) }
+```
+
+`seen` is empty-safe: `forEach` on an empty list asserts nothing, so replacing `onProgress` with a
+no-op reddens nothing here. The reasoning is sound and the alternative really is a flaky test.
+
+**No action on the body.** The name says `AndReportsProgress` and the body says it does not check
+that, which is the `probeForConcat` shape from `CLAUDE.md` — *a passing test with a wrong
+explanation is its own failure mode* — in its mildest form, since here the KDoc immediately corrects
+the name.
+
+**Contrast the FFmpeg side, which is a real gap and is filed as #229**: `FFmpegEngine`'s percentage
+arithmetic is executed by every FFmpeg test and observed by none, because every call site omits
+`onProgress` entirely. Media3's is unasserted; FFmpeg's is unobserved. Only the second is a ticket.
+
+---
+
+## E4 — the marker's KDoc says removing it grows the gating leg by two; three tests carry it
+
+**Severity: low · Confirmed by inspection · one line**
+
+```
+app/src/androidTest/java/org/libremediaconverter/FailsOnEmulatorApi37.kt:20
+```
+
+> Delete the annotation from the tests, and the advisory job goes empty and the gating one grows by
+> **two**.
+
+Three tests carry it — `Media3EngineTest:72`, `Media3EngineTest:135`, `SafPickerRoundTripTest:320` —
+and `FAILS_ON_EMULATOR_API37_BASELINE = 3` eleven lines further down the same file, where the count
+is machine-checked by `.github/scripts/e2e-report-shape.sh`.
+
+The third marker was added when the SAF rotation test was excluded; the sentence was not updated
+with it. **Everything that is checked is consistent at three**; only the prose says two, which is
+exactly why it drifted — and a good argument for the baseline const being a const.
+
+---
+
+## E5 — `coverage-read-findings.md`'s F7 calls covered code uncovered
+
+**Severity: low · Confirmed by inspection · half of F7 is stale**
+
+F7 says `probeWithExtractor`'s catch (`MediaProbe.kt:180-182`) is unreachable on Robolectric and
+"stays device-only", measured across four URI shapes. **The unreachability claim is correct and
+stands.** The implication readers take from it — that nothing exercises it — does not:
+
+```
+app/src/androidTest/java/org/libremediaconverter/convert/RemuxTest.kt:111
+```
+
+`probeDistinguishesAudioFromImagesFromRubbish` feeds it a file of random bytes and asserts
+`InputKind.UNPARSEABLE`, on a device, on every gating leg.
+
+**"Device-only" holds; "uncovered" does not** — and the difference matters, because F7 is one of the
+six entries that document calls "no action", on the grounds that a test would not help. A test
+already exists. The entry should say so.
+
+**This is the failure mode the split between the two documents was meant to prevent**, and it caught
+this repo out: a JaCoCo-derived document cannot see `androidTest`, so it will keep re-deriving
+"uncovered" for anything the instrumented suite covers. That is a structural reason for this
+document to exist, not a one-off correction.
+
+---
+
+## E6 — the suite's one device-capability assertion derives its expectation from the call it is testing
+
+**Severity: low · Confirmed by inspection · no independent oracle exists**
+
+```
+app/src/androidTest/java/org/libremediaconverter/work/ConversionWorkerTest.kt:151-152
+```
+
+```kotlin
+val hasHardwareHevc = AndroidDeviceCodecs.get().canEncode(VideoCodec.H265)
+```
+
+and then the expectation is `if (hasHardwareHevc) MEDIA3 else FFMPEG`. The test asks
+`AndroidDeviceCodecs` what to expect and then checks that the router agreed with
+`AndroidDeviceCodecs`. **If the whole enumeration returned empty, this would still pass** — and
+empty is precisely what the `runCatching` fallback returns (the reason `#194` was worth cutting;
+it logs "assuming permissive" while making `canEncode` answer *no* for everything).
+
+Its KDoc defends the choice, and the defence is good:
+
+> Asserting MEDIA3 unconditionally tests the test machine, not the router.
+
+That is true, and there is no third source of truth on a device: `MediaCodecList` is what
+`AndroidDeviceCodecs` reads, so any oracle built from it is the same oracle.
+
+**No action, but read it with #223.** It is the same missing oracle that makes the
+vacuous-test fix a judgement call rather than a one-liner — you cannot assert "this device has
+hardware HEVC" from inside the suite without asking the class under test. The honest options are a
+visible skip or a red test, and that decision is the ticket's.
+
+---
+
+## Summary
+
+| ID | Finding | Severity | Evidence | Action |
+|---|---|---|---|---|
+| E1 | `RemuxTest`'s KDoc claims engine assertions three of its tests correctly omit | low | confirmed by inspection; traced through `MEDIA3_CONTAINERS` | **one line of KDoc** — the tests are right |
+| E2 | Three of the 60 instrumented tests assert nothing; two never run | n/a | confirmed by inspection; `docs/local-emulator.md:305` | **no action** — deliberate; but 60 ≠ 60 |
+| E3 | `…AndReportsProgress` does not assert progress fired | low | confirmed by inspection; reason inline | **no action** — the name overstates, the KDoc corrects it |
+| E4 | The API 37 marker's KDoc says "two"; three tests carry it | low | confirmed by inspection; baseline const says 3 | **fix the sentence** |
+| E5 | `coverage-read-findings.md` F7's "uncovered" half is stale | low | confirmed by inspection; `RemuxTest.kt:111` drives it | **amend F7** — "device-only" stands, "uncovered" does not |
+| E6 | The device-capability assertion asks the class under test what to expect | low | confirmed by inspection; no third oracle exists on a device | **no action** — read with **#223** |
+
+**Five of the six are prose, not code**, and that is the shape of this read. The instrumented suite
+is in good condition: 57 of its 60 tests bite, the fixtures are committed with their generation
+recipes, and the one class that asserts nothing says so in its first line. What this read found is
+that **the suite's self-description has drifted from the suite** in five small places and one large
+one.
+
+**The large one is not in this table**, because a test fixes it: **#223**.
+
+## Not covered here
+
+**The vacuous test.** `HardwareFallbackTest.aFileMedia3CannotDecodeStillConvertsViaFfmpeg` passes on
+every CI leg without ever entering the fallback it exists to prove. It is **#223**, not an entry
+here, because a test fixes it — and it is the reason this read happened rather than an aside from it.
+
+Measured, not inferred, on run **`34004304566`** (all legs green), from each leg's own
+`e2e-diagnostics-api*` logcat:
+
+```
+I/AndroidDeviceCodecs: Hardware video encoders: []
+I/RealMediaBenchmark: BENCH can-encode: COPY=true, H264=false, H265=false, VP9=false, VP8=false, AV1=false
+I/ConversionWorker: Routing sample_h264_444.mp4 -> OutputSpec(container=MP4, videoCodec=H265,
+                    audioCodec=AAC) via FFMPEG (NO_HARDWARE_ENCODER)
+```
+
+Identical on **API 33, 34, 35 and 37**. (API 36's logcat artifact on that run is truncated to 838 KB
+and carries no test output at all, so it is unread rather than different.) The job is routed
+**straight to FFmpeg before Media3 is attempted**, the `catch` in `runMedia3OrFallBack` is never
+entered, and the test's two assertions — `SUCCEEDED`, output non-empty — are true anyway. It ran in
+448 ms.
+
+**The repository already knew.** `ForcedFailureTest.hardwareFailureFallsBackToSoftware`, in the same
+package, pins `ConversionDependencies.deviceCodecs = { DeviceCodecs.PERMISSIVE }` and says why:
+
+> most emulators expose no hardware video encoder at all -- so the router would legitimately send
+> the job straight to FFmpeg and the hardware path would never be attempted. Without this the test
+> passes on a Pixel and fails on every emulator, which says nothing about the code under test.
+
+`ConversionWorkerTest.routesAFastMp4JobByDeviceCapability` records the same fact a third time. The
+knowledge is in two sibling files; `HardwareFallbackTest` is the one that walked into it — and
+because its assertions are about the *output* rather than the *path*, it passes where
+`ForcedFailureTest` would have failed. **That asymmetry is why nobody noticed.**
+
+**State it precisely.** The fallback *wiring* is covered on every leg by `ForcedFailureTest`, with
+fakes. What has never run on any emulator is a fallback triggered by a **real** mid-export codec
+failure — which is the case `HardwareFallbackTest` exists for, and the only reason
+`sample_h264_444.mp4` is committed at all. That fixture, generated with x264 because Fedora's
+ffmpeg ships openh264 and cannot produce High 4:4:4, does nothing on any CI leg today.
+
+The fix is not one assertion. `KEY_ENGINE_USED` is `FFMPEG` **whether the fallback fired or the
+router went straight there** — asserting it changes nothing. The vacuity guard is two facts
+together: the router chose `MEDIA3` for this request on this device, *and* the worker reported
+`FFMPEG`. Whether to reach that with `assumeTrue` (a visible skip on emulators, and the "2 skipped"
+becomes 3) or with an assertion (red on emulators, announcing it cannot test what it claims) is a
+decision, not a detail — see **E6** for why no third option exists — and **#223** leaves it open.
+
+**The other e2e gaps this read found are tickets too**, and are not repeated here:
+
+| # | Gap |
+|---|---|
+| **#223** | `HardwareFallbackTest` never attempts the hardware path on any emulator leg |
+| **#224** | Cancelling a *running* native session, in any of the three engines |
+| **#225** | No `content://` input has reached a *successful* conversion — the ffkitsaf bridge |
+| **#226** | `OutputPublisher.publish` against a real `DocumentsProvider`, and the SAF premise it rests on |
+| **#227** | The notification's Cancel action has never been fired |
+| **#228** | `encodesFlacLosslessAudio` and `encodesOpus` pass on any non-empty file |
+| **#229** | FFmpeg's progress percentage is computed everywhere and asserted nowhere |
+| **#230** | *(spike)* whether a running conversion's process can be killed under instrumentation |
+
+**Nothing here was filed as a coverage delta.** Each names the mutation that has to go red, which is
+the acceptance criterion wave 4 established and which caught two vacuous tests in that wave before
+they shipped. #223 is the one that shows why the criterion matters: it has two passing assertions and
+still tests nothing.