LibreMediaConverter/CLAUDE.md

# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

LibreMediaConverter is an Android media converter (Kotlin, Jetpack Compose, Material 3) with two
conversion engines: Media3 Transformer for the hardware path and FFmpeg for everything the platform
cannot do. See `@README.md` for the architecture and `@LICENSES/README.md` for the split license —
this file covers only what is not obvious from the code.

## Build, test, lint

**Everything is on Java 25** — daemon, CI, IDE, and the app's own bytecode. Four places say so and
they must not drift apart:

| Where | What sets it |
|---|---|
| Gradle daemon | `gradle/gradle-daemon-jvm.properties` → `toolchainVersion=25` |
| CI | `java-version: '25'` in both workflows |
| IDE | `.idea/misc.xml` |
| App bytecode | `compileOptions` in `app/build.gradle.kts` |

**Do not pick a JDK for the daemon — the repo does.** `gradle-daemon-jvm.properties` carries foojay
download URLs per platform, so Gradle provisions and runs the daemon on Java 25 regardless of what
`JAVA_HOME` says (that only sets the *launcher* — `./gradlew --version` prints both). Change it with
`./gradlew updateDaemonJvm --jvm-version=NN`, never by hand.

**Reaching 25 in the bytecode row took a deliberate build change.** AGP 9's built-in Kotlin compiles
with the KGP it bundles — 2.2.10 for AGP 9.3.1 — and that caps `jvmTarget` at 24. The root
`build.gradle.kts` puts KGP (and the lockstep Compose compiler plugin) on the buildscript classpath
so AGP picks up 2.4.10 instead, which supports up to 26. That is why the module applies
`com.android.application` and the Compose plugin by `id()` rather than from the catalog. Verified end
to end, not assumed: compiled classes report major version 69, D8 dexes them, and R8 minifies them.

Consequences worth knowing before touching any of it:

- Raising `kotlin` requires a matching `compose-compiler-gradle-plugin`; they are one version.
- Still **do not** apply `org.jetbrains.kotlin.android` — incompatible with AGP 9's DSL.
- Java 24 is *not* an option even though Kotlin allows it: Adoptium dropped the EOL non-LTS, so
  there is no installable temurin-24. 25 is LTS and in the repo.

The Gradle wrapper does **not** float and cannot: `distributionUrl` names one archive and
`distributionSha256Sum` is that file's checksum. Bump it with `./gradlew wrapper --gradle-version X
--gradle-distribution-sha256-sum <sha>` so the two stay consistent.

```bash
./gradlew :app:assembleDebug          # build debug APK
./gradlew :app:testDebugUnitTest      # JVM unit tests
./gradlew :app:ktlintCheck            # formatting
./gradlew :app:ktlintFormat           # fix formatting in place
./gradlew :app:detekt                 # static analysis
./gradlew :app:lintDebug              # Android lint
./gradlew :app:jacocoTestReport       # coverage (XML+HTML under app/build/reports/jacoco/)
# single unit test:
./gradlew :app:testDebugUnitTest --tests "org.libremediaconverter.model.ConversionRouterTest"
```

CI's "Static analysis" gate is exactly `./gradlew :app:ktlintCheck :app:detekt :app:lintDebug
--continue`. Run it with `--continue` locally too: one round trip gives you all three lists instead
of the first one that fails.

**Before treating a change as done**, run: `assembleDebug` + `testDebugUnitTest` +
`compileDebugAndroidTestKotlin` + `ktlintCheck` + `detekt` + `lintDebug`.
`compileDebugAndroidTestKotlin` matters more here than it looks — the instrumented suite cannot run
on this machine (below), so without it an androidTest compile error is not discovered until CI.
ktlint and detekt also cover the `test`/`androidTest` source sets that `lintDebug` skips.

## Instrumented tests: where they actually run

This section said the opposite until 2026-08-24, and both of its claims had been false for two
days. Read it as the current answer, and see the git history if you need the old one.

- **Local emulators work, for API 33-36.** `tools/local-emulator/run-e2e.sh` runs them on this
  host. The segfault that made this look impossible was not a broken machine: SwiftShader's Reactor
  JIT writes generated shader code onto the heap and executes it, Fedora's SELinux policy denies
  `execheap`, and qemu dies. Choosing a different renderer avoids it entirely — `-gpu host`,
  `angle_indirect` and `swangle_indirect` all boot, while `auto`, `off`, `guest` and
  `swiftshader_indirect` do not. `docs/local-emulator.md` has the evidence and the per-API renderer
  table.
- **CI runs API 37, and it gates.** The matrix is 33/34/35/36/37. **Three** of the 60 instrumented
  tests cannot pass on that image, for two unrelated reasons: two Media3 hardware transcodes fail
  inside the emulator's own `c2.goldfish.h264.decoder`, and one SAF test takes the framework down
  when it rotates the display. All three carry `@FailsOnEmulatorApi37` and run in a separate
  `continue-on-error` job; the gating leg runs the other 57.

  That job is still called `E2E API 37 Media3 hardware transcode (advisory)`, which no longer
  describes everything in it. The name is kept deliberately — it is not a required context and
  people have learned to look for it — so **read the marker, not the name**, for what it holds.
  **It is red on every PR, by design**: do not read it as your change breaking something, and do
  not read a green run as evidence those three tests pass.
  `docs/api-37-emulator-crash.md` has the measurements.

  **That instruction is also why nobody looks, so the job now reports its own shape** — expected,
  received, failed, and whether the run completed — to the job summary, and compares it against
  `FAILS_ON_EMULATOR_API37_BASELINE`, committed beside the marker. A deviation is a `::notice::`;
  the job stays advisory and its conclusion is untouched. **Add or remove a `@FailsOnEmulatorApi37`
  and that number changes in the same diff**, or the next run says so. A bare failure count would
  not have worked: the run is usually truncated by an `INSTRUMENTATION_ABORTED`, and the test XML
  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.

On a device or emulator, build only the ABI it can execute:

```bash
./gradlew :app:connectedDebugAndroidTest -PabiFilters=x86_64
```

FFmpeg's native libraries dominate the APK, so shipping arm64 to an x86_64 emulator doubles the
install for code that can never run — and on API 37 the full APK does not fit at all.

## Conventions

- **ktlint owns formatting, detekt owns static analysis.** detekt's formatting ruleset is off, so
  the two can never disagree about the same line. Never hand-fix a formatting complaint — run
  `ktlintFormat`. Style is `intellij_idea` at 120 columns, set in `.editorconfig`.
- **detekt config is `config/detekt/detekt.yml`**, merged onto detekt's defaults
  (`buildUponDefaultConfig = true`), so it carries only the rules this codebase legitimately
  breaks — each with the reason written next to it. Relax a rule that way or fix the code; never a
  bare `@Suppress`. Do not invent config keys: unknown ones are rejected.
- 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** — **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 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
  `isIncludeNoLocationClasses` block in `app/build.gradle.kts` is what fixes it; **do not delete
  it as stray config**, and re-run the numbers if you ever touch it. Same commit, same 335 tests:
  29.7% -> 69.2% with that block alone.

  The old entry also explained the wrong thing. It said coverage **fell** as the suite grew from 11
  test files to 43 because "the denominator outran the numerator" on framework-edge code "the JVM
  cannot reach". The JVM reaches that code fine. What actually happened is that the new tests were
  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 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.

- **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.

  Three things make that a real bar rather than a slogan here:

  - **Unit-testable is broader than it looks.** The pure-seam pattern — `work/FailureOutcome.kt`
    documents the reasoning — turns "needs a device" into "a pure function plus a thin edge".
    Robolectric is in the JVM source set, `compose-ui-test-junit4` with it, so Compose screens are
    unit testable too. Reach for the seam before concluding something cannot be unit tested.
  - **E2E is runnable locally**, API 33-36, via `tools/local-emulator/run-e2e.sh` — see
    "Instrumented tests: where they actually run" above. That was believed impossible until the
    SELinux/renderer cause was found, and it is what makes the e2e half of this norm enforceable.
  - **A test has to bite.** Revert the line it covers, confirm it goes red, restore. A review of
    this codebase ran 46 mutations against a 257-test suite and **9 were vacuous** — five of them
    passing the whole suite over a completely unguarded code path. Green is not evidence.

  Name what you did not cover and why. Genuine exemptions exist; implied coverage is the problem.

- `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:
  eight issues filed as a scripted batch all reached the board; one filed as a one-off minutes
  later did not. A batch carries the board step in its loop; **one-offs are where it slips**, which
  is what the script is for. It resolves the project and Status ids by name rather than caching
  them, and it **reads the item back** — a mutation returning 200 is not evidence the board shows
  what was asked for. Exit 3 means the issue was created but did not reach the board, and prints
  the number so it cannot be lost quietly.

  `above-cut` and `backlog` are **labels from the 2026-08-22 triage pass** — "worked autonomously
  overnight" and "held for manual review". They are not board columns. Status carries board state;
  do not put a cut label on a newly filed ticket.

- **shellcheck runs in CI**, inside the Static analysis job, over `git ls-files '*.sh'` so a new
  script is covered without editing the workflow. It runs at full severity, `info` included: the
  two findings that raises today are answered with targeted `disable` directives carrying their
  reason, exactly as `config/detekt/detekt.yml` carries only the rules this codebase legitimately
  breaks. Do not silence it with `--severity=warning` — that hides the next real finding too.
  **It is pinned by image digest, and joins ktlint/detekt/JaCoCo in the "Dependency versions"
  rule above** — for exactly the reason stated there, demonstrated the day it was added. The first
  cut used the runner's ambient shellcheck. That is **0.9.0**, while the container used to check
  locally was 0.11.0, and the two disagree about how to report a trap handler: 0.11.0 says
  `SC2329` once on the declaration, 0.9.0 says `SC2317` on each of seven lines in the body. Same
  script, same directive, one green and one red. Directives that must survive both name both codes.

  Locally, use the same pin rather than whatever is installed:
  `podman run --rm -v "$PWD:/mnt:z" docker.io/koalaman/shellcheck@sha256:61862eba... <files>`
  (the digest is in `status_check.yml`; there is no shellcheck system package on this host).

  **`actionlint` covers the half shellcheck cannot see** — the inline `run:` blocks, where a good
  deal of this repo's bash lives. It runs shellcheck over each `run:` plus its own checks on
  expression syntax, `needs:` references, matrix keys and action inputs. It sits in the same job,
  **pinned by digest** for the reason above and one of its own: its documented installer is a
  `curl | bash` off a moving branch, which does not belong in a repo that pins every action by SHA.
  Locally: `podman run --rm -v "$PWD:/repo:z" -w /repo docker.io/rhysd/actionlint@sha256:9d360886... -color`.

## Dependency versions

Libraries **float on minor + patch** (`coreKtx = "1.+"`). Three groups deliberately do not:

- **`agp`, `kotlin`, `ksp` are version-locked to each other.** AGP 9.3.1's POM declares
  `kotlin-gradle-plugin` 2.2.10, and that is what AGP's built-in Kotlin compiles with. Android lint
  will suggest Kotlin 2.4.10; taking it breaks the Compose compiler unless KGP is *also* forced onto
  the root buildscript classpath. Move all three together, by hand, or none.
- **ktlint, detekt and JaCoCo are pinned.** A new rule in a linter makes files nobody touched stop
  passing, so CI goes red on a PR whose diff cannot explain it. Upgrading them is its own commit:
  run the tool, read the new findings, fix or relax them.
- **The FFmpeg AAR** is a committed file, not a coordinate.

**`+` does not mean "newest stable" on its own** — Gradle will happily resolve it to an alpha, and
androidx routinely publishes alphas numbered above the current stable (at last check: lifecycle,
navigation, work, datastore and annotation all did). The `componentSelection` block in
`app/build.gradle.kts` rejects prereleases, which is the only reason `2.+` means 2.11.0 rather than
2.12.0-alpha01. **Do not remove it.** To try a prerelease, name the exact version in the catalog —
that pins it, which is the right way round.

Because versions float, a build can change without a commit. `./gradlew :app:dependencies
--configuration debugRuntimeClasspath` shows what actually resolved.

## Traps

- **Do not apply `org.jetbrains.kotlin.android`.** AGP 9 has built-in Kotlin; applying the legacy
  plugin fails the build. This is why `libs.versions.toml` pins `kotlin` to AGP's bundled KGP
  version rather than the newest Kotlin release — the Compose compiler plugin must match it.
- **The FFmpeg AAR is committed** under `bin/`, deliberately. It is not on any Maven repo
  (ffmpeg-kit was archived and delisted). Rebuilding per CI run made red builds ambiguous: broken
  code, or a cross-compile that hiccuped? `bin/README.md` has provenance and how to regenerate it.
- **Anything touching Media3 carries `@UnstableApi`** rather than swallowing the marker with
  `@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.