diff --git a/app/src/test/java/org/libremediaconverter/join/JoinStateAffordancesTest.kt b/app/src/test/java/org/libremediaconverter/join/JoinStateAffordancesTest.kt
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+++ b/app/src/test/java/org/libremediaconverter/join/JoinStateAffordancesTest.kt
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+package org.libremediaconverter.join
+
+import android.net.Uri
+import androidx.compose.ui.semantics.ProgressBarRangeInfo
+import androidx.compose.ui.semantics.SemanticsProperties
+import androidx.compose.ui.semantics.getOrNull
+import androidx.compose.ui.test.SemanticsMatcher
+import androidx.compose.ui.test.assertRangeInfoEquals
+import androidx.compose.ui.test.assertTextEquals
+import androidx.compose.ui.test.onNodeWithTag
+import androidx.compose.ui.test.onNodeWithText
+import androidx.compose.ui.test.performClick
+import androidx.compose.ui.test.performScrollTo
+import androidx.media3.common.util.UnstableApi
+import org.junit.Assert.assertEquals
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.convert.InputFile
+import org.libremediaconverter.createDrainedComposeRule
+import org.libremediaconverter.model.ConcatStrategy
+import org.libremediaconverter.ui.TestTags
+import org.robolectric.RobolectricTestRunner
+import java.io.File
+
+/**
+ * Every `JoinState` renders its own affordances, wired to its own callback.
+ *
+ * The defect is a branch of `JoinScreenContent`'s `when` that reads the wrong thing: a count taken
+ * from a literal rather than from `inputs`, a strategy line that describes the other strategy, a
+ * button wired to the neighbouring branch's callback, a `Failed` that drops the message it carries.
+ * None of that is visible at compile time -- every branch of the `when` type-checks against the
+ * same `JoinScreenContent` signature -- and none of it is visible from the leaf tests either, which
+ * compose `FileRow` on its own and never see a state.
+ *
+ * `JoinScreenContentTest` deliberately asks only whether the seam exists, using `Joined`. This is
+ * the matrix behind it: seven states, each pinned to what it lets the user do next.
+ *
+ * ### Two assertions here that nothing else in the suite makes
+ *
+ * **Order.** A join is the one flow where the order of the inputs is the content of the output --
+ * the empty state promises "in the order you want them" -- so the rows are read back sorted by
+ * their position on screen and compared as a list, not as a set. `JoinLeafTagsTest` proves a row
+ * tags itself with the file it shows; nothing proved the rows come out in the order they went in.
+ *
+ * **Indeterminate.** The join progress bar carries no percentage, on purpose: FFmpeg reports
+ * progress against one input's duration, which means nothing across a concatenation. The converter
+ * screen's bar is determinate, so "it has a progress bar" is the assertion that would not notice a
+ * fabricated percentage arriving here.
+ *
+ * ### Not asserted here, deliberately
+ *
+ * `JoinState.Joined.mimeType` is not rendered by this composable at all -- it is read by the entry
+ * point, to open the save dialog with a type that matches the finished job. The colour of the
+ * `Failed` message is `MaterialTheme.colorScheme.error`, which is theme lookup rather than state
+ * logic, so it is left to the eye. The `is JoinState.Idle -> Unit` arm inside the scrolling branch
+ * is unreachable by construction: the outer `when` peels `Idle` off first.
+ */
+@UnstableApi
+@RunWith(RobolectricTestRunner::class)
+class JoinStateAffordancesTest {
+
+    // Not `createComposeRule()` directly: see [org.libremediaconverter.drainEscapedCoroutineErrors].
+    @get:Rule
+    val composeRule = createDrainedComposeRule()
+
+    /** Which callback the screen invoked, in order, with what it passed. Empty until one fires. */
+    private val events = mutableListOf<String>()
+
+    @Test
+    fun `the empty state asks for files in order and offers the picker`() {
+        setContent(JoinState.Idle)
+
+        composeRule.onNodeWithText("Pick two or more files to join, in the order you want them.").assertExists()
+        // No `performScrollTo` on this one: `Idle` is the centred branch, outside the scrolling
+        // column every other state renders into, so there is nothing to scroll.
+        composeRule.onNodeWithTag(TestTags.Join.CHOOSE_FILES).performClick()
+
+        assertEquals(listOf("pickInputs"), events)
+    }
+
+    /**
+     * The rows come out in the order the inputs went in.
+     *
+     * Sorted by position rather than trusting the order `fetchSemanticsNodes` happens to return, so
+     * the assertion is about what the user sees down the screen. Three inputs, with names whose
+     * alphabetical order is not their picked order, so a list that had been sorted anywhere on the
+     * way through would not be able to pass this.
+     */
+    @Test
+    fun `the picked inputs are listed in the order they were picked`() {
+        val picked = listOf("intro.mp4", "middle.mp4", "outro.mp4")
+        setContent(JoinState.Ready(inputs = picked.map(::input)))
+
+        val topToBottom = composeRule.onAllNodes(isFileRow)
+            .fetchSemanticsNodes()
+            .sortedBy { it.positionInRoot.y }
+            .map { it.config[SemanticsProperties.TestTag] }
+
+        assertEquals(picked.map(TestTags.Join::fileRow), topToBottom)
+    }
+
+    /**
+     * Three inputs, not two: two is the minimum a join accepts, so a button that had been
+     * hardcoded to the smallest legal join would still read correctly with two on screen.
+     */
+    @Test
+    fun `the join button counts the files it will join`() {
+        setContent(JoinState.Ready(inputs = listOf(input("intro.mp4"), input("middle.mp4"), input("outro.mp4"))))
+
+        composeRule.onNodeWithTag(TestTags.Join.JOIN).assertTextEquals("Join 3 files")
+        composeRule.onNodeWithTag(TestTags.Join.JOIN).performScrollTo().performClick()
+
+        assertEquals(listOf("join"), events)
+    }
+
+    /** `Ready` is the one working state that still offers the picker, to replace the selection. */
+    @Test
+    fun `a ready join can be repicked`() {
+        setContent(JoinState.Ready(inputs = listOf(input("intro.mp4"), input("outro.mp4"))))
+
+        composeRule.onNodeWithTag(TestTags.Join.CHOOSE_DIFFERENT_FILES).performScrollTo().performClick()
+
+        assertEquals(listOf("pickInputs"), events)
+    }
+
+    @Test
+    fun `a running join names the count and shows a bar with no percentage`() {
+        setContent(JoinState.Joining(inputs = listOf(input("intro.mp4"), input("outro.mp4"))))
+
+        composeRule.onNodeWithText("Joining 2 files…").assertExists()
+        composeRule.onNodeWithTag(TestTags.Join.PROGRESS).assertRangeInfoEquals(ProgressBarRangeInfo.Indeterminate)
+        composeRule.onNodeWithTag(TestTags.CANCEL).performScrollTo().performClick()
+
+        assertEquals(listOf("cancel"), events)
+    }
+
+    /**
+     * The paragraph is byte-identical to the converter screen's, which is the point of asserting
+     * the whole of it rather than a fragment: the two branches were worded together, and a reword
+     * that lands on one screen only is the failure this notices.
+     */
+    @Test
+    fun `a paused join explains itself and still offers cancel`() {
+        setContent(JoinState.Waiting(inputs = listOf(input("intro.mp4"), input("outro.mp4"))))
+
+        composeRule.onNodeWithText(PAUSED_PARAGRAPH).assertExists()
+        composeRule.onNodeWithTag(TestTags.CANCEL).performScrollTo().performClick()
+
+        assertEquals(listOf("cancel"), events)
+    }
+
+    @Test
+    fun `a stream copied join says nothing was re-encoded`() {
+        setContent(joined(ConcatStrategy.STREAM_COPY))
+
+        composeRule.onNodeWithText(STREAM_COPY_EXPLANATION).assertExists()
+        composeRule.onNodeWithText(REENCODE_EXPLANATION).assertDoesNotExist()
+    }
+
+    /**
+     * The other half of the pair. Asserting the absence of the stream-copy line as well, because a
+     * branch that had collapsed to one answer would still render *an* explanation.
+     */
+    @Test
+    fun `a re-encoded join says the files differed`() {
+        setContent(joined(ConcatStrategy.REENCODE))
+
+        composeRule.onNodeWithText(REENCODE_EXPLANATION).assertExists()
+        composeRule.onNodeWithText(STREAM_COPY_EXPLANATION).assertDoesNotExist()
+    }
+
+    /** The size comes from the staged file, which is missing here, so `length()` answers `0L`. */
+    @Test
+    fun `a finished join reports the size of what it produced`() {
+        setContent(joined(ConcatStrategy.STREAM_COPY))
+
+        composeRule.onNodeWithText("Joined — 0 MB.").assertExists()
+    }
+
+    @Test
+    fun `a finished join offers save and start over, and they are not the same button`() {
+        setContent(joined(ConcatStrategy.STREAM_COPY))
+
+        composeRule.onNodeWithTag(TestTags.SAVE_FILE).performScrollTo().performClick()
+        composeRule.onNodeWithTag(TestTags.START_OVER).performScrollTo().performClick()
+
+        assertEquals(listOf("save:joined.mp4", "reset"), events)
+    }
+
+    @Test
+    fun `a saved join names the file and offers to join more`() {
+        setContent(JoinState.Saved(displayName = "holiday-joined.mp4"))
+
+        composeRule.onNodeWithText("Saved holiday-joined.mp4.").assertExists()
+        composeRule.onNodeWithTag(TestTags.Join.JOIN_MORE).assertTextEquals("Join more")
+        composeRule.onNodeWithTag(TestTags.Join.JOIN_MORE).performScrollTo().performClick()
+
+        assertEquals(listOf("reset"), events)
+    }
+
+    /**
+     * The message is the whole content of this state -- it is the only thing that says why the job
+     * stopped -- and it arrives as a string the failure produced, so a branch that rendered a fixed
+     * apology instead would look correct on screen.
+     */
+    @Test
+    fun `a failed join renders the message it carries`() {
+        setContent(JoinState.Failed(message = "The second file has no audio track, so joining stopped."))
+
+        composeRule.onNodeWithText("The second file has no audio track, so joining stopped.").assertExists()
+    }
+
+    @Test
+    fun `a failed join offers start over`() {
+        setContent(JoinState.Failed(message = "The second file has no audio track, so joining stopped."))
+
+        composeRule.onNodeWithTag(TestTags.START_OVER).performScrollTo().performClick()
+
+        assertEquals(listOf("reset"), events)
+    }
+
+    /** Anything `FileRow` tagged, whichever file it is showing. The prefix comes from the table. */
+    private val isFileRow = SemanticsMatcher("is a join file row") { node ->
+        node.config.getOrNull(SemanticsProperties.TestTag)?.startsWith(TestTags.Join.fileRow("")) == true
+    }
+
+    private fun input(displayName: String) = InputFile(
+        uri = Uri.parse("content://test/$displayName"),
+        displayName = displayName,
+        sizeBytes = 4_000_000L,
+    )
+
+    /** `staged` names a missing file deliberately -- see the same helper in `JoinScreenContentTest`. */
+    private fun joined(strategy: ConcatStrategy) = JoinState.Joined(
+        staged = File("no-such-staged-output.mp4"),
+        strategy = strategy,
+        suggestedName = "joined.mp4",
+        mimeType = "video/mp4",
+    )
+
+    private fun setContent(state: JoinState) {
+        composeRule.setContent {
+            JoinScreenContent(
+                state = state,
+                actions = JoinActions(
+                    onPickInputs = { events += "pickInputs" },
+                    onJoin = { events += "join" },
+                    onCancel = { events += "cancel" },
+                    onSave = { suggestedName -> events += "save:$suggestedName" },
+                    onReset = { events += "reset" },
+                ),
+            )
+        }
+    }
+
+    private companion object {
+        /** Byte-identical to the converter screen's, and split the same way `main` splits it. */
+        const val PAUSED_PARAGRAPH =
+            "Paused. Android limits background media processing, so this will " +
+                "resume automatically — keeping the app open helps it along."
+
+        const val STREAM_COPY_EXPLANATION =
+            "Files matched, so they were joined without " +
+                "re-encoding — no quality loss."
+
+        const val REENCODE_EXPLANATION =
+            "Files differed in format, so they were re-encoded " +
+                "to match."
+    }
+}