address review: anchor relocation and empty-context positions

- clamp anchorPlay to the context size, so an empty context with a queued
  block puts the block at combined 0 (previously position 0 was
  unresolvable and the queue view emptied).
- move/removeAt now derive the playing track's new position from its id
  after an edit, instead of assuming a fixed list delta; moving or
  removing the anchor relocates the whole block, which movedCursor
  ignored.
- removing the currently playing track reloads at the model's next
  position (the anchor may have relocated) rather than removing it from
  the player in place.
- the queue view's remove reconciles its rows from the repository instead
  of decrementing positions by one, so a block relocation stays in sync.

app/src/test/kotlin/com/subtracks/playback/PlaybackControllerTest.kt

@@ -926,6 +926,81 @@ class PlaybackControllerTest {
         assertFalse(handle.operations.any { it.startsWith("setWindow") })
     }
 
+    @Test
+    fun removingTheLastContextTrackWithAQueuedTrackKeepsTheQueue() {
+        seedAlbum(1, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 0)
+        await {
+            controller.state.value.item
+                ?.id == "s1"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        runBlocking { controller.removeAt(0) }
+        await {
+            controller.state.value.item
+                ?.id == "x1"
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(1L, snapshot.size)
+        assertTrue(runBlocking { queues.range(snapshot, 0, 0) }.isNotEmpty())
+    }
+
+    @Test
+    fun movingTheAnchorKeepsThePlayingTrackPositionInSync() {
+        seedAlbum(4, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 1)
+        await {
+            controller.state.value.item
+                ?.id == "s2"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+        controller.next()
+        await {
+            controller.state.value.item
+                ?.id == "x1"
+        }
+
+        runBlocking { controller.move(1, 0) }
+        await {
+            controller.state.value.item
+                ?.id == "x1"
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(runBlocking { queues.combinedIndexOf(snapshot, "x1") }, controller.state.value.position)
+    }
+
+    @Test
+    fun removingThePlayingAnchorTrackPlaysTheModelNextTrack() {
+        seedAlbum(4, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 1)
+        await {
+            controller.state.value.item
+                ?.id == "s2"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        runBlocking { controller.removeAt(1) }
+        await {
+            controller.state.value.item
+                ?.id == "s3"
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(runBlocking { queues.combinedIndexOf(snapshot, "s3") }, controller.state.value.position)
+    }
+
     @Test
     fun nextFollowsTheReorderedQueue() {
         seedAlbum(3, sourceId = 1)

app/src/test/kotlin/com/subtracks/ui/playback/QueueViewModelTest.kt

@@ -6,6 +6,7 @@ import androidx.sqlite.driver.bundled.BundledSQLiteDriver
 import androidx.test.core.app.ApplicationProvider
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.subtracks.data.db.SubtracksDatabase
+import com.subtracks.data.model.QueueKind
 import com.subtracks.data.model.Song
 import com.subtracks.data.model.SongListItem
 import com.subtracks.data.model.Source
@@ -225,6 +226,37 @@ class QueueViewModelTest {
         assertEquals((220L..399L).toList(), viewModel.rows.map { it.position })
     }
 
+    @Test
+    fun removingATrackUnderTheBlockReloadsRows() {
+        runBlocking { seedSongs(10) }
+        runBlocking {
+            db.libraryDao().upsertSongs(
+                listOf(Song(1, "x1", "al2", "ar1", "X", "Other Album", "Artist", 100, 1, 1, null, null)),
+            )
+        }
+        controller.playAlbum(1, "al1", 2)
+        await { controller.state.value.position == 2L }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        viewModel.open()
+        await { viewModel.ready }
+
+        viewModel.remove(2)
+        await {
+            val positions = viewModel.rows.map { it.position }
+            positions.isNotEmpty() && positions == (positions.first()..positions.last()).toList()
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        val expected =
+            runBlocking {
+                (viewModel.rows.first().position..viewModel.rows.last().position)
+                    .mapNotNull { queues.itemAt(snapshot, it)?.song?.id }
+            }
+        assertEquals(expected, viewModel.rows.map { it.song.song.id })
+    }
+
     @Test
     fun removeAndUndoKeepTheBoundedWindowConsistent() {
         runBlocking { seedSongs(400) }