library: bound the detail scroll restore and cover grids

Gate the restore on the caller's content being ready instead of an item
count that can include only the header, clear it after clamping to the
available rows, and include offset-only positions. Apply the same saver
to the artist grid so a process-death restore does not open at the top.

app/src/main/kotlin/com/subtracks/ui/components/HeroDetailScaffold.kt

@@ -84,7 +84,6 @@ import com.subtracks.ui.theme.ArtworkTheme
 import com.subtracks.ui.theme.HeroGradient
 import com.subtracks.ui.theme.baseArtworkColors
 import com.subtracks.ui.theme.heroBarColor
-import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.launch
 
 private const val FADE_DISTANCE_DP = 64
@@ -105,12 +104,13 @@ fun HeroDetailScaffold(
     header: @Composable (controlsModifier: Modifier, topInset: Dp) -> Unit,
     content: LazyListScope.(rowModifier: Modifier) -> Unit,
     modifier: Modifier = Modifier,
+    contentReady: Boolean = true,
 ) {
     val effectiveArtwork = artwork ?: baseArtworkColors()
     CompositionLocalProvider(LocalContentColor provides MaterialTheme.colorScheme.onBackground) {
         ArtworkTheme(effectiveArtwork) {
             BoxWithConstraints(modifier.fillMaxSize().background(MaterialTheme.colorScheme.background)) {
-                val listState = rememberRestorableLazyListState()
+                val listState = rememberRestorableLazyListState(contentReady)
                 val scope = rememberCoroutineScope()
                 val fill = rememberViewportFill(listState)
                 val density = LocalDensity.current
@@ -214,16 +214,15 @@ fun HeroDetailScaffold(
 }
 
 @Composable
-private fun rememberRestorableLazyListState(): LazyListState {
+private fun rememberRestorableLazyListState(contentReady: Boolean): LazyListState {
     val state = rememberLazyListState()
     var index by rememberSaveable { mutableIntStateOf(state.firstVisibleItemIndex) }
     var offset by rememberSaveable { mutableIntStateOf(state.firstVisibleItemScrollOffset) }
-    var pending by remember { mutableStateOf(index > 0) }
-    LaunchedEffect(state) {
-        if (pending) {
-            state.requestScrollToItem(index, offset)
-            snapshotFlow { state.layoutInfo.totalItemsCount }.first { it > index }
-            state.scrollToItem(index, offset)
+    var pending by remember { mutableStateOf(index > 0 || offset > 0) }
+    LaunchedEffect(contentReady) {
+        if (contentReady && pending) {
+            val target = index.coerceAtMost((state.layoutInfo.totalItemsCount - 1).coerceAtLeast(0))
+            state.scrollToItem(target, offset)
             pending = false
         }
     }

app/src/main/kotlin/com/subtracks/ui/library/AlbumDetailScreen.kt

@@ -215,6 +215,7 @@ fun AlbumDetailScreen(
         artwork = artwork,
         title = album?.name.orEmpty(),
         onBack = onBack,
+        contentReady = songs.isNotEmpty(),
         header = { controlsModifier, topInset ->
             val artistName = album?.albumArtist?.takeIf { it.isNotBlank() }
             HeroHeader(

app/src/main/kotlin/com/subtracks/ui/library/ArtistDetailScreen.kt

@@ -28,6 +28,7 @@ import androidx.compose.foundation.layout.statusBars
 import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.lazy.grid.GridCells
 import androidx.compose.foundation.lazy.grid.GridItemSpan
+import androidx.compose.foundation.lazy.grid.LazyGridState
 import androidx.compose.foundation.lazy.grid.LazyVerticalGrid
 import androidx.compose.foundation.lazy.grid.itemsIndexed
 import androidx.compose.foundation.lazy.grid.rememberLazyGridState
@@ -55,10 +56,13 @@ import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
+import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
+import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.alpha
@@ -302,7 +306,7 @@ fun ArtistDetailScreen(
     onCollapseBio: () -> Unit = {},
     onShuffle: () -> Unit = {},
 ) {
-    val listState = rememberLazyGridState()
+    val listState = rememberRestorableLazyGridState(albumsLoaded)
     val fill = rememberViewportFill(listState)
     val density = LocalDensity.current
     val lineHeight =
@@ -570,6 +574,31 @@ fun ArtistDetailScreen(
     }
 }
 
+@Composable
+private fun rememberRestorableLazyGridState(contentReady: Boolean): LazyGridState {
+    val state = rememberLazyGridState()
+    var index by rememberSaveable { mutableIntStateOf(state.firstVisibleItemIndex) }
+    var offset by rememberSaveable { mutableIntStateOf(state.firstVisibleItemScrollOffset) }
+    var pending by remember { mutableStateOf(index > 0 || offset > 0) }
+    LaunchedEffect(contentReady) {
+        if (contentReady && pending) {
+            val target = index.coerceAtMost((state.layoutInfo.totalItemsCount - 1).coerceAtLeast(0))
+            state.scrollToItem(target, offset)
+            pending = false
+        }
+    }
+    LaunchedEffect(state) {
+        snapshotFlow { state.firstVisibleItemIndex to state.firstVisibleItemScrollOffset }
+            .collect { (currentIndex, currentOffset) ->
+                if (!pending) {
+                    index = currentIndex
+                    offset = currentOffset
+                }
+            }
+    }
+    return state
+}
+
 // Only the lines that overlap the action buttons are narrowed; later lines use the full width.
 @Composable
 private fun ArtistBio(

app/src/main/kotlin/com/subtracks/ui/library/ArtistDetailViewModel.kt

@@ -25,7 +25,6 @@ import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.filterNotNull
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.flow.flatMapLatest
-import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.launch
 
@@ -58,7 +57,6 @@ class ArtistDetailViewModel(
     val albums: StateFlow<ArtistAlbums?> =
         combine(sourceId, sourceRepository.offline) { id, offline -> id to offline }
             .flatMapLatest { (id, offline) -> libraryRepository.artistAlbums(id, artistId, offline) }
-            .map<ArtistAlbums, ArtistAlbums?> { it }
             .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), null)
 
     val downloadStatus: StateFlow<ListDownloadStatus> =

app/src/main/kotlin/com/subtracks/ui/library/PlaylistDetailScreen.kt

@@ -55,7 +55,7 @@ fun PlaylistDetailRoute(
     viewModel: PlaylistDetailViewModel = koinViewModel(key = playlistId) { parametersOf(playlistId) },
     playbackController: PlaybackController = koinInject(),
 ) {
-    val playlist by viewModel.playlist.collectAsStateWithLifecycle(initialValue = null)
+    val playlist by viewModel.playlist.collectAsStateWithLifecycle()
     val downloads by viewModel.downloads.collectAsStateWithLifecycle()
     val downloadStatus by viewModel.downloadStatus.collectAsStateWithLifecycle()
     val offline by viewModel.offline.collectAsStateWithLifecycle()
@@ -138,6 +138,7 @@ fun PlaylistDetailScreen(
         artwork = artwork,
         title = playlist?.name.orEmpty(),
         onBack = onBack,
+        contentReady = songs.itemCount > 0,
         header = { controlsModifier, topInset ->
             HeroHeader(
                 art = coverArt(playlist?.coverArt, false),

app/src/main/kotlin/com/subtracks/ui/library/PlaylistDetailViewModel.kt

@@ -37,7 +37,10 @@ class PlaylistDetailViewModel(
 ) : ViewModel() {
     private val sourceId = libraryRepository.activeSourceId.filterNotNull()
 
-    val playlist: Flow<Playlist?> = sourceId.flatMapLatest { libraryRepository.playlist(it, playlistId) }
+    val playlist: StateFlow<Playlist?> =
+        sourceId
+            .flatMapLatest { libraryRepository.playlist(it, playlistId) }
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), null)
 
     val songs: Flow<PagingData<PlaylistSongItem>> =
         combine(sourceId, sourceRepository.offline) { id, offline -> id to offline }

app/src/test/kotlin/com/subtracks/ui/components/HeroDetailScaffoldRestoreTest.kt

@@ -46,6 +46,7 @@ class HeroDetailScaffoldRestoreTest {
                             onBack = {},
                             header = { _, _ -> Box(Modifier.height(200.dp)) },
                             content = { rowModifier -> rowItems(rows, rowModifier) },
+                            contentReady = rows > 0,
                         )
                     }
                 }