library: restore detail scroll positions after the rows load

The lazy list restores its saved position before a paging-backed playlist
has replayed its cached pages, so a zero-item first frame clamped it back
to the top. Track the position separately and reapply it once the item
exists, which covers paged playlists as well as the album and artist
lists. The test disposes and restores the scaffold with an empty first
frame to keep this from regressing.

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

@@ -24,6 +24,7 @@ import androidx.compose.foundation.layout.statusBars
 import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.LazyListScope
+import androidx.compose.foundation.lazy.LazyListState
 import androidx.compose.foundation.lazy.rememberLazyListState
 import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.foundation.shape.RoundedCornerShape
@@ -47,12 +48,17 @@ import androidx.compose.material3.TopAppBar
 import androidx.compose.material3.TopAppBarDefaults
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.CompositionLocalProvider
+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.clip
@@ -78,6 +84,7 @@ 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
@@ -103,7 +110,7 @@ fun HeroDetailScaffold(
     CompositionLocalProvider(LocalContentColor provides MaterialTheme.colorScheme.onBackground) {
         ArtworkTheme(effectiveArtwork) {
             BoxWithConstraints(modifier.fillMaxSize().background(MaterialTheme.colorScheme.background)) {
-                val listState = rememberLazyListState()
+                val listState = rememberRestorableLazyListState()
                 val scope = rememberCoroutineScope()
                 val fill = rememberViewportFill(listState)
                 val density = LocalDensity.current
@@ -206,6 +213,32 @@ fun HeroDetailScaffold(
     }
 }
 
+@Composable
+private fun rememberRestorableLazyListState(): 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)
+            pending = false
+        }
+    }
+    LaunchedEffect(state) {
+        snapshotFlow { state.firstVisibleItemIndex to state.firstVisibleItemScrollOffset }
+            .collect { (currentIndex, currentOffset) ->
+                if (!pending) {
+                    index = currentIndex
+                    offset = currentOffset
+                }
+            }
+    }
+    return state
+}
+
 @Composable
 private fun HeroSubtitle(
     text: String,

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

@@ -0,0 +1,82 @@
+package com.subtracks.ui.components
+
+import androidx.activity.ComponentActivity
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.lazy.LazyListScope
+import androidx.compose.material3.Text
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableIntStateOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.saveable.rememberSaveableStateHolder
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.test.assertIsDisplayed
+import androidx.compose.ui.test.hasScrollAction
+import androidx.compose.ui.test.hasText
+import androidx.compose.ui.test.junit4.v2.createAndroidComposeRule
+import androidx.compose.ui.test.onNodeWithText
+import androidx.compose.ui.test.performScrollToNode
+import androidx.compose.ui.unit.dp
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.subtracks.ui.theme.SubtracksTheme
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.annotation.GraphicsMode
+
+@RunWith(AndroidJUnit4::class)
+@GraphicsMode(GraphicsMode.Mode.NATIVE)
+class HeroDetailScaffoldRestoreTest {
+    @get:Rule
+    val composeRule = createAndroidComposeRule<ComponentActivity>()
+
+    @Test
+    fun aRestoredScreenWaitsForItsRowsBeforeRestoringTheScrollPosition() {
+        var visible by mutableStateOf(true)
+        var rows by mutableIntStateOf(100)
+        composeRule.setContent {
+            SubtracksTheme {
+                val holder = rememberSaveableStateHolder()
+                if (visible) {
+                    holder.SaveableStateProvider("detail") {
+                        HeroDetailScaffold(
+                            artwork = null,
+                            title = "Album",
+                            onBack = {},
+                            header = { _, _ -> Box(Modifier.height(200.dp)) },
+                            content = { rowModifier -> rowItems(rows, rowModifier) },
+                        )
+                    }
+                }
+            }
+        }
+
+        composeRule.onNode(hasScrollAction()).performScrollToNode(hasText("Item 20"))
+        composeRule.onNodeWithText("Item 20").assertIsDisplayed()
+
+        composeRule.runOnIdle { visible = false }
+        composeRule.runOnIdle {
+            rows = 0
+            visible = true
+        }
+        composeRule.waitForIdle()
+        composeRule.runOnIdle { rows = 100 }
+        composeRule.waitForIdle()
+
+        composeRule.onNodeWithText("Item 20").assertIsDisplayed()
+    }
+
+    private fun LazyListScope.rowItems(
+        count: Int,
+        modifier: Modifier,
+    ) {
+        repeat(count) { index ->
+            item(key = index) {
+                Box(modifier.height(48.dp)) {
+                    Text("Item $index")
+                }
+            }
+        }
+    }
+}