fix(player): smooth the expanded player swipe

Read sheet progress in draw/derived scopes instead of composition so a
drag no longer recomposes the player every frame, keep the expanded
player, floating player and bottom bar composed once shown instead of
building them mid-drag, and settle with a spring that carries fling
velocity. Add a drag test for CompactPlayerOverlay.

composeApp/src/commonMain/kotlin/org/spotube/spotube/modules/shell/AppShell.kt

@@ -32,17 +32,23 @@ import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.navigationBars
 import androidx.compose.foundation.layout.padding
 import androidx.compose.runtime.Composable
+import androidx.compose.animation.core.spring
+import androidx.compose.animation.core.Spring
 import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.key
+import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.setValue
 import androidx.compose.runtime.staticCompositionLocalOf
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.graphicsLayer
+import androidx.compose.ui.layout.layout
 import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.layout.onSizeChanged
 import androidx.compose.ui.unit.dp
@@ -299,6 +305,15 @@ internal fun CompactPlayerOverlay(
     onAlternativeSource: () -> Unit,
     onExpandLyrics: () -> Unit,
     floatingPlayer: @Composable (Modifier) -> Unit = { AppFloatingPlayer(modifier = it) },
+    expandedPlayer: @Composable (modifier: Modifier, onCollapse: () -> Unit) -> Unit = { modifier, onCollapse ->
+        AppExpandedPlayer(
+            modifier = modifier,
+            onCollapse = onCollapse,
+            onQueue = onQueue,
+            onAlternativeSource = onAlternativeSource,
+            onExpandLyrics = onExpandLyrics
+        )
+    },
     bottomBar: @Composable (Modifier) -> Unit = {
         AppBottombar(navigator = navigator, navigationState = navigationState, modifier = it)
     },
@@ -325,16 +340,39 @@ internal fun CompactPlayerOverlay(
     val sheetState = key(peekHeight) {
         rememberBottomSheetState(
             initialDetent = partialDetent,
-            detents = listOf(partialDetent, SheetDetent.FullyExpanded)
+            detents = listOf(partialDetent, SheetDetent.FullyExpanded),
+            // A spring carries the fling velocity into the settle, so releasing mid-drag
+            // glides to the nearest detent instead of stalling and then easing like a tween.
+            animationSpec = spring(
+                dampingRatio = Spring.DampingRatioNoBouncy,
+                stiffness = Spring.StiffnessMediumLow)
         )
     }
 
-    val progress = if (!sheetState.offset.isFinite() ||
+    // Read only inside draw/derived scopes: the sheet offset changes every drag frame, and
+    // reading it in composition would recompose the whole player on each one.
+    val progress by remember(sheetState, partialDetent) {
+        derivedStateOf {
+            val value = if (!sheetState.offset.isFinite() ||
                 (sheetState.isIdle && sheetState.currentDetent == partialDetent)
             ) 0f else runCatching {
                 sheetState.progress(partialDetent, SheetDetent.FullyExpanded)
             }.getOrDefault(0f).takeIf { it.isFinite() } ?: 0f
-    val uiState = viewModel.compactSheetUiState(progress.coerceIn(0f, 1f))
+            value.coerceIn(0f, 1f)
+        }
+    }
+    // Keyed on the sheet state: it is recreated when the peek height changes, and a derived
+    // value left reading the previous sheet's progress would never update again.
+    val expandedPlayerVisible by remember(sheetState) {
+        derivedStateOf { viewModel.compactSheetUiState(progress).showExpandedPlayer }
+    }
+    val floatingPlayerVisible by remember(sheetState) {
+        derivedStateOf { viewModel.compactSheetUiState(progress).floatingPlayerAlpha > 0.01f }
+    }
+    // Once shown, keep the expanded player composed: composing it (view models, lyrics, cover
+    // art) on the first drag frame of every gesture is the hitch the user feels as lag.
+    var expandedPlayerComposed by remember { mutableStateOf(false) }
+    if (expandedPlayerVisible) expandedPlayerComposed = true
 
     Box(modifier = Modifier.fillMaxSize()) {
         content()
@@ -345,44 +383,52 @@ internal fun CompactPlayerOverlay(
         ) {
             Sheet(modifier = Modifier.fillMaxSize()) {
                 Box(modifier = Modifier.fillMaxSize()) {
-                    if (uiState.showExpandedPlayer) {
+                    if (expandedPlayerComposed) {
-                        AppExpandedPlayer(
+                        expandedPlayer(
-                            modifier = Modifier
+                            Modifier
                                 .fillMaxSize()
-                                .graphicsLayer { alpha = uiState.expandedPlayerAlpha },
+                                .graphicsLayer {
-                            onCollapse = {
+                                    alpha = viewModel.compactSheetUiState(progress).expandedPlayerAlpha
-                                scope.launch { sheetState.animateTo(partialDetent) }
                                 },
-                            onQueue = onQueue,
+                            { scope.launch { sheetState.animateTo(partialDetent) } }
-                            onAlternativeSource = onAlternativeSource,
-                            onExpandLyrics = onExpandLyrics
                         )
                     }
 
-                    if (uiState.floatingPlayerAlpha > 0.01f) {
+                    // Always composed: building it (view model, cover art, player state) only once
+                    // progress drops below the fade-out point stalls the collapse mid-drag.
                     Column(modifier = Modifier.fillMaxWidth()) {
                         floatingPlayer(
                             Modifier
+                                .zeroSizeUnless(floatingPlayerVisible)
                                 .graphicsLayer {
-                                        alpha = uiState.floatingPlayerAlpha
+                                    alpha = viewModel.floatingPlayerAlpha(progress)
                                 })
                     }
                 }
             }
         }
-        }
 
-        if (uiState.floatingPlayerAlpha > 0.01f) {
         bottomBar(
             Modifier
                 .fillMaxWidth()
                 .align(Alignment.BottomCenter)
+                .zeroSizeUnless(floatingPlayerVisible)
                 .onSizeChanged { size ->
                     viewModel.onBottomBarMeasured(with(density) { size.height.toDp() })
                 }
                 .graphicsLayer {
-                        alpha = uiState.floatingPlayerAlpha
+                    alpha = viewModel.floatingPlayerAlpha(progress)
                 })
     }
-    }
 }
+
+/**
+ * Keeps the content composed but takes it out of layout, drawing and hit testing while it is
+ * fully faded out, so it neither steals touches from what is above it nor costs a re-composition
+ * when it comes back.
+ */
+private fun Modifier.zeroSizeUnless(visible: Boolean): Modifier =
+    if (visible) this else layout { measurable, constraints ->
+        measurable.measure(constraints)
+        layout(0, 0) {}
+    }

composeApp/src/jvmTest/kotlin/org/spotube/spotube/modules/shell/CompactPlayerOverlayDragTest.kt

@@ -0,0 +1,113 @@
+package org.spotube.spotube.modules.shell
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.runtime.DisposableEffect
+import androidx.compose.runtime.remember
+import androidx.compose.ui.ExperimentalComposeUiApi
+import androidx.compose.ui.ImageComposeScene
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.layout.onGloballyPositioned
+import androidx.compose.ui.input.pointer.PointerEventType
+import androidx.compose.ui.input.pointer.PointerType
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.use
+import org.spotube.spotube.core.navigation.Navigator
+import org.spotube.spotube.core.navigation.Routes
+import org.spotube.spotube.core.navigation.TOP_LEVEL_ROUTES
+import org.spotube.spotube.core.navigation.rememberNavigationState
+import org.spotube.spotube.core.ui.theming.SpotubeTheme
+import org.spotube.spotube.modules.settings.Theme
+import org.spotube.spotube.modules.settings.UserSettings
+import kotlin.test.Test
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+@OptIn(ExperimentalComposeUiApi::class)
+class CompactPlayerOverlayDragTest {
+    @Test
+    fun draggingTheSheetUpShowsTheExpandedPlayer() {
+        var expandedComposed = false
+        var floatingCompositions = 0
+        var floatingPlaced = false
+        val width = 400
+        val height = 800
+        ImageComposeScene(width, height) {
+            SpotubeTheme(UserSettings(theme = Theme.DARK)) {
+                val navigation = rememberNavigationState(Routes.Home, TOP_LEVEL_ROUTES)
+                val navigator = remember(navigation) { Navigator(navigation) }
+                CompactPlayerOverlay(
+                    navigator = navigator,
+                    navigationState = navigation,
+                    viewModel = remember { AppShellViewModel() },
+                    onQueue = {}, onAlternativeSource = {}, onExpandLyrics = {},
+                    floatingPlayer = { modifier ->
+                        DisposableEffect(Unit) {
+                            floatingCompositions++
+                            onDispose {}
+                        }
+                        Box(
+                            modifier.fillMaxWidth().height(64.dp).background(Color.Gray)
+                                .onGloballyPositioned { floatingPlaced = true })
+                    },
+                    expandedPlayer = { modifier, _ ->
+                        DisposableEffect(Unit) {
+                            expandedComposed = true
+                            onDispose { expandedComposed = false }
+                        }
+                        Box(modifier.fillMaxSize().background(Color.Red))
+                    },
+                    bottomBar = { modifier ->
+                        Box(modifier.fillMaxWidth().height(56.dp).background(Color.Blue))
+                    }
+                ) { Box(Modifier.fillMaxSize()) }
+            }
+        }.use { scene ->
+            var t = 0L
+            fun frame() { scene.render(t); t += 16_000_000L }
+            repeat(10) { frame() }
+            assertTrue(floatingPlaced, "floating player should show while collapsed")
+            assertFalse(expandedComposed, "expanded player should not be composed while collapsed")
+
+            // grab the floating player (just above the bottom bar) and drag to the top
+            val startY = height - 120f + 20f
+            val start = Offset(width / 2f, startY)
+            scene.sendPointerEvent(PointerEventType.Press, start, type = PointerType.Touch, timeMillis = t / 1_000_000L)
+            repeat(5) { frame() }
+            var y = startY
+            repeat(30) {
+                y -= (startY - 40f) / 30f
+                scene.sendPointerEvent(PointerEventType.Move, Offset(width / 2f, y), type = PointerType.Touch, timeMillis = t / 1_000_000L)
+                frame()
+            }
+            scene.sendPointerEvent(PointerEventType.Release, Offset(width / 2f, y), type = PointerType.Touch, timeMillis = t / 1_000_000L)
+            repeat(120) { frame() }
+
+            assertTrue(expandedComposed, "expanded player should be composed after dragging up")
+            // out of layout while the expanded player covers it, but never re-composed
+            floatingPlaced = false
+            repeat(5) { frame() }
+            assertFalse(floatingPlaced, "floating player should be out of layout when fully expanded")
+
+            // drag back down: the floating player must return without being composed again
+            val downStart = Offset(width / 2f, 60f)
+            scene.sendPointerEvent(PointerEventType.Press, downStart, type = PointerType.Touch, timeMillis = t / 1_000_000L)
+            repeat(5) { frame() }
+            var dy = downStart.y
+            repeat(30) {
+                dy += (height - 160f) / 30f
+                scene.sendPointerEvent(PointerEventType.Move, Offset(width / 2f, dy), type = PointerType.Touch, timeMillis = t / 1_000_000L)
+                frame()
+            }
+            scene.sendPointerEvent(PointerEventType.Release, Offset(width / 2f, dy), type = PointerType.Touch, timeMillis = t / 1_000_000L)
+            repeat(120) { frame() }
+            assertTrue(floatingPlaced, "floating player should be back after collapsing")
+            assertTrue(floatingCompositions == 1, "floating player was composed $floatingCompositions times, expected 1")
+        }
+    }
+}