fix back popping the route under now playing; disc header spacing; dial back the fab glyph

- register the now playing and queue back callback from a LaunchedEffect instead of BackHandler: the NavHost registers its own callback from a LaunchedEffect, so a BackHandler composed before it loses the back and pops the route underneath
- NowPlayingBackTest registers the overlay handler before the NavHost (the losing order) and asserts back closes the overlay without popping the route
- more top padding on disc headers, except the first which sits flush
- LIST_OPTIONS_WEIGHT 0.15 -> 0.11

app/src/main/kotlin/com/subtracks/ui/AppRoot.kt

@@ -1,7 +1,8 @@
 package com.subtracks.ui
 
 import android.net.Uri
-import androidx.activity.compose.BackHandler
+import androidx.activity.OnBackPressedCallback
+import androidx.activity.compose.LocalOnBackPressedDispatcherOwner
 import androidx.compose.animation.AnimatedVisibility
 import androidx.compose.animation.core.animate
 import androidx.compose.animation.core.animateDpAsState
@@ -32,6 +33,7 @@ import androidx.compose.foundation.layout.navigationBars
 import androidx.compose.foundation.layout.statusBars
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.SideEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.mutableStateOf
@@ -72,6 +74,7 @@ import com.subtracks.ui.settings.SettingsRoute
 import com.subtracks.ui.theme.ArtworkTheme
 import com.subtracks.ui.theme.rememberArtworkColors
 import kotlinx.coroutines.Job
+import kotlinx.coroutines.awaitCancellation
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.stateIn
@@ -157,6 +160,30 @@ private fun MainNavigation() {
                 if (!open) nowPlayingOpen = false
             }
     }
+    // The NavHost registers its back callback from a LaunchedEffect, so a BackHandler composed
+    // before it can lose the back to the NavHost (which would pop the route under the overlay).
+    // Register ours from a LaunchedEffect after the NavHost so it lands later and wins.
+    val backDispatcher = LocalOnBackPressedDispatcherOwner.current?.onBackPressedDispatcher
+    val overlayBack =
+        remember {
+            object : OnBackPressedCallback(false) {
+                override fun handleOnBackPressed() {
+                    when {
+                        showingQueue -> showingQueue = false
+                        nowPlayingOpen -> settleNowPlaying(false)
+                    }
+                }
+            }
+        }
+    SideEffect { overlayBack.isEnabled = showingQueue || nowPlayingOpen }
+    LaunchedEffect(backDispatcher, overlayBack) {
+        backDispatcher?.addCallback(overlayBack)
+        try {
+            awaitCancellation()
+        } finally {
+            overlayBack.remove()
+        }
+    }
     LaunchedEffect(Unit) { playbackController.connect() }
     LaunchedEffect(Unit) { if (nowPlayingOpen) nowPlayingProgress = 1f }
 
@@ -370,9 +397,4 @@ private fun MainNavigation() {
             }
         }
     }
-
-    BackHandler(enabled = showingQueue) { showingQueue = false }
-    BackHandler(enabled = nowPlayingOpen && !showingQueue) {
-        settleNowPlaying(false)
-    }
 }

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

@@ -128,7 +128,7 @@ fun AlbumDetailScreen(
                 val disc = song.disc ?: 1L
                 val discLabel = discLabels[disc]
                 if (disc != lastDisc && (multiDisc || discLabel != null)) {
-                    item(key = "disc:$disc") { DiscHeader(discLabel ?: "Disc $disc") }
+                    item(key = "disc:$disc") { DiscHeader(discLabel ?: "Disc $disc", first = lastDisc == null) }
                 }
                 lastDisc = disc
                 item(key = song.id) {
@@ -146,10 +146,13 @@ fun AlbumDetailScreen(
 }
 
 @Composable
-private fun DiscHeader(text: String) {
+private fun DiscHeader(
+    text: String,
+    first: Boolean,
+) {
     Row(
         verticalAlignment = Alignment.CenterVertically,
-        modifier = Modifier.padding(start = 16.dp, end = 16.dp, top = 28.dp, bottom = 4.dp),
+        modifier = Modifier.padding(start = 16.dp, end = 16.dp, top = if (first) 0.dp else 40.dp, bottom = 4.dp),
     ) {
         Icon(
             imageVector = Icons.Outlined.FiberSmartRecord,

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

@@ -653,7 +653,7 @@ private val TAB_BAR_CONTENT_HEIGHT = 52.dp
 private val TAB_ICON_SIZE = 24.dp
 private val TAB_VERTICAL_PADDING = 6.dp
 private val TOUCH_TARGET = 48.dp
-private const val LIST_OPTIONS_WEIGHT = 0.15f
+private const val LIST_OPTIONS_WEIGHT = 0.11f
 private val LIST_OPTIONS_BAR_WIDTHS = listOf(1f, 0.62f, 0.34f)
 
 @Composable