feat(collections): unify selection-aware screens

lib/widgets/album_scaffold_body.dart

@@ -1,65 +0,0 @@
-import 'package:flutter/material.dart';
-
-/// Shared body for album-style screens: a [PopScope] that exits selection
-/// mode instead of popping, a scrollable track list, and a bottom-select bar
-/// that slides in over it.
-class AlbumScaffoldBody extends StatelessWidget {
-  const AlbumScaffoldBody({
-    super.key,
-    required this.scrollController,
-    required this.isSelectionMode,
-    required this.onExitSelectionMode,
-    required this.appBar,
-    required this.trackList,
-    required this.bottomBar,
-    required this.bottomInset,
-    required this.bottomPadding,
-  });
-
-  final ScrollController scrollController;
-  final bool isSelectionMode;
-  final VoidCallback onExitSelectionMode;
-  final Widget appBar;
-  final Widget trackList;
-  final Widget bottomBar;
-  final double bottomInset;
-  final double bottomPadding;
-
-  @override
-  Widget build(BuildContext context) {
-    return PopScope(
-      canPop: !isSelectionMode,
-      onPopInvokedWithResult: (didPop, result) {
-        if (!didPop && isSelectionMode) {
-          onExitSelectionMode();
-        }
-      },
-      child: Scaffold(
-        body: Stack(
-          children: [
-            CustomScrollView(
-              controller: scrollController,
-              slivers: [
-                appBar,
-                trackList,
-                SliverToBoxAdapter(
-                  child: SizedBox(height: isSelectionMode ? 120 : 32),
-                ),
-                SliverToBoxAdapter(child: SizedBox(height: bottomInset)),
-              ],
-            ),
-
-            AnimatedPositioned(
-              duration: const Duration(milliseconds: 250),
-              curve: Curves.easeOutCubic,
-              left: 0,
-              right: 0,
-              bottom: isSelectionMode ? 0 : -(200 + bottomPadding),
-              child: bottomBar,
-            ),
-          ],
-        ),
-      ),
-    );
-  }
-}

lib/widgets/collection_scaffold.dart

@@ -0,0 +1,124 @@
+import 'package:flutter/material.dart';
+import 'package:spotiflac_android/widgets/selection_bottom_bar.dart';
+
+/// Shared shell for every track-collection screen: album, playlist, local
+/// album, downloaded album and library folders.
+///
+/// Before this existed each of those screens built its own `Scaffold` +
+/// `PopScope` + selection plumbing, which is why the same four screens had three
+/// different selection-bar mechanisms and why the playlist screen ended up with
+/// none at all. Screens now supply their header, their content slivers and the
+/// bar contents; everything else is shared:
+///
+/// * back gesture exits selection mode instead of popping the route,
+/// * the selection bar is mounted in the root overlay so it floats above the
+///   shell navigation bar,
+/// * trailing space is reserved so the last row stays reachable while the bar
+///   is up.
+class CollectionScaffold extends StatefulWidget {
+  const CollectionScaffold({
+    super.key,
+    required this.scrollController,
+    required this.isSelectionMode,
+    required this.onExitSelectionMode,
+    required this.appBar,
+    required this.slivers,
+    required this.bottomInset,
+    this.selectionBar,
+    this.backgroundColor,
+  });
+
+  final ScrollController scrollController;
+  final bool isSelectionMode;
+  final VoidCallback onExitSelectionMode;
+
+  /// The collapsing header sliver.
+  final Widget appBar;
+
+  /// Content slivers below the header (track list, error card, footer, ...).
+  final List<Widget> slivers;
+
+  /// Bottom inset needed to clear the shell navigation bar.
+  final double bottomInset;
+
+  /// Rebuilt by the caller each frame; shown only while [isSelectionMode] is
+  /// true. Null means the screen has no multi-select.
+  final Widget? selectionBar;
+
+  final Color? backgroundColor;
+
+  @override
+  State<CollectionScaffold> createState() => _CollectionScaffoldState();
+}
+
+class _CollectionScaffoldState extends State<CollectionScaffold> {
+  final SelectionOverlayController _selectionOverlay =
+      SelectionOverlayController();
+  final GlobalKey _selectionBarKey = GlobalKey();
+
+  // Used for the first frame only. Once the overlay has laid out, the spacer
+  // tracks its real height so multi-row actions and large text cannot cover the
+  // final collection rows.
+  double _selectionBarHeight = 160;
+
+  @override
+  void dispose() {
+    _selectionOverlay.dispose();
+    super.dispose();
+  }
+
+  void _syncSelectionOverlay() {
+    if (!mounted) return;
+    final bar = widget.selectionBar;
+    if (!widget.isSelectionMode || bar == null) {
+      _selectionOverlay.hide();
+      return;
+    }
+    _selectionOverlay.show(
+      context,
+      (_) => KeyedSubtree(key: _selectionBarKey, child: bar),
+    );
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted || !widget.isSelectionMode) return;
+      final height = _selectionBarKey.currentContext?.size?.height;
+      if (height == null || height <= 0) return;
+      if ((height - _selectionBarHeight).abs() < 0.5) return;
+      setState(() => _selectionBarHeight = height);
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (widget.isSelectionMode || _selectionOverlay.isVisible) {
+      WidgetsBinding.instance.addPostFrameCallback(
+        (_) => _syncSelectionOverlay(),
+      );
+    }
+
+    return PopScope(
+      canPop: !widget.isSelectionMode,
+      onPopInvokedWithResult: (didPop, result) {
+        if (!didPop && widget.isSelectionMode) {
+          widget.onExitSelectionMode();
+        }
+      },
+      child: Scaffold(
+        backgroundColor: widget.backgroundColor,
+        body: CustomScrollView(
+          controller: widget.scrollController,
+          slivers: [
+            widget.appBar,
+            ...widget.slivers,
+            SliverToBoxAdapter(
+              child: SizedBox(
+                key: const ValueKey('collection-selection-spacer'),
+                height: widget.isSelectionMode ? _selectionBarHeight : 32,
+              ),
+            ),
+            SliverToBoxAdapter(child: SizedBox(height: widget.bottomInset)),
+          ],
+        ),
+      ),
+    );
+  }
+}