fix(libraries): keep a sidebar library selection that mounts the Libraries tab

Choosing a hidden library from the sidebar on a fresh launch opened the
topmost visible library instead, and reordering the sidebar changed which
wrong library appeared. A second attempt worked.

The first visit to the Libraries tab mounts LibrariesScreen, and two
post-frame callbacks then run in registration order: MainScreen's, which
applies the requested library, and the one initState registers during that
frame's build, which picks the saved or topmost-visible default. The second
one overwrote the first — and a hidden library can be neither default, so
the selection was always lost.

Initialization now treats an existing selection as authoritative, both on
entry and after each of its awaits, so a selection arriving while it is
suspended (the refresh() stale-resume path) also survives.

close #2207

lib/screens/libraries/libraries_screen.dart

@@ -136,10 +136,22 @@ class _LibrariesScreenState extends State<LibrariesScreen>
 
   /// Initialize the screen with libraries from the provider.
   /// This handles initial library selection and content loading.
+  ///
+  /// An existing selection is authoritative. A sidebar library selection that
+  /// mounts this screen (`MainScreen._selectLibrary`) applies its target from a
+  /// post-frame callback registered *before* the one in [initState], so this
+  /// runs second and must not replace the requested library — which may be
+  /// hidden, and so is never the saved or topmost-visible default — with a
+  /// default. The re-checks cover the reverse interleaving, where a selection
+  /// lands while this is suspended on an await ([refresh]'s stale-resume
+  /// entry point).
   Future<void> _initializeWithLibraries() async {
+    if (_selectedLibraryGlobalKey != null) return;
+
     final librariesProvider = context.read<LibrariesProvider>();
     final hiddenLibrariesProvider = context.read<HiddenLibrariesProvider>();
     await hiddenLibrariesProvider.ensureInitialized();
+    if (!mounted || _selectedLibraryGlobalKey != null) return;
     final allLibraries = librariesProvider.libraries;
 
     if (allLibraries.isEmpty) {
@@ -151,6 +163,7 @@ class _LibrariesScreenState extends State<LibrariesScreen>
     final visibleLibraries = allLibraries.where((lib) => !hiddenKeys.contains(lib.globalKey)).toList();
 
     final storage = await StorageService.getInstance();
+    if (!mounted || _selectedLibraryGlobalKey != null) return;
     final savedLibraryKey = storage.getSelectedLibraryKey();
 
     String? libraryGlobalKeyToLoad;
@@ -165,7 +178,7 @@ class _LibrariesScreenState extends State<LibrariesScreen>
       libraryGlobalKeyToLoad = visibleLibraries.first.globalKey;
     }
 
-    if (libraryGlobalKeyToLoad != null && mounted) {
+    if (libraryGlobalKeyToLoad != null) {
       unawaited(_loadLibraryContent(libraryGlobalKeyToLoad));
     }
   }

test/screens/libraries/libraries_screen_test.dart

@@ -18,6 +18,7 @@ import 'package:plezy/providers/hidden_libraries_provider.dart';
 import 'package:plezy/providers/libraries_provider.dart';
 import 'package:plezy/providers/multi_server_provider.dart';
 import 'package:plezy/screens/libraries/libraries_screen.dart';
+import 'package:plezy/screens/libraries/tabs/base_library_tab.dart';
 import 'package:plezy/services/multi_server_manager.dart';
 import 'package:plezy/services/settings_service.dart';
 import 'package:plezy/services/storage_service.dart';
@@ -68,6 +69,36 @@ void main() {
 
   tearDown(() => TvDetectionService.debugSetAppleTVOverride(null));
 
+  testWidgets('a sidebar selection at first mount survives initialization', (tester) async {
+    final preferences = _GatedPreferences({'selected_library_key': _libraryA.globalKey});
+    final harness = await _Harness.create(preferences);
+    addTearDown(harness.dispose);
+    // The requested library is hidden, so it can be neither the saved key nor
+    // the topmost visible default — exactly the reported repro.
+    await harness.hiddenLibraries.hideLibrary(_libraryB.globalKey);
+    final selected = <String>[];
+
+    // MainScreen._selectLibrary registers its loadLibraryByKey callback before
+    // the frame that mounts this screen, so it runs ahead of the post-frame
+    // callback initState registers during that very frame's build.
+    await harness.pump(
+      tester,
+      onLibrarySelected: selected.add,
+      onFirstPostFrame: () =>
+          (tester.state(find.byType(LibrariesScreen)) as LibraryLoadable).loadLibraryByKey(_libraryB.globalKey),
+    );
+
+    // Initialization must not follow up with the default library.
+    expect(selected, [_libraryB.globalKey]);
+    // ...and the content on screen is the requested library's.
+    final mountedTabs = tester
+        .widgetList(find.byWidgetPredicate((widget) => widget is BaseLibraryTab))
+        .cast<BaseLibraryTab>()
+        .toList();
+    expect(mountedTabs, isNotEmpty);
+    expect(mountedTabs.map((tab) => tab.library.globalKey).toSet(), {_libraryB.globalKey});
+  });
+
   testWidgets('stale saved tab cannot replace the current library tab', (tester) async {
     final preferences = _GatedPreferences({
       'selected_library_key': _libraryB.globalKey,
@@ -193,12 +224,23 @@ final class _Harness {
     return _Harness(libraries: libraries, hiddenLibraries: hiddenLibraries, multiServer: multiServer);
   }
 
-  Future<void> pump(WidgetTester tester, {ValueChanged<String>? onLibrarySelected, bool settle = true}) async {
+  Future<void> pump(
+    WidgetTester tester, {
+    ValueChanged<String>? onLibrarySelected,
+    bool settle = true,
+    VoidCallback? onFirstPostFrame,
+  }) async {
     tester.view.physicalSize = const Size(1280, 720);
     tester.view.devicePixelRatio = 1;
     addTearDown(tester.view.resetPhysicalSize);
     addTearDown(tester.view.resetDevicePixelRatio);
 
+    // Registered before the mounting frame, so it runs ahead of the callback
+    // LibrariesScreen.initState adds during that frame's build.
+    if (onFirstPostFrame != null) {
+      WidgetsBinding.instance.addPostFrameCallback((_) => onFirstPostFrame());
+    }
+
     await tester.pumpWidget(
       MultiProvider(
         providers: [