Fixed changing supportedLocales fails to update the locale (flutter#178526)

<!--
Thanks for filing a pull request!
Reviewers are typically assigned within a week of filing a request.
To learn more about code review, see our documentation on Tree Hygiene:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
-->

Recompute the resolved locale in LocalizationsResolver.update(...) and
notify listeners
only when the resolved locale actually changes. Previously, updating
WidgetsApp.supportedLocales
did not always propagate to the app locale because the resolver did not
re-resolve and
notify listeners. This change fixes that bug and adds unit tests.

Fixes
[https://github.com/flutter/flutter/issues/117210](https://github.com/flutter/flutter/issues/117210)

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
authoritative feedback from the Flutter team. If you find its comments
useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
addressed.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

---------

Co-authored-by: anuj.kumar <anuj.kumar@powerup.money>

Author: 2 people

packages/flutter/lib/src/widgets/app.dart

@@ -1650,14 +1650,24 @@ class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
     supportedLocales: widget.supportedLocales,
   );
 
-  void _updateLocalizations({WidgetsApp? oldWidget}) {
-    _localizationsResolver.update(
-      locale: widget.locale,
-      localeListResolutionCallback: widget.localeListResolutionCallback,
-      localeResolutionCallback: widget.localeResolutionCallback,
-      supportedLocales: widget.supportedLocales,
-      localizationsDelegates: widget.localizationsDelegates,
-    );
+  bool _shouldUpdateLocalizations(WidgetsApp oldWidget) {
+    return widget.locale != oldWidget.locale ||
+        widget.localeListResolutionCallback != oldWidget.localeListResolutionCallback ||
+        widget.localeResolutionCallback != oldWidget.localeResolutionCallback ||
+        widget.supportedLocales != oldWidget.supportedLocales ||
+        widget.localizationsDelegates != oldWidget.localizationsDelegates;
+  }
+
+  void _updateLocalizations({required WidgetsApp oldWidget}) {
+    if (_shouldUpdateLocalizations(oldWidget)) {
+      _localizationsResolver.update(
+        locale: widget.locale,
+        localeListResolutionCallback: widget.localeListResolutionCallback,
+        localeResolutionCallback: widget.localeResolutionCallback,
+        localizationsDelegates: widget.localizationsDelegates,
+        supportedLocales: widget.supportedLocales,
+      );
+    }
   }
 
   // BUILDER

packages/flutter/lib/src/widgets/localizations.dart

@@ -808,7 +808,10 @@ class LocalizationsResolver extends ChangeNotifier with WidgetsBindingObserver {
     _localeListResolutionCallback = localeListResolutionCallback;
     _localeResolutionCallback = localeResolutionCallback;
     _localizationsDelegates = localizationsDelegates;
-    _supportedLocales = supportedLocales;
+    if (_supportedLocales != supportedLocales) {
+      _supportedLocales = supportedLocales;
+      _updateResolvedLocale(WidgetsBinding.instance.platformDispatcher.locales);
+    }
   }
 
   /// The currently resolved [Locale] based on the current platform locale and
@@ -867,7 +870,17 @@ class LocalizationsResolver extends ChangeNotifier with WidgetsBindingObserver {
 
   @override
   void didChangeLocales(List<Locale>? locales) {
-    final Locale newLocale = _resolveLocales(locales, supportedLocales);
+    _updateResolvedLocale(locales);
+  }
+
+  /// Recompute the resolved locale based on [preferredLocales] and
+  /// [supportedLocales], update the resolved locale, and notify listeners
+  /// only if the resolved locale changed.
+  ///
+  /// This method is called from both [update] (when locale resolution
+  /// parameters change) and [didChangeLocales] (when system locales change).
+  void _updateResolvedLocale(List<Locale>? preferredLocales) {
+    final Locale newLocale = _resolveLocales(preferredLocales, supportedLocales);
     if (newLocale != _resolvedLocale) {
       _resolvedLocale = newLocale;
       notifyListeners();

packages/flutter/test/widgets/localizations_test.dart

@@ -106,6 +106,55 @@ void main() {
     expect(Localizations.maybeLocaleOf(contextKey.currentContext!), isNull);
   });
 
+  testWidgets('LocalizationsResolver.update notifies listeners when supportedLocales changes', (
+    WidgetTester tester,
+  ) async {
+    final LocalizationsResolver resolver = LocalizationsResolver(
+      supportedLocales: <Locale>[const Locale('en')],
+    );
+
+    bool notified = false;
+    resolver.addListener(() {
+      notified = true;
+    });
+
+    resolver.update(
+      locale: null,
+      localeListResolutionCallback: null,
+      localeResolutionCallback: null,
+      localizationsDelegates: null,
+      supportedLocales: <Locale>[const Locale('de')],
+    );
+
+    expect(notified, isTrue);
+    resolver.dispose();
+  });
+
+  testWidgets('LocalizationsResolver.update does not notify when resolved locale unchanged', (
+    WidgetTester tester,
+  ) async {
+    final LocalizationsResolver resolver = LocalizationsResolver(
+      supportedLocales: <Locale>[const Locale('en'), const Locale('de')],
+    );
+
+    bool notified = false;
+    resolver.addListener(() {
+      notified = true;
+    });
+
+    // Update with the same effective supportedLocales shouldn't change resolved locale.
+    resolver.update(
+      locale: null,
+      localeListResolutionCallback: null,
+      localeResolutionCallback: null,
+      localizationsDelegates: null,
+      supportedLocales: <Locale>[const Locale('en'), const Locale('de')],
+    );
+
+    expect(notified, isFalse);
+    resolver.dispose();
+  });
+
   group('Semantics', () {
     testWidgets('set locale semantics', (WidgetTester tester) async {
       await tester.pumpWidget(