Add platform check to FocusManager app lifecycle listener (#144718)

This PR implements a temporary fix for the mobile device keyboard bug reported in [this comment](flutter/flutter#142930 (comment)).

CC @gspencergoog

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

@@ -1677,8 +1677,16 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
     if (kFlutterMemoryAllocationsEnabled) {
       ChangeNotifier.maybeDispatchObjectCreation(this);
     }
-    _appLifecycleListener = _AppLifecycleListener(_appLifecycleChange);
-    WidgetsBinding.instance.addObserver(_appLifecycleListener);
+    if (kIsWeb || defaultTargetPlatform != TargetPlatform.android) {
+      // It appears that some Android keyboard implementations can cause
+      // app lifecycle state changes: adding this listener would cause the
+      // text field to unfocus as the user is trying to type.
+      //
+      // Until this is resolved, we won't be adding the listener to Android apps.
+      // https://github.com/flutter/flutter/pull/142930#issuecomment-1981750069
+      _appLifecycleListener = _AppLifecycleListener(_appLifecycleChange);
+      WidgetsBinding.instance.addObserver(_appLifecycleListener!);
+    }
     rootScope._manager = this;
   }
 
@@ -1695,7 +1703,9 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
 
   @override
   void dispose() {
-    WidgetsBinding.instance.removeObserver(_appLifecycleListener);
+    if (_appLifecycleListener != null) {
+      WidgetsBinding.instance.removeObserver(_appLifecycleListener!);
+    }
     _highlightManager.dispose();
     rootScope.dispose();
     super.dispose();
@@ -1856,7 +1866,7 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
 
   // Allows FocusManager to respond to app lifecycle state changes,
   // temporarily suspending the primaryFocus when the app is inactive.
-  late final _AppLifecycleListener _appLifecycleListener;
+  _AppLifecycleListener? _appLifecycleListener;
 
   // Stores the node that was focused before the app lifecycle changed.
   // Will be restored as the primary focus once app is resumed.

packages/flutter/test/widgets/focus_manager_test.dart

@@ -354,7 +354,44 @@ void main() {
       logs.clear();
     }, variant: KeySimulatorTransitModeVariant.all());
 
+    testWidgets('FocusManager ignores app lifecycle changes on Android.', (WidgetTester tester) async {
+      final bool shouldRespond = kIsWeb || defaultTargetPlatform != TargetPlatform.android;
+      if (shouldRespond) {
+        return;
+      }
+
+      Future<void> setAppLifecycleState(AppLifecycleState state) async {
+        final ByteData? message = const StringCodec().encodeMessage(state.toString());
+        await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+            .handlePlatformMessage('flutter/lifecycle', message, (_) {});
+      }
+
+      final BuildContext context = await setupWidget(tester);
+      final FocusScopeNode scope = FocusScopeNode(debugLabel: 'Scope');
+      addTearDown(scope.dispose);
+      final FocusAttachment scopeAttachment = scope.attach(context);
+      final FocusNode focusNode = FocusNode(debugLabel: 'Focus Node');
+      addTearDown(focusNode.dispose);
+      final FocusAttachment focusNodeAttachment = focusNode.attach(context);
+      scopeAttachment.reparent(parent: tester.binding.focusManager.rootScope);
+      focusNodeAttachment.reparent(parent: scope);
+      focusNode.requestFocus();
+      await tester.pump();
+      expect(focusNode.hasPrimaryFocus, isTrue);
+
+      await setAppLifecycleState(AppLifecycleState.paused);
+      expect(focusNode.hasPrimaryFocus, isTrue);
+
+      await setAppLifecycleState(AppLifecycleState.resumed);
+      expect(focusNode.hasPrimaryFocus, isTrue);
+    });
+
     testWidgets('FocusManager responds to app lifecycle changes.', (WidgetTester tester) async {
+      final bool shouldRespond = kIsWeb || defaultTargetPlatform != TargetPlatform.android;
+      if (!shouldRespond) {
+        return;
+      }
+
       Future<void> setAppLifecycleState(AppLifecycleState state) async {
         final ByteData? message = const StringCodec().encodeMessage(state.toString());
         await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
@@ -402,8 +439,6 @@ void main() {
       expect(focusNode.hasPrimaryFocus, isTrue);
 
       await setAppLifecycleState(AppLifecycleState.paused);
-      expect(focusNode.hasPrimaryFocus, isFalse);
-
       focusNodeAttachment.detach();
       expect(focusNode.hasPrimaryFocus, isFalse);