Fix AnimatedCrossFade would focus on a hidden widget (#87618)

Moving the focus with the tab key on the external keyboard will move the focus to the widget that is hidden by AnimatedCrossFade.

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

@@ -6,6 +6,7 @@ import 'package:flutter/rendering.dart';
 
 import 'animated_size.dart';
 import 'basic.dart';
+import 'focus_scope.dart';
 import 'framework.dart';
 import 'ticker_provider.dart';
 import 'transitions.dart';
@@ -343,9 +344,11 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
       enabled: _isTransitioning,
       child: ExcludeSemantics(
         excluding: true, // Always exclude the semantics of the widget that's fading out.
-        child: FadeTransition(
-          opacity: bottomAnimation,
-          child: bottomChild,
+        child: ExcludeFocus(
+          child: FadeTransition(
+            opacity: bottomAnimation,
+            child: bottomChild,
+          ),
         ),
       ),
     );
@@ -354,9 +357,12 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
       enabled: true, // Top widget always has its animations enabled.
       child: ExcludeSemantics(
         excluding: false, // Always publish semantics for the widget that's fading in.
-        child: FadeTransition(
-          opacity: topAnimation,
-          child: topChild,
+        child: ExcludeFocus(
+          excluding: false,
+          child: FadeTransition(
+            opacity: topAnimation,
+            child: topChild,
+          ),
         ),
       ),
     );

packages/flutter/test/widgets/animated_cross_fade_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -360,6 +361,30 @@ void main() {
     expect(find.text('BBB'), findsOneWidget);
     expect(find.text('AAA'), findsNothing);
   });
+
+  testWidgets('AnimatedCrossFade test focus', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: AnimatedCrossFade(
+          firstChild: TextButton(onPressed: () {}, child: const Text('AAA')),
+          secondChild: TextButton(onPressed: () {}, child: const Text('BBB')),
+          crossFadeState: CrossFadeState.showFirst,
+          duration: const Duration(milliseconds: 50),
+        ),
+      ),
+    );
+
+    final FocusNode visibleNode = Focus.of(tester.element(find.text('AAA')), scopeOk: true);
+    visibleNode.requestFocus();
+    await tester.pump();
+    expect(visibleNode.hasPrimaryFocus, isTrue);
+
+    final FocusNode hiddenNode = Focus.of(tester.element(find.text('BBB')), scopeOk: true);
+    hiddenNode.requestFocus();
+    await tester.pump();
+    expect(hiddenNode.hasPrimaryFocus, isFalse);
+  });
 }
 
 class _TickerWatchingWidget extends StatefulWidget {