Add ability to control if bottom child focus can be excluded in `AnimatedCrossFade` (#96593)

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

@@ -130,6 +130,7 @@ class AnimatedCrossFade extends StatefulWidget {
     required this.duration,
     this.reverseDuration,
     this.layoutBuilder = defaultLayoutBuilder,
+    this.excludeBottomFocus = true,
   }) : assert(firstChild != null),
        assert(secondChild != null),
        assert(firstCurve != null),
@@ -139,6 +140,7 @@ class AnimatedCrossFade extends StatefulWidget {
        assert(crossFadeState != null),
        assert(duration != null),
        assert(layoutBuilder != null),
+       assert(excludeBottomFocus != null),
        super(key: key);
 
   /// The child that is visible when [crossFadeState] is
@@ -206,6 +208,15 @@ class AnimatedCrossFade extends StatefulWidget {
   /// result in the widgets jumping about when the cross-fade state is changed.
   final AnimatedCrossFadeBuilder layoutBuilder;
 
+  /// When true, this is equivalent to wrapping the bottom widget with an [ExcludeFocus]
+  /// widget while it is at the bottom of the cross-fade stack.
+  ///
+  /// Defaults to true. When it is false, the bottom widget in the cross-fade stack
+  /// can remain in focus until the top widget requests focus. This is useful for
+  /// animating between different [TextField]s so the keyboard remains open during the
+  /// cross-fade animation.
+  final bool excludeBottomFocus;
+
   /// The default layout algorithm used by [AnimatedCrossFade].
   ///
   /// The top child is placed in a stack that sizes itself to match the top
@@ -345,6 +356,7 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
       child: IgnorePointer(
         child: ExcludeSemantics( // Always exclude the semantics of the widget that's fading out.
           child: ExcludeFocus(
+            excluding: widget.excludeBottomFocus,
             child: FadeTransition(
               opacity: bottomAnimation,
               child: bottomChild,

packages/flutter/test/widgets/animated_cross_fade_test.dart

@@ -385,6 +385,31 @@ void main() {
     expect(hiddenNode.hasPrimaryFocus, isFalse);
   });
 
+  testWidgets('AnimatedCrossFade bottom child can have 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),
+          excludeBottomFocus: false,
+        ),
+      ),
+    );
+
+    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, isTrue);
+  });
+
   testWidgets('AnimatedCrossFade second child do not receive touch events',
       (WidgetTester tester) async {
     int numberOfTouchEventNoticed = 0;