Treat hidden `IndexedStack` children as offstage for test finder (#123129)

Treat hidden `IndexedStack` children as offstage for test finder

examples/api/test/material/dropdown/dropdown_button.style.0_test.dart

@@ -16,13 +16,15 @@ void main() {
       ),
     );
 
-    expect(find.text('One'), findsNWidgets(4));
+    expect(find.text('One'), findsOneWidget);
+    expect(find.text('One', skipOffstage: false), findsNWidgets(4));
 
     await tester.tap(find.text('One').first);
     await tester.pumpAndSettle();
     expect(find.text('Two'), findsOneWidget);
     await tester.tap(find.text('Two'));
     await tester.pumpAndSettle();
-    expect(find.text('Two'), findsNWidgets(4));
+    expect(find.text('Two'), findsOneWidget);
+    expect(find.text('Two', skipOffstage: false), findsNWidgets(4));
   });
 }

examples/api/test/widgets/basic/indexed_stack.0_test.dart

@@ -11,11 +11,11 @@ void main() {
     await tester.pumpWidget(const example.IndexedStackApp());
 
     final Finder gesture2 = find.byKey(const Key('gesture2'));
-    final Element containerFinder = find.byKey(const Key('Dash')).evaluate().first;
+    final Element containerFinder = find.byKey(const Key('Dash'), skipOffstage: false).evaluate().first;
     expect(containerFinder.renderObject!.debugNeedsPaint, false);
-    final Element containerFinder1 = find.byKey(const Key('John')).evaluate().first;
+    final Element containerFinder1 = find.byKey(const Key('John'), skipOffstage: false).evaluate().first;
     expect(containerFinder1.renderObject!.debugNeedsPaint, true);
-    final Element containerFinder2 = find.byKey(const Key('Mary')).evaluate().first;
+    final Element containerFinder2 = find.byKey(const Key('Mary'), skipOffstage: false).evaluate().first;
     expect(containerFinder2.renderObject!.debugNeedsPaint, true);
 
     await tester.tap(gesture2);
@@ -34,9 +34,9 @@ void main() {
     await tester.pumpWidget(const example.IndexedStackApp());
 
     final Finder gesture1 = find.byKey(const Key('gesture1'));
-    final Element containerFinder = find.byKey(const Key('Dash')).evaluate().first;
-    final Element containerFinder1 = find.byKey(const Key('John')).evaluate().first;
-    final Element containerFinder2 = find.byKey(const Key('Mary')).evaluate().first;
+    final Element containerFinder = find.byKey(const Key('Dash'), skipOffstage: false).evaluate().first;
+    final Element containerFinder1 = find.byKey(const Key('John'), skipOffstage: false).evaluate().first;
+    final Element containerFinder2 = find.byKey(const Key('Mary'), skipOffstage: false).evaluate().first;
 
     await tester.tap(gesture1);
     await tester.pump();
@@ -53,17 +53,18 @@ void main() {
   testWidgets('has correct element addition handling', (WidgetTester tester) async {
     await tester.pumpWidget(const example.IndexedStackApp());
 
-    expect(find.byType(example.PersonTracker), findsNWidgets(3));
+    expect(find.byType(example.PersonTracker), findsOneWidget);
+    expect(find.byType(example.PersonTracker, skipOffstage: false), findsNWidgets(3));
     final Finder textField = find.byType(TextField);
     await tester.enterText(textField, 'hello');
     await tester.testTextInput.receiveAction(TextInputAction.done);
     await tester.pump();
-    expect(find.byType(example.PersonTracker), findsNWidgets(4));
+    expect(find.byType(example.PersonTracker, skipOffstage: false), findsNWidgets(4));
 
     await tester.enterText(textField, 'hello1');
     await tester.testTextInput.receiveAction(TextInputAction.done);
     await tester.pump();
-    expect(find.byType(example.PersonTracker), findsNWidgets(5));
+    expect(find.byType(example.PersonTracker, skipOffstage: false), findsNWidgets(5));
   });
   testWidgets('has state preservation', (WidgetTester tester) async {
     await tester.pumpWidget(const example.IndexedStackApp());

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

@@ -4075,6 +4075,28 @@ class _RawIndexedStack extends Stack {
       ..alignment = alignment
       ..textDirection = textDirection ?? Directionality.maybeOf(context);
   }
+
+  @override
+  MultiChildRenderObjectElement createElement() {
+    return _IndexedStackElement(this);
+  }
+}
+
+class _IndexedStackElement extends MultiChildRenderObjectElement {
+  _IndexedStackElement(_RawIndexedStack super.widget);
+
+  @override
+  _RawIndexedStack get widget => super.widget as _RawIndexedStack;
+
+  @override
+  void debugVisitOnstageChildren(ElementVisitor visitor) {
+    final int? index = widget.index;
+    // If the index is null, no child is onstage. Otherwise, only the child at
+    // the selected index is.
+    if (index != null && children.isNotEmpty) {
+      visitor(children.elementAt(index));
+    }
+  }
 }
 
 /// A widget that controls where a child of a [Stack] is positioned.

packages/flutter/test/widgets/stack_test.dart

@@ -264,16 +264,28 @@ void main() {
       );
     }
 
+    void expectFindsChild(int n) {
+      for (int i = 0; i < 3; i++) {
+        expect(find.text('$i', skipOffstage: false), findsOneWidget);
+
+        if (i == n) {
+          expect(find.text('$i'), findsOneWidget);
+        } else {
+          expect(find.text('$i'), findsNothing);
+        }
+      }
+    }
+
     await tester.pumpWidget(buildFrame(0));
-    expect(find.text('0'), findsOneWidget);
-    expect(find.text('1'), findsOneWidget);
-    expect(find.text('2'), findsOneWidget);
+    expectFindsChild(0);
     expect(itemsPainted, equals(<int>[0]));
 
     await tester.pumpWidget(buildFrame(1));
+      expectFindsChild(1);
     expect(itemsPainted, equals(<int>[1]));
 
     await tester.pumpWidget(buildFrame(2));
+      expectFindsChild(2);
     expect(itemsPainted, equals(<int>[2]));
   });
 
@@ -473,6 +485,40 @@ void main() {
     expect(tapped, isNull);
   });
 
+  testWidgets('IndexedStack reports hidden children as offstage', (WidgetTester tester) async {
+    final List<Widget> children = <Widget>[
+      for (int i = 0; i < 5; i++) Text('child $i'),
+    ];
+
+    Future<void> pumpIndexedStack(int? activeIndex) async{
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: IndexedStack(
+            index: activeIndex,
+            children: children,
+          ),
+        )
+      );
+    }
+
+    final Finder finder = find.byType(Text);
+    final Finder finderIncludingOffstage = find.byType(Text, skipOffstage: false);
+
+    await pumpIndexedStack(null);
+    expect(finder, findsNothing); // IndexedStack with null index shows nothing
+    expect(finderIncludingOffstage, findsNWidgets(5));
+
+    for (int i = 0; i < 5; i++) {
+      await pumpIndexedStack(i);
+
+      expect(finder, findsOneWidget);
+      expect(finderIncludingOffstage, findsNWidgets(5));
+
+      expect(find.text('child $i'), findsOneWidget);
+    }
+  });
+
   testWidgets('Stack clip test', (WidgetTester tester) async {
     await tester.pumpWidget(
       const Directionality(

packages/flutter_test/test/finders_test.dart

@@ -318,8 +318,9 @@ void main() {
           ],
         )),
       );
-      expect(find.byType(GestureDetector), findsNWidgets(2));
-      final Finder hitTestable = find.byType(GestureDetector).hitTestable();
+      expect(find.byType(GestureDetector), findsOneWidget);
+      expect(find.byType(GestureDetector, skipOffstage: false), findsNWidgets(2));
+      final Finder hitTestable = find.byType(GestureDetector, skipOffstage: false).hitTestable();
       expect(hitTestable, findsOneWidget);
       expect(tester.widget(hitTestable).key, const ValueKey<int>(0));
     });