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

Treat hidden `IndexedStack` children as offstage for test finder

Author: simolus3

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.