Flutter test finder doesn't find my stateful widget

Finale

Change your code to:

              // titleSection,
              // buttonsSection,
              // textSection,
              statesSection

and the test will pass for

  testWidgets('State management tests', (WidgetTester tester) async {
    // Build our app and trigger a frame.
    await tester.pumpWidget(const MyApp());
    expect(find.byType(Scaffold), findsOneWidget);
    expect(find.byType(StatefulParentWidget), findsOneWidget);

    expect(find.text("Inactive"), findsOneWidget);
    expect(find.text("Active"), findsNothing);
    //StatefulParentWidget statefulParentWidget = const StatefulParentWidget();
    //tester.state(find.byWidget(statefulParentWidget));
    //expect(find.byWidget(tapboxB), findsOneWidget);
  });

So the test only can find widgets rendered already, and in your case, the widgets in statesSection were once off the stage.

Former Discussion

1 With Scaffod

If you were facing the same exception message as follows:

flutter: (The following exception is now available via WidgetTester.takeException:)
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building MainPage:
flutter: No MediaQuery widget ancestor found.
flutter: Scaffold widgets require a MediaQuery widget ancestor.

Change your test code to this, it should work

void main() {
  testWidgets('State management tests', (WidgetTester tester) async {
    // Build our app and trigger a frame.
    await tester.pumpWidget(MaterialApp(
      home: const MainPage(),
    ));
    expect(find.byType(ParentWidget), findsOneWidget); // Fails!
    expect(find.text("Inactive"), findsOneWidget); // Fails!
    expect(find.text("Active"), findsNothing); // Fails!
  });
}

Full code is here:

void main() {
  testWidgets('State management tests', (WidgetTester tester) async {
    // Build our app and trigger a frame.
    await tester.pumpWidget(MaterialApp(home: const MainPage(),));
    expect(find.byType(ParentWidget), findsOneWidget); // Fails!
    expect(find.text("Inactive"), findsOneWidget); // Fails!
    expect(find.text("Active"), findsNothing); // Fails!
  });
}

class MainPage extends StatelessWidget {
  const MainPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Hello"),
        ),
        body: Container(
            padding: const EdgeInsets.all(32),
            child: Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: const [ParentWidget()])));
  }
}

// ParentWidget manages the state for TapboxB.

//------------------------ ParentWidget --------------------------------

class ParentWidget extends StatefulWidget {
  const ParentWidget({super.key});

  @override
  _ParentWidgetState createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  bool _active = false;

  void _handleTapboxChanged(bool newValue) {
    setState(() {
      _active = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return SizedBox(
      child: TapboxB(
        active: _active,
        onChanged: _handleTapboxChanged,
      ),
    );
  }
}

//------------------------- TapboxB ----------------------------------

class TapboxB extends StatelessWidget {
  const TapboxB({
    super.key,
    this.active = false,
    required this.onChanged,
  });

  final bool active;
  final ValueChanged<bool> onChanged;

  void _handleTap() {
    onChanged(!active);
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: _handleTap,
      child: Container(
        child: Center(
          child: Text(
            active ? 'Active' : 'Inactive',
            style: const TextStyle(fontSize: 32.0, color: Colors.white),
          ),
        ),
        width: 200.0,
        height: 200.0,
        decoration: BoxDecoration(
          color: active ? Colors.lightGreen[700] : Colors.grey[600],
        ),
      ),
    );
  }
}

2 Without Scaffod

If you want to test the widget solely, then you have to replace your Text widget with this:

          child: Text(
            active ? 'Active' : 'Inactive',
            textDirection: TextDirection.ltr,
            style: const TextStyle(fontSize: 32.0, color: Colors.white),
          ),

In my first example, the Scallfold does the trick so you can test without telling textDirection attribution. Please refer to TextDirection enum for further reading.

Full code is here:

void main() {
  testWidgets('State management tests', (WidgetTester tester) async {
    // Build our app and trigger a frame.
    await tester.pumpWidget(statesSection);
    expect(find.byType(ParentWidget), findsOneWidget); // Fails!
    expect(find.text("Inactive"), findsOneWidget); // Fails!
    expect(find.text("Active"), findsNothing); // Fails!
  });
}

Widget statesSection = Container(
    padding: const EdgeInsets.all(32),
    child: Row(
        mainAxisAlignment: MainAxisAlignment.center,
        children: const [ParentWidget()]));

class MainPage extends StatelessWidget {
  const MainPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Hello", textDirection: TextDirection.ltr),
        ),
        body: Container(
            padding: const EdgeInsets.all(32),
            child: Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: const [ParentWidget()])));
  }
}

// ParentWidget manages the state for TapboxB.

//------------------------ ParentWidget --------------------------------

class ParentWidget extends StatefulWidget {
  const ParentWidget({super.key});

  @override
  _ParentWidgetState createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  bool _active = false;

  void _handleTapboxChanged(bool newValue) {
    setState(() {
      _active = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return SizedBox(
      child: TapboxB(
        active: _active,
        onChanged: _handleTapboxChanged,
      ),
    );
  }
}

//------------------------- TapboxB ----------------------------------

class TapboxB extends StatelessWidget {
  const TapboxB({
    super.key,
    this.active = false,
    required this.onChanged,
  });

  final bool active;
  final ValueChanged<bool> onChanged;

  void _handleTap() {
    onChanged(!active);
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: _handleTap,
      child: Container(
        child: Center(
          child: Text(
            active ? 'Active' : 'Inactive',
            textDirection: TextDirection.ltr,
            style: const TextStyle(fontSize: 32.0, color: Colors.white),
          ),
        ),
        width: 200.0,
        height: 200.0,
        decoration: BoxDecoration(
          color: active ? Colors.lightGreen[700] : Colors.grey[600],
        ),
      ),
    );
  }
}