Repository: flutter
Issue: [a11y] VoiceOver can't check `Checkbox` using `MergeSemantics`
Labels: framework,f: material design,a: accessibility,platform-web,has reproducible steps,P2,found in release: 2.10,found in release: 2.11,customer: troy,team-web,triaged-web

If a `Checkbox` is labeled using [`MergeSemantics`](https://api.flutter.dev/flutter/widgets/MergeSemantics-class.html) (often the go-to example for the `MergeSemantics` feature), VoiceOver can't interact with it using **Control**-**Option**-**Space**.

## Steps to Reproduce

<!-- Please include full steps to reproduce so that we can reproduce the problem. -->

1. Execute `flutter run -d chrome` on the code sample.
2. Turn on VoiceOver.
3. Try to check the first checkbox using **Control**-**Option**-**Space**.

**Expected results:**

The checkbox is checked.

**Actual results:**

Nothing happens. 

**Workaround:**

Use `Semantics` and `ExcludeSemantics` instead of `MergeSemantics`, as demonstrated by the second checkbox.

<details>
<summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();

  // Auto-enable accessibility for our Blind and Low Vision customers (see
  // https://docs.flutter.dev/development/accessibility-and-localization/accessibility#screen-readers).
  RendererBinding.instance!.setSemanticsEnabled(true);

  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    const appTitle = 'Checkbox Demo';

    return MaterialApp(
      title: appTitle,
      home: Scaffold(
        appBar: AppBar(
          title: const Text(appTitle),
        ),
        body: const MyCustomForm(),
      ),
    );
  }
}

class MyCustomForm extends StatefulWidget {
  const MyCustomForm({Key? key}) : super(key: key);

  @override
  State<MyCustomForm> createState() => _MyCustomFormState();
}

class _MyCustomFormState extends State<MyCustomForm> {
  final _formKey = GlobalKey<FormState>();

  bool _checkbox1Value = false;
  bool _checkbox2Value = false;

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: <Widget>[
          MergeSemantics(
            child: Row(
              children: <Widget>[
                Checkbox(
                  value: _checkbox1Value,
                  onChanged: (bool? newValue) => setState(() => _checkbox1Value = newValue!),
                ),
                const Text('Checkbox using MergeSemantics'),
              ],
            ),
          ),
          Row(
            children: <Widget>[
              Semantics(
                label: 'Checkbox using ExcludeSemantics',
                child: Checkbox(
                  value: _checkbox2Value,
                  onChanged: (bool? newValue) => setState(() => _checkbox2Value = newValue!),
                ),
              ),
              const ExcludeSemantics(child: Text('Checkbox using ExcludeSemantics')),
            ],
          ),
        ],
      ),
    );
  }
}
```

</details>

<details>
  <summary>Logs</summary>

```
[✓] Flutter (Channel stable, 2.10.3, on macOS 11.6.2 20G314 darwin-x64, locale en-US)
    • Flutter version 2.10.3 at /Users/vsomayaji/Dev/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 7e9793dee1 (5 days ago), 2022-03-02 11:23:12 -0600
    • Engine revision bd539267b4
    • Dart version 2.16.1
    • DevTools version 2.9.2

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at /Users/vsomayaji/Library/Android/sdk
    • Platform android-31, build-tools 30.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 13.2.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • CocoaPods version 1.11.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2020.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)

[✓] IntelliJ IDEA Ultimate Edition (version 2021.1.2)
    • IntelliJ at /Applications/IntelliJ IDEA.app
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart

[✓] Connected device (1 available)
    • Chrome (web) • chrome • web-javascript • Google Chrome 98.0.4758.109

[✓] HTTP Host Availability
    • All required HTTP hosts are available

• No issues found!
```

</details>


https://user-images.githubusercontent.com/1007109/157132766-2ed6feab-6244-4068-8baa-b008b62b9a29.mov