Repository: flutter
Issue: Accessibility: Cannot scroll if SliverPersistentHeader is full height
Labels: platform-ios,framework,a: accessibility,has reproducible steps,P2,found in release: 2.8,found in release: 2.10,team-ios,triaged-ios

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## Steps to Reproduce


<!-- Please include full steps to reproduce so that we can reproduce the problem. -->

1. Turn on VoiceOver <!-- (see "Code sample" section below) -->
2. Execute `flutter run` on the code sample <!-- describe steps to demonstrate bug -->
3. Select title or body, then three finger swipe to scroll <!-- for example "Tap on X and see a crash" -->


**Expected results:** <!-- what did you want to see? -->

Scrollable

**Actual results:** <!-- what did you see? -->

Non-scrollable


https://user-images.githubusercontent.com/666539/152012670-832fda05-1422-4d77-a7f6-8073a9215c4d.MP4



<details>
<summary>Code sample</summary>

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```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Scrollbar(
        child: CustomScrollView(
          slivers: [
            SliverAppBar(
              title: Text("Title"),
              excludeHeaderSemantics: true,
              flexibleSpace: Container(
                color: Colors.red,
                alignment: Alignment.center,
                padding: EdgeInsets.all(25),
                child: Text(
                  'Select me with VoiceOver, '
                  'then three finger swipe to try to scroll down',
                ),
              ),
              expandedHeight: 1200,
            ),
            SliverList(
              delegate: SliverChildListDelegate.fixed(
                [
                  for (final i in List.generate(100, (index) => index))
                    ListTile(
                      title: Text('$i'),
                    ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}
```

</details>

<details>
  <summary>Logs</summary>

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```
```

<!--
     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
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```
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 2.8.1, on macOS 12.1 21C52 darwin-x64, locale en-US)
[✓] Android toolchain - develop for Android devices (Android SDK version 32.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 13.2.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2020.3)
[✓] VS Code (version 1.63.2)
[✓] Connected device (3 available)

• No issues found!
```


</details>


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