Repository: flutter
Issue: [VoiceOver] Can not focus on TextField when inside sliver list that is scrolled. 
Labels: a: text input,platform-ios,framework,engine,f: material design,a: accessibility,customer: money (g3),P2,team-ios,triaged-ios

Internal: b/151285964

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When a text field is in the body of a sliver list, and you scroll the sliver list under a sliver app bar, the text field is no longer editable through iOS VoiceOver.

Seems similar to https://github.com/flutter/flutter/issues/45394.

## Steps to Reproduce

Using the following example app: 

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(Demo());
}

class Demo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: CustomScrollView(
          slivers: [
            SliverAppBar(
              pinned: true,
              expandedHeight: 200,
              flexibleSpace: Container(height: 100),
            ),
            SliverList(
              delegate: SliverChildListDelegate([
                Container(
                  height: 400,
                  child: Center(child: TextField()),
                ),
                Container(
                  height: 400,
                  child: Center(child: Text('empty box')),
                ),
              ]),
            ),
          ],
        ),
      ),
    );
  }
}
```

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Run the above app on an iOS device. 
1. Enable VoiceOver mode. 
1. Scroll the app down so that the `SliverAppBar` is collapsed. 
1. Attempt to edit the text field, notice that you can not focus on the text field, despite the VoiceOver saying "Text field, double tap to edit".

Also notice that if the app is _not_ scrolled, the `TextField` is editable even with VoiceOver enabled. 

**Expected results:** <!-- what did you want to see? -->
Text field can be focused on and text can be entered.

**Actual results:** <!-- what did you see? -->
Text field can not be focused on or edited. 

<details>
  <summary>Flutter doctor output</summary>

<!--
      Run your application with `flutter run --verbose` and attach all the
      log output below between the lines with the backticks. If there is an
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      to explain how to solve the issue.
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```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel fix-defaultTextStyleAboveMaterialApp, v1.15.12-pre.4, on Mac OS X 10.14.6 18G3020, locale en-US)

[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    ! Some Android licenses not accepted.  To resolve this, run: flutter doctor --android-licenses
[!] Xcode - develop for iOS and macOS (Xcode 11.3)
    ! CocoaPods 1.7.5 out of date (1.8.0 is recommended).
        CocoaPods is used to retrieve the iOS and macOS platform side's plugin code that responds to your plugin usage on the Dart side.
        Without CocoaPods, plugins will not work on iOS or macOS.
        For more info, see https://flutter.dev/platform-plugins
      To upgrade:
        sudo gem install cocoapods
[✓] Chrome - develop for the web
[✓] Android Studio (version 3.6)
[✓] Android Studio (version 3.5)
[✓] Connected device (5 available)

! Doctor found issues in 2 categories.
```

<!--
     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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```
```