Repository: flutter
Issue: NestedScrollView incorrect semantic  items traversal
Labels: framework,a: accessibility,f: scrolling,P2,found in release: 3.0,found in release: 3.1,team-framework,triaged-framework

Hello!
I faced with issue of traversing semantics nodes for a list that child of a NestedScrollView.

The problem is that when the semantics focus reaches the end of the screen border, it does not go to the elements that are outside the screen, so we cannot pass through the entire list. 

More details: In fact, the list scrolls once when it reaches the end of the screen, but this does not happen afterwards, and if we have lists of more than 2 screen heights, we will not be able to reach its end.

## Steps to Reproduce

<!-- Please include full steps to reproduce so that we can reproduce the problem. -->

1. Execute `flutter run` on the code sample
2. Enable TackBack/VoiceOver
3. Swipe to right to go at the next semantic element
4. Swipe right until you reach the end of the screen
5. Swipe right to go next item out of screen 
6. After you reached the end of the screen, list might automatically scrolled down to show next list items 
7. Swipe right until you reach the end of the screen again:

**Expected results:** list should be automatically scrolled down to show next list items 

**Actual results:** traversal stoped, next  swipe move focus at the start of page

<details>
<summary>Code sample</summary>

These is two widgets/scenario 

- `DemoWithNestedScrollView` - with traversal issue
- `DemoWithoutNestedScrollView` - traversal works fine all items are accessible    

```dart
import 'package:flutter/material.dart';

void main() => runApp(const MaterialApp(home: DemoWithNestedScrollView()));

class DemoWithNestedScrollView extends StatelessWidget {
  const DemoWithNestedScrollView({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: NestedScrollView(
        headerSliverBuilder: (context, innerBoxIsScrolled) {
          return [
            const SliverAppBar(
              pinned: true,
              expandedHeight: 250.0,
              flexibleSpace: FlexibleSpaceBar(
                title: Text('Demo with NestedScrollView'),
              ),
            )
          ];
        },
        body: CustomScrollView(
          slivers: <Widget>[
            SliverPadding(
              padding: const EdgeInsets.all(8.0),
              sliver: SliverFixedExtentList(
                itemExtent: 50,
                delegate: SliverChildBuilderDelegate(
                  (BuildContext context, int index) {
                    return Builder(builder: (context) {
                      return ListTile(
                        title: Text(index < 19 ? 'Item $index' : 'End'),
                      );
                    });
                  },
                  childCount: 20,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class DemoWithoutNestedScrollView extends StatelessWidget {
  const DemoWithoutNestedScrollView({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          const SliverAppBar(
            pinned: true,
            expandedHeight: 250.0,
            flexibleSpace: FlexibleSpaceBar(
              title: Text('Demo without NestedScrollView'),
            ),
          ),
          SliverPadding(
            padding: const EdgeInsets.all(8.0),
            sliver: SliverFixedExtentList(
              itemExtent: 50,
              delegate: SliverChildBuilderDelegate(
                (BuildContext context, int index) {
                  return Builder(builder: (context) {
                    return ListTile(
                      title: Text(index < 19 ? 'Item $index' : 'End'),
                    );
                  });
                },
                childCount: 20,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

```

</details>

<details>

```
[✓] Flutter (Channel stable, 3.0.0, on macOS 11.5.1 20G80 darwin-x64, locale en-BY)
    • Flutter version 3.0.0 at /Users/Aliaksei_Katsura/fvm/versions/3.0.0
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision ee4e09cce0 (2 weeks ago), 2022-05-09 16:45:18 -0700
    • Engine revision d1b9a6938a
    • Dart version 2.17.0
    • DevTools version 2.12.2

[✓] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at /Users/Aliaksei_Katsura/Library/Android/sdk
    • Platform android-31, build-tools 31.0.0
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.11+0-b60-7590822)
    • All Android licenses accepted.

[!] Xcode - develop for iOS and macOS (Xcode 13.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    ! CocoaPods 1.10.2 out of date (1.11.0 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 see https://guides.cocoapods.org/using/getting-started.html#installation for instructions.

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2021.1)
    • 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 11.0.11+0-b60-7590822)

[✓] Connected device (3 available)
    • Pixel 3 (mobile) • 95GX20PGC • android-arm64  • Android 12 (API 31)
    • macOS (desktop)  • macos     • darwin-x64     • macOS 11.5.1 20G80 darwin-x64
    • Chrome (web)     • chrome    • web-javascript • Google Chrome 101.0.4951.64

[✓] HTTP Host Availability
    • All required HTTP hosts are available

```

</details>

<details>
<summary>Videos</summary>

`DemoWithNestedScrollView` - with traversal issue

https://user-images.githubusercontent.com/26206011/170558252-25fc8c0b-3def-4ec4-995c-777057dd2ccd.mp4

`DemoWithoutNestedScrollView` - traversal works fine all items are accessible    

https://user-images.githubusercontent.com/26206011/170558448-215c32f6-af11-4248-b63f-b8393c7a6e7b.mp4



</details>