[Accessibility] PinnedHeaderSliver captures semantic focus before list is fully scrolled

### Steps to reproduce

1. Enable accessibility services (TalkBack or VoiceOver) on the device.
2. Open a screen containing a CustomScrollView with a PinnedHeaderSliver and a list of items (e.g., SliverList).
3. Scroll down the list so that the top items are off-screen.
4. Navigate backwards (swipe left / previous item gesture) to scroll up toward the top of the list.

### Expected results

The list should be **fully scrolled to the top** (displaying the very first item) before the semantic focus moves to the widget preceding the list or the pinned header. The user should be able to navigate through all remaining list items before leaving the list context.

### Actual results

The PinnedHeaderSliver takes the semantic focus immediately when navigating up, effectively "skipping" the remaining items at the top of the list or preventing the list from scrolling all the way back to the start.

### Code sample

<details open><summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(home: AccessibilityTestPage()));
}

class AccessibilityTestPage extends StatelessWidget {
  const AccessibilityTestPage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          const SliverAppBar(
            pinned: true,
            expandedHeight: 100.0,
            title: Text('Pinned Semantic Focus'),
            backgroundColor: Colors.blue,
          ),
          // The Pinned Header
          const PinnedHeaderSliver(
            child: ColoredBox(
              color: Colors.amber,
              child: ListTile(title: Text('Pinned Semantic Focus')),
            ),
          ),
          // The List
          SliverList(
            delegate: SliverChildBuilderDelegate((BuildContext context, int index) {
              return ListTile(
                title: Text('Item $index'),
                subtitle: const Text('Scroll down then navigate back up'),
              );
            }, childCount: 50),
          ),
        ],
      ),
    );
  }
}
```

https://github.com/user-attachments/assets/6c2b1fbe-066c-4371-9291-d3eda82dd4ea

</details>


### Screenshots or Video

<details open>
<summary>Screenshots / Video demonstration</summary>

[Upload media here]

</details>


### Logs

_No response_

### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
[✓] Flutter (Channel master, 3.39.0-1.0.pre-261, on macOS 15.7.2 24G325 darwin-arm64, locale fr-FR) [1 826ms]
    • Flutter version 3.39.0-1.0.pre-261 on channel master at /Users/emmanuel_lefebvre/fvm/versions/master
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 12b5c1cfdf (3 days ago), 2025-11-22 03:12:08 +0200
    • Engine revision 216e5124a0
    • Dart version 3.11.0 (build 3.11.0-162.0.dev)
    • DevTools version 2.52.0
    • Pub download mirror https://artifact.socrate.vsct.fr/artifactory/api/pub/all-pub
    • Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations, enable-native-assets, omit-legacy-version-file,
      enable-lldb-debugging, enable-uiscene-migration

[✓] Android toolchain - develop for Android devices (Android SDK version 35.0.1) [1 816ms]
    • Android SDK at /Users/emmanuel_lefebvre/Library/Android/sdk
    • Emulator version 36.2.12.0 (build_id 14214601) (CL:N/A)
    • Platform android-36, build-tools 35.0.1
    • Java binary at: /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
      This is the JDK bundled with the latest Android Studio installation on this machine.
      To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.8+-14196175-b1038.72)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 26.1.1) [1 598ms]
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 17B100
    • CocoaPods version 1.16.2

[✓] Chrome - develop for the web [7ms]
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Connected device (4 available) [6,8s]
    • Pixel 7 Pro (mobile)        • 35061FDH30024V • android-arm64  • Android 16 (API 36)
    • sdk gphone64 arm64 (mobile) • emulator-5554  • android-arm64  • Android 15 (API 35) (emulator)
    • macOS (desktop)             • macos          • darwin-arm64   • macOS 15.7.2 24G325 darwin-arm64
    • Chrome (web)                • chrome         • web-javascript • Google Chrome 142.0.7444.176
    ! Error: Browsing on the local area network for iPhone XS. Ensure the device is unlocked and attached with a cable or associated with the same local area network as this Mac.
      The device must be opted into Developer Mode to connect wirelessly. (code -27)

[✓] Network resources [547ms]
    • All expected network resources are available.

• No issues found!
```

</details>


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Uh oh!

[Accessibility] PinnedHeaderSliver captures semantic focus before list is fully scrolled #179022

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Uh oh!

[Accessibility] PinnedHeaderSliver captures semantic focus before list is fully scrolled #179022

Description

Steps to reproduce

Expected results

Actual results

Code sample

Screenshots or Video

Logs

Flutter Doctor output

Metadata

Metadata

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Labels

Type

Fields

Projects

Milestone

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Development

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