Repository: flutter
Issue: Multiple Pinned headers should work within a SliverMainAxisGroup
Labels: framework,f: scrolling,has reproducible steps,P2,team-framework,triaged-framework,found in release: 3.24,found in release: 3.26

### Steps to reproduce

Put multiple `PinnedHeaderSliver` in a `SliverMainAxisGroup` widget

It looks like this [issue](https://github.com/flutter/flutter/issues/155395) is talking about the same use case, but it has been closed without further investigation.

### Expected results

All the pinned widgets should be pinned to the top of the scroll view.

### Actual results

Only the first pinned widget is pinned to the top of the scroll view.

### Code sample

<details open><summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: const Text('SliverMainAxisGroup pinned headers'),
        ),
        body: const CustomScrollView(
          slivers: [
            SliverMainAxisGroup(
              slivers: [
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.green, pinned: true),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.green, pinned: true),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.green, pinned: true),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
                _Sliver(color: Colors.red, pinned: false),
                _Sliver(color: Colors.yellow, pinned: false),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

class _Sliver extends StatelessWidget {
  const _Sliver({
    required this.color,
    required this.pinned,
  });

  final Color color;
  final bool pinned;

  @override
  Widget build(BuildContext context) {
    final child = Container(
      height: 80,
      color: color,
      child: Padding(
        padding: const EdgeInsets.all(16),
        child: Center(
          child: Text(
            pinned ? 'Pinned' : 'Not pinned',
            style: const TextStyle(fontSize: 30),
          ),
        ),
      ),
    );
    if (pinned) {
      return PinnedHeaderSliver(child: child);
    } else {
      return SliverToBoxAdapter(child: child);
    }
  }
}
```

</details>


### Screenshots or Video

<details open>
<summary>Video demonstration</summary>

https://github.com/user-attachments/assets/3a318000-d298-4f04-9a75-f97fc4190782

</details>


### Logs

<details open><summary>Logs</summary>

```console
```

</details>


### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
[✓] Flutter (Channel stable, 3.24.3, on macOS 14.1.1 23B81 darwin-arm64, locale fr-FR)
    • Flutter version 3.24.3 on channel stable at /Users/xxxx/.puro/envs/stable/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 2663184aa7 (2 weeks ago), 2024-09-11 16:27:48 -0500
    • Engine revision 36335019a8
    • Dart version 3.5.3
    • DevTools version 2.37.3

[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at /Users/xxxx/Library/Android/sdk
    • Platform android-34, build-tools 34.0.0
    • Java binary at: /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 17.0.7+0-17.0.7b1000.6-10550314)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.4)
    • Xcode at /Applications/Xcode-15.4.0.app/Contents/Developer
    • Build 15F31d
    • CocoaPods version 1.15.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2023.1)
    • 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 17.0.7+0-17.0.7b1000.6-10550314)

[✓] VS Code (version 1.93.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.96.0

[✓] Connected device (4 available)
    • IN2023 (mobile)                 • 87950fce              • android-arm64  • Android 13 (API 33)
    • macOS (desktop)                 • macos                 • darwin-arm64   • macOS 14.1.1 23B81 darwin-arm64
    • Mac Designed for iPad (desktop) • mac-designed-for-ipad • darwin         • macOS 14.1.1 23B81 darwin-arm64
    • Chrome (web)                    • chrome                • web-javascript • Google Chrome 129.0.6668.60
    ! Error: Browsing on the local area network for iPhone. 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
    • All expected network resources are available.

• No issues found!
```

</details>