Repository: flutter
Issue: `NestedScrollView`: `ScrollBar` is behind `SliverAppBar`
Labels: framework,f: scrolling,has reproducible steps,P2,found in release: 3.3,found in release: 3.6,team-framework,triaged-framework

## Steps to Reproduce

1. Execute `flutter run` on the code sample <!-- (see "Code sample" section below) -->
2. Scroll via `ScrollBar`

**Expected results:**
`ScrollBar` should be fully visible as if I had used `CustomScrollView`

**Actual results:**
`ScrollBar` isn't fully visible

<details>
<summary>Code sample</summary>

```dart
import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyScrollBehavior extends MaterialScrollBehavior {
  const MyScrollBehavior();

  @override
  Widget buildScrollbar(BuildContext context, Widget child, ScrollableDetails details) {
    // regardless of platform, always use a scrollbar
    return Scrollbar(
      controller: details.controller,
      child: child,
    );
  }

  @override
  Set<PointerDeviceKind> get dragDevices => const {...PointerDeviceKind.values};
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      // TODO: change MyNestedScrollView() to MyCustomScrollView() and everything working fine
      home: MyNestedScrollView(),
      scrollBehavior: MyScrollBehavior(),
    );
  }
}

class MyNestedScrollView extends StatelessWidget {
  const MyNestedScrollView({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: NestedScrollView(
        headerSliverBuilder: (context, innerBoxIsScrolled) {
          return <Widget>[
            SliverOverlapAbsorber(
              handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
              sliver: SliverAppBar(
                title: const Text('SliverAppBar'),
                pinned: true,
                expandedHeight: 150,
                forceElevated: innerBoxIsScrolled,
                bottom: const PreferredSize(
                  preferredSize: Size.fromHeight(50),
                  child: Text('123'),
                ),
              ),
            ),
          ];
        },
        body: Builder(builder: (context) {
          return CustomScrollView(
            slivers: <Widget>[
              SliverOverlapInjector(
                handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
              ),
              SliverFixedExtentList(
                itemExtent: 48,
                delegate: SliverChildBuilderDelegate(
                  childCount: 100,
                  (context, index) {
                    return ListTile(
                      title: Text('Item $index'),
                    );
                  },
                ),
              ),
            ],
          );
        }),
      ),
    );
  }
}

class MyCustomScrollView extends StatelessWidget {
  const MyCustomScrollView({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: [
          const SliverAppBar(
            title: Text('SliverAppBar'),
            pinned: true,
            expandedHeight: 150,
            bottom: PreferredSize(
              preferredSize: Size.fromHeight(50),
              child: Text('123'),
            ),
          ),
          SliverFixedExtentList(
            itemExtent: 48,
            delegate: SliverChildBuilderDelegate(
              childCount: 100,
              (context, index) {
                return ListTile(
                  title: Text('Item $index'),
                );
              },
            ),
          ),
        ],
      ),
    );
  }
}
```

</details>

<details>
  <summary>flutter doctor -v</summary>

```
[√] Flutter (Channel stable, 3.3.8, on Microsoft Windows [Version 10.0.19044.2251], locale hu-HU)
    • Flutter version 3.3.8 on channel stable at C:\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 52b3dc25f6 (6 days ago), 2022-11-09 12:09:26 +0800
    • Engine revision 857bd6b74c
    • Dart version 2.18.4
    • DevTools version 2.15.0

[√] Android toolchain - develop for Android devices (Android SDK version 33.0.0)
    • Android SDK at C:\Users\maRci002\AppData\Local\Android\sdk
    • Platform android-33, build-tools 33.0.0
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Community 2022 17.3.4)
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.3.32901.215
    • Windows 10 SDK version 10.0.20348.0

[√] Android Studio (version 2021.3)
    • Android Studio at C:\Program Files\Android\Android Studio
    • 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 11.0.13+0-b1751.21-8125866)

[√] VS Code (version 1.73.1)
    • VS Code at C:\Users\maRci002\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.44.0

[√] Connected device (3 available)
    • Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.19044.2251]
    • Chrome (web)      • chrome  • web-javascript • Google Chrome 107.0.5304.107
    • Edge (web)        • edge    • web-javascript • Microsoft Edge 107.0.1418.35

[√] HTTP Host Availability
    • All required HTTP hosts are available

• No issues found!
```

</details>

|`NestedScrollView` version | `CustomScrollView` version |
| -- | -- |
| ( `MaterialApp(home: const MyNestedScrollView(), ...)` ) | ( `MaterialApp(home: const MyCustomScrollView(), ...)` ) |
| <video src="https://user-images.githubusercontent.com/8436039/201918345-bddf1a3a-95e9-4ac2-9eb9-bd75ec475736.mp4" /> | <video src="https://user-images.githubusercontent.com/8436039/201918426-fa1e4139-ce76-4386-bdee-bfebdf019686.mp4" /> |