Repository: flutter
Issue: Overscrolling outer scrollview / SliverAppBar.stretch does not work in NestedScrollView
Labels: framework,f: material design,f: scrolling,has reproducible steps,P2,workaround available,team-design,triaged-design,found in release: 3.16,found in release: 3.20

<details>
<summary>code sample</summary>

```bash
          DefaultTabController(
              length: 2,
              child: NestedScrollView(
                  physics: BouncingScrollPhysics(),
                  headerSliverBuilder: (context, innerScrolled) => <Widget>[
                    SliverOverlapAbsorber(
                      handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                      child: SliverAppBar(
                        pinned: true,
                        stretch: true, 
                        title: Text('username'),
                        expandedHeight: 325,
                        flexibleSpace: FlexibleSpaceBar(
                          stretchModes: <StretchMode>[
                            StretchMode.zoomBackground,
                            StretchMode.blurBackground,
                          ],
                          background: Image.asset('images/splash.png', fit: BoxFit.cover)
                        ),
                        bottom: 
                          TabBar(
                            tabs: <Widget>[
                              Text('test1'),
                              Text('test2')
                            ]
                          )
                      ),
                    )
                  ],
              body: TabBarView(
                  children: [
                    Builder( 
                      builder: (context) => CustomScrollView(
                          slivers: <Widget>[
                            SliverOverlapInjector(
                                handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context)),
                                SliverFixedExtentList(
                                    delegate: SliverChildBuilderDelegate(
                                        (_, index) => Text('not working'),
                                        childCount: 100),
                                    itemExtent: 25
                                  )
                          ],
                        ),
                    ),
                    Text('working')
                  ]
            )));
```
</details>


![skiun-x5ux6](https://user-images.githubusercontent.com/26580242/78512407-d0529080-77d6-11ea-804c-9442f3405ccb.gif)


<details>
<summary>flutter doctor -v</summary>

```bash
[✓] Flutter (Channel stable, v1.12.13+hotfix.9, on Mac OS X 10.15.1 19B88, locale en-CN)
    • Flutter version 1.12.13+hotfix.9 at /Users/lalawila/flutter
    • Framework revision f139b11009 (6 days ago), 2020-03-30 13:57:30 -0700
    • Engine revision af51afceb8
    • Dart version 2.7.2

[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/lalawila/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-29, build-tools 29.0.2
    • Java binary at: /Library/Java/JavaVirtualMachines/jdk-10.0.2.jdk/Contents/Home/bin/java
    • Java version Java(TM) SE Runtime Environment 18.3 (build 10.0.2+13)
    ✗ Android license status unknown.
      Try re-installing or updating your Android SDK Manager.
      See https://developer.android.com/studio/#downloads or visit https://flutter.dev/setup/#android-setup for detailed instructions.

 
[✓] Xcode - develop for iOS and macOS (Xcode 11.2.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.2.1, Build version 11B53
    • CocoaPods version 1.8.0

[!] Android Studio (not installed)
    • Android Studio not found; download from https://developer.android.com/studio/index.html
      (or visit https://flutter.dev/setup/#android-setup for detailed instructions).

[✓] VS Code (version 1.43.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.9.0

[✓] Connected device (1 available)
    • iPhone 11 • 0BFEAC24-164A-4F59-B67D-EBDBF1A499D1 • ios • com.apple.CoreSimulator.SimRuntime.iOS-13-2 (simulator)

! Doctor found issues in 2 categories.
```
</details>