Repository: flutter
Issue: NestedScrollView doesn't respect ballistic drags
Labels: framework,f: scrolling,has reproducible steps,P2,found in release: 2.1,team-framework,triaged-framework

this issue is quite similar to https://github.com/flutter/flutter/issues/63978
if this was already fixed, this might be a regression again, but i didn't try triaging the applied PR https://github.com/flutter/flutter/pull/68644

i attached a video and added to the sample how i expect it to work - this is how it works with CustomScrollView

the bug is even more obvious with BouncingScrollPhysics

<details>
<summary>code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: NestedScrollViewExample(),
      // home: CustomScrollViewExample(),
    );
  }
}

class NestedScrollViewExample extends StatelessWidget {
  NestedScrollViewExample({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: NestedScrollView(
        floatHeaderSlivers: true,
        headerSliverBuilder: (context, innerBoxIsScrolled) => [
          SliverAppBar(
            floating: true,
            expandedHeight: 250.0,
            elevation: 0.0,
            forceElevated: false,
            flexibleSpace: FlexibleSpaceBar(
              title: Text('Demo'),
            ),
          ),
        ],
        body: CustomScrollView(
          slivers: <Widget>[
            SliverPadding(
              padding: const EdgeInsets.all(8.0),
              sliver: SliverFixedExtentList(
                itemExtent: 48.0,
                delegate: SliverChildBuilderDelegate(
                  (BuildContext context, int index) {
                    return ListTile(
                      title: Text('Item $index'),
                    );
                  },
                  childCount: 30,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class CustomScrollViewExample extends StatelessWidget {
  CustomScrollViewExample({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            floating: true,
            expandedHeight: 250.0,
            elevation: 0.0,
            forceElevated: false,
            flexibleSpace: FlexibleSpaceBar(
              title: Text('Demo'),
            ),
          ),
          SliverPadding(
            padding: const EdgeInsets.all(8.0),
            sliver: SliverFixedExtentList(
              itemExtent: 48.0,
              delegate: SliverChildBuilderDelegate(
                (BuildContext context, int index) {
                  return ListTile(
                    title: Text('Item $index'),
                  );
                },
                childCount: 30,
              ),
            ),
          ),
        ],
      ),
    );
  }
}
```
</details>

<details>
<summary>flutter doctor -v</summary>

```
[✓] Flutter (Channel master, 2.1.0-13.0.pre.223, on Microsoft Windows [Version 10.0.19041.867], locale ru-RU)
    • Flutter version 2.1.0-13.0.pre.223 at c:\dev\src\flutter
    • Framework revision 891511d58f (2 days ago), 2021-03-23 14:57:01 -0700
    • Engine revision d59a01d2d3
    • Dart version 2.13.0 (build 2.13.0-150.0.dev)

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at C:\Users\danya\AppData\Local\Android\sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • 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 2019 16.7.7)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
    • Visual Studio Community 2019 version 16.7.30621.155
    • Windows 10 SDK version 10.0.18362.0

[✓] Android Studio (version 4.0)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 50.0.1
    • Dart plugin version 193.7547
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[✓] VS Code (version 1.54.3)
    • VS Code at C:\Users\danya\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.20.0

[✓] Connected device (5 available)
    • Redmi Note 5 (mobile)   • 40c9f14       • android-arm64  • Android 9 (API 28)
    • sdk gphone x86 (mobile) • emulator-5554 • android-x86    • Android 11 (API 30) (emulator)
    • Windows (desktop)       • windows       • windows-x64    • Microsoft Windows [Version 10.0.19041.867]
    • Chrome (web)            • chrome        • web-javascript • Google Chrome 89.0.4389.90
    • Edge (web)              • edge          • web-javascript • Microsoft Edge 89.0.774.57

• No issues found!
```
</details>


https://user-images.githubusercontent.com/39104740/112519626-9d205d00-8dab-11eb-8ef1-da7ffbf909ae.mp4

https://user-images.githubusercontent.com/39104740/112519697-ab6e7900-8dab-11eb-95a7-2d2d362e76f8.mp4