Repository: flutter
Issue: floatHeaderSlivers true behaves abnormally at the bottom of NestedScollView
Labels: framework,f: scrolling,has reproducible steps,P2,found in release: 2.1,team-framework,triaged-framework

steps to reproduce

* set `floatHeaderSlivers` to true
* scroll to bottom of NestedScollView and make sure that floating slivers are fully hidden
* scroll up a little bit so that header slivers appear
* scroll down with fling (not necessary with fling it illustrates it better)

unexpected results:
* the NestedScollView will be overscrolled (despite ClampingScrollPhysics), which makes it a consistent way to reproduce https://github.com/flutter/flutter/issues/76760
* header slivers will react to consequent scroll gestures abnormally

additional observations:
there are no error logs, however one time when it was overscrolled i somehow managed it to hit the https://github.com/flutter/flutter/issues/63825

cc @Piinks 

<details>
<summary>code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

/// This is the main application widget.
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sample',
      home: MyStatelessWidget(),
    );
  }
}

/// This is the stateless widget that the main application instantiates.
class MyStatelessWidget extends StatelessWidget {
  MyStatelessWidget({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: NestedScrollView(
        floatHeaderSlivers: true,
        headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
          return <Widget>[
            SliverAppBar(
              title: const Text('Snapping Nested SliverAppBar'),
              expandedHeight: 200.0,
            ),
          ];
        },
        body: Builder(
          builder: (BuildContext context) {
            return CustomScrollView(
              slivers: <Widget>[
                SliverFixedExtentList(
                  itemExtent: 48.0,
                  delegate: SliverChildBuilderDelegate(
                    (BuildContext context, int index) => ListTile(title: Text('Item $index')),
                    childCount: 30,
                  ),
                ),
              ],
            );
          },
        ),
      ),
    );
  }
}
```
</details>


<details>
<summary>flutter doctor -v</summary>

```
[✓] Flutter (Channel nt4f04und, 2.1.0-13.0.pre.86, on Microsoft Windows [Version 10.0.19041.867], locale ru-RU)
    • Flutter version 2.1.0-13.0.pre.86 at c:\dev\src\flutter
    • Framework revision 40e3489254 (6 days ago), 2021-03-13 17:43:03 -0500
    • Engine revision 9ec46417bd
    • Dart version 2.13.0 (build 2.13.0-134.0.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
    ✗ Visual Studio is missing necessary components. Please re-run the Visual Studio installer for the "Desktop
      development with C++" workload, and include these components:
        MSVC v142 - VS 2019 C++ x64/x86 build tools
         - If there are multiple build tool versions available, install the latest
        C++ CMake tools for Windows
        Windows 10 SDK

[✓] 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 (4 available)
    • Redmi Note 5 (mobile) • 40c9f14 • android-arm64  • Android 9 (API 28)
    • 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 89.0.774.54

! Doctor found issues in 1 category.
```
</details>

https://user-images.githubusercontent.com/39104740/111855988-c9198980-8938-11eb-8a2e-60f8bbd51366.mp4

https://user-images.githubusercontent.com/39104740/111855669-40e6b480-8937-11eb-91df-c338d6d4057a.mp4