Repository: flutter
Issue: On scroll to the top, CustomScrollview snaps on the pinned app bar in a NestedScrollView
Labels: framework,f: scrolling,P2,team-framework,triaged-framework

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## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

https://dartpad.dev/39d852c23bc573bb7160fad795720a54?

**Expected results:** <!-- what did you want to see? -->
The scroll does not stop when it reaches to the top and continues to expand the app bar without that slight "pause".

**Actual results:** <!-- what did you see? -->
The scroll pauses when it reaches to the top if its slow enough, and needs further pulling down to expand the flexible space of the app bar. 

**Explanation**
When a customscrollview is scrolled to the top, there is a snap when the first element is reached at the app bar. Then if the inertia was high, it will continue to expand the flexible space of the app bar. This behavior seems natural if there was no flexible space, but the problem I'm facing is that even at a decent enough flick speed, I can see a pause in the scroll and then it continues to expand the flexible space.


<details>
  <summary>Logs</summary>

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
 deepashok λ flutter doctor -v
[✓] Flutter (Channel stable, 1.22.5, on Mac OS X 10.15.7 19H2 darwin-x64, locale en-US)
    • Flutter version 1.22.5 at /Users/deepashok/Documents/programming/flutter/flutter
    • Framework revision 7891006299 (6 weeks ago), 2020-12-10 11:54:40 -0800
    • Engine revision ae90085a84
    • Dart version 2.10.4

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/deepashok/Library/Android/sdk
    • Platform android-29, build-tools 29.0.2
    • ANDROID_HOME = /Users/deepashok/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 12.2)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.2, Build version 12B45b
    • CocoaPods version 1.10.0

[✓] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin installed
    • Dart plugin version 201.9317
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] VS Code (version 1.52.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.18.1

[✓] Connected device (1 available)
    • iPhone SE (2nd generation) (mobile) • 3C94BC6D-4A4F-49E8-88B4-B6733C677453 • ios • com.apple.CoreSimulator.SimRuntime.iOS-14-2
      (simulator)

• No issues found!
```

</details>