Repository: flutter
Issue: SelectableText with custom scrollPhysics inside SingleChildScrollView inconsistent scroll behaviour
Labels: a: text input,framework,f: scrolling,d: api docs,has reproducible steps,P2,found in release: 3.7,found in release: 3.9,team-framework,triaged-framework

## Steps to Reproduce

1. Wrap `SelectableText` with `SingleChildScrollView` - all is ok
2. Add some custom scrollPhysics - scrolling either doesn't work at all, or scrolls with unexpected behavior

Code repoducible also available here
https://github.com/nt4f04unds-archive/issues/tree/flutter_71342 

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(App());
}

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'SelectableText with AlwaysScrollableScrollPhysics',
      localizationsDelegates: [],
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  const HomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SingleChildScrollView(
        child: SelectableText(
          List.generate(1000, (index) => index.toString()).join(" "),
          scrollPhysics: AlwaysScrollableScrollPhysics(),
          // or
          // scrollPhysics: AlwaysScrollableScrollPhysics(parent: ClampingScrollPhysics()),
          // or
          // scrollPhysics: ClampingScrollPhysics(parent: AlwaysScrollableScrollPhysics()),
          // or
          // scrollPhysics: AlwaysScrollableScrollPhysics(parent: BouncingScrollPhysics()),
          // or
          // scrollPhysics: BouncingScrollPhysics(parent: AlwaysScrollableScrollPhysics()),
          // or ...
        ),
      ),
    );
  }
}
```

<details>
  <summary>Logs</summary>

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
PS C:\dev\projects\issues> flutter doctor -v
[√] Flutter (Channel master, 1.24.0-8.0.pre.374, on Microsoft Windows [Version 10.0.19041.630], locale ru-RU)
    • Flutter version 1.24.0-8.0.pre.374 at c:\dev\src\flutter
    • Framework revision 183f0e797a (2 days ago), 2020-11-26 19:12:28 +0100
    • Engine revision 20caf54969
    • Dart version 2.12.0 (build 2.12.0-76.0.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.

[√] 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 1.51.1)
    • VS Code at C:\Users\danya\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.16.0

[√] Connected device (2 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)

• No issues found!
```

</details>