Repository: flutter
Issue: Horizontal edge scrolling is broken
Labels: a: text input,team,platform-android,platform-ios,framework,f: material design,has reproducible steps,c: tech-debt,team: skip-test,found in release: 3.3,found in release: 3.7,team-design,triaged-design

Horizontal edge scrolling appears to be broken.  I've tried it on iOS and Android emulators and on a physical Pixel 2.  I've tried TextField and SelectableText.  The bug is present in all of these cases.

CupertinoTextField seems to have slightly better luck. It sometimes feels ok on the physical Pixel 2. On the Android emulator, I see some movement, but it's not really possible to scroll to the far right in one movement.

![edge_scroll](https://user-images.githubusercontent.com/389558/90545483-922e8b80-e13d-11ea-91c5-43e0e561d777.gif)



## Steps to Reproduce

  1. Run an app with a one-line TextField (code below).
  1. Enter enough text to go beyond the edge of the screen.
  1. Scroll back to the beginning.
  1. Start a selection, and drag the right selection handle to the right edge of the input.
  1. The input doesn't scroll, and it's not possible to select the rightmost text without stopping the gesture.

<details>
<summary>Code</summary>

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Material App',
      theme: ThemeData.dark(),
      home: Home(),
    );
  }
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Material'),
      ),
      body: ListView(
        children: <Widget>[
          TextField(
            maxLines: 1,
          ),
        ],
      ),
    );
  }
}
```

</details>

<details>
<summary>flutter doctor -v</summary>

```
flutter doctor -v
[✓] Flutter (Channel master, 1.21.0-10.0.pre.114, on Mac OS X 10.15.6 19G73, locale en-US)
    • Flutter version 1.21.0-10.0.pre.114 at /Users/jmccandless/Projects/flutter
    • Framework revision b2c0970aa7 (2 days ago), 2020-08-16 06:11:02 -0400
    • Engine revision b300be3df3
    • Dart version 2.10.0 (build 2.10.0-34.0.dev)


[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/jmccandless/Library/Android/sdk
    • Platform android-29, build-tools 28.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 12.0)
    • Xcode at /Applications/Xcode-beta.app/Contents/Developer
    • Xcode 12.0, Build version 12A6159
    • CocoaPods version 1.8.4

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 3.5)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 42.1.1
    • Dart plugin version 191.8593
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] Connected device (6 available)
    • Pixel 2 XL (mobile)                 • 709KPNY0023559                       • android-arm64  • Android 10 (API 29)
    • sdk gphone x86 64 (mobile)          • emulator-5554                        • android-x64    • Android 11 (API 30) (emulator)
    • iPhone SE (2nd generation) (mobile) • 95E2AF70-6B9B-4290-931D-94E05637CBE8 • ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-0 (simulator)
    • macOS (desktop)                     • macos                                • darwin-x64     • Mac OS X 10.15.6 19G73
    • Web Server (web)                    • web-server                           • web-javascript • Flutter Tools
    • Chrome (web)                        • chrome                               • web-javascript • Google Chrome 84.0.4147.125

• No issues found!
```

</details>