Repository: flutter
Issue: SelectableText background offsetted from the selection by 1 pixel
Labels: framework,f: material design,a: quality,has reproducible steps,found in release: 2.0,found in release: 2.1,team-design,triaged-design

`background` and the selection have a small 1 pixel offset from each other

<details>
<summary>screenshot</summary>

![woow](https://user-images.githubusercontent.com/39104740/112696383-c669e780-8e96-11eb-9b3d-4ce4b76a541a.png)
</details>

<details>
<summary>code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    const title = 'code sample';
    return MaterialApp(
      title: title,
      home: Scaffold(
        backgroundColor: Colors.black,
        body: Home(),
      ),
    );
  }
}

class Home extends StatelessWidget {
  const Home({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Center(
      child: SingleChildScrollView(
        child: Theme(
          data: ThemeData(
            textSelectionTheme: TextSelectionThemeData(selectionColor: Colors.red),
          ),
          child: SelectableText.rich(
            TextSpan(
              children: [
                TextSpan(text: 'wo',
                  style: TextStyle(
                    backgroundColor: Colors.green,
                    fontSize: 400.0,
                  ),
                ),
                TextSpan(text: 'rd',
                  style: TextStyle(
                    color: Colors.white,
                    fontSize: 400.0,
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

```
</details>


<details>
<summary>flutter doctor -v</summary>

```
[√] Flutter (Channel master, 2.1.0-13.0.pre.288, on Microsoft Windows [Version 10.0.19041.867], locale ru-RU)
    • Flutter version 2.1.0-13.0.pre.288 at c:\dev\src\flutter
    • Framework revision e384ca7979 (80 minutes ago), 2021-03-26 17:34:03 +0100
    • Engine revision b5e15d055d
    • Dart version 2.13.0 (build 2.13.0-162.0.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 (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 89.0.774.57

• No issues found!
```
</details>