Repository: flutter
Issue: [web] text selection is flickering and has wrong thresholds
Labels: framework,f: material design,a: typography,has reproducible steps,found in release: 2.0,found in release: 2.1,team-design,triaged-design

affects any character from https://www.gnu.org/software/freefont/ranges/Sinhala.html

<details>
<summary>code samples</summary>

```html
<!DOCTYPE html>
<html lang="en">
<head>
   <meta charset="UTF-8">
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>Document</title>
   <style>
      @import url('https://fonts.googleapis.com/css2?family=Roboto&display=swap');
      html, body, div {
         height: 100%; 
      }
      div {
         font-family: 'Roboto', sans-serif;
         font-size: 200px;
         display: flex;
         align-items: center;
         justify-content: center;
      }
   </style>
</head>
<body>
   <div>
      text ඞ
   </div>
</body>
</html>
```

```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(
        body: Home(),
      ),
    );
  }
}

class Home extends StatelessWidget {
  const Home({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Center(
      child: SelectableText(
        'text ඞ',
        style: TextStyle(
          fontSize: 200.0
        ),
      ),
    );
  }
}
```
</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>


https://user-images.githubusercontent.com/39104740/112761473-2a5dee80-9004-11eb-8b4a-9f238c790458.mp4