Repository: flutter
Issue: [web, desktop] SelectableText - right click selects a word
Labels: framework,f: material design,platform-web,a: desktop,has reproducible steps,P2,found in release: 3.0,found in release: 3.1,team-design,triaged-design

### expected

on right click over SelectableText, if nothing is selected:
- on web - generic context menu
- on desktop (windows) - nothing to happen. i'm not sure about macos and linux

### actual

on both platforms a word is selected and selection context menu opened

<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(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Home(),
      ),
    );
  }
}

class Home extends StatelessWidget {
  const Home({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: [
          SelectableText(List.generate(100, (index) => 'word$index').join(' ')),
        ],
      ),
    );
  }
}
```
</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/39104740/112683512-b7793a00-8e82-11eb-9afc-ad9cea3b840d.mp4