Repository: flutter
Issue: [Desktop] Incorrect backspace handling with combined Khmer Unicode characters
Labels: a: text input,engine,platform-mac,a: desktop,has reproducible steps,P2,found in release: 3.0,found in release: 3.1,fyi-text-input,team-macos,triaged-macos

## Steps to Reproduce

1. Execute `flutter run` on the code sample
2. Using a Khmer Unicode keyboard, type កុំ (K + U + Shift + M) into the text field and pressing Backspace to delete each character

**Expected results:** <!-- what did you want to see? -->

Pressing backspace should only remove one character at a time.

**Actual results:** <!-- what did you see? -->

Pressing backspace deletes all of the combined characters (only on Desktop).

The incorrect backspace behaviour on Desktop seems to apply to any combined characters, e.g. កី (K + Shift + I), តើ (T + ;), as it seems to consider them as one character.

_(I haven't had the occasion to test on any mobile platform.)_

I've also attached a demonstration video below:

https://user-images.githubusercontent.com/6465682/180672657-61c947ed-0023-431f-a232-074fc6da8c46.mov

<details>
<summary>Code sample</summary>

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```dart
import 'package:flutter/material.dart';
// import 'package:flutter_tts/flutter_tts.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
        home: Scaffold(
            body: Padding(
      padding: EdgeInsets.all(8.0),
      child: TextField(),
    )));
  }
}
```

</details>

<details>
  <summary>Logs</summary>

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     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
-->

```
Analyzing flutter_tuto...                                               
No issues found! (ran in 1.2s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
[✓] Flutter (Channel stable, 3.0.5, on macOS 12.4 21F79 darwin-arm, locale en-FR)
    • Flutter version 3.0.5 at /Users/fidele007/Projects/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision f1875d570e (11 days ago), 2022-07-13 11:24:16 -0700
    • Engine revision e85ea0e79c
    • Dart version 2.17.6
    • DevTools version 2.12.2

[✓] Xcode - develop for iOS and macOS (Xcode 13.4.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • CocoaPods version 1.11.3

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] IntelliJ IDEA Ultimate Edition (version 2022.1)
    • IntelliJ at /Applications/IntelliJ IDEA.app
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart

[✓] VS Code (version 1.69.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.44.0

[✓] Connected device (2 available)
    • macOS (desktop) • macos  • darwin-arm64   • macOS 12.4 21F79 darwin-arm
    • Chrome (web)    • chrome • web-javascript • Google Chrome 103.0.5060.134

[✓] HTTP Host Availability
    • All required HTTP hosts are available

• No issues found!
```

</details>


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