Repository: flutter
Issue: Starting selection handle disappears on rich text when TextDirection is set to RTL
Labels: platform-ios,framework,a: typography,has reproducible steps,P2,found in release: 2.5,found in release: 2.6,team-ios,triaged-ios

## Steps to Reproduce
```dart
import 'package:flutter/material.dart';


void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.white,
        body: SafeArea(
            child: Directionality(
                child: Home(),
                textDirection: TextDirection.rtl,
            ),
        ),
      ),
    );
  }
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: SelectableText.rich(
        TextSpan(
          children: <InlineSpan>[
            TextSpan(text: 'Hello', style: TextStyle(fontSize: 40.0)),
            TextSpan(text: ' World', style: TextStyle(fontSize: 50.0)),
          ],
        ),
      ),
    );
  }
}
```

1. Long press first word to select first word.

Expected: Start and end selection handles show up.
![Screen Shot 2021-07-20 at 11 18 16 PM](https://user-images.githubusercontent.com/948037/126440625-4ef8c04b-7fea-4a78-a36e-86a45ef48d32.png)


Actual: Start handle is missing at the beginning of the line. When moved passed the first character it reappears.
![richrtl](https://user-images.githubusercontent.com/948037/126440737-b3b59c02-5807-4af4-90f7-a61d7e4a7b39.gif)

This behavior is observed on iOS but on Android the start handle appears normally.

<details> 
<summary>flutter doctor -v</summary> 

```bash
[✓] Flutter (Channel master, 2.4.0-5.0.pre.122, on macOS 11.4 20F71 darwin-x64,
    locale en)
    • Flutter version 2.4.0-5.0.pre.122 at /Users/roliv/flutter
    • Upstream repository git@github.com:Renzo-Olivares/flutter.git
    • Framework revision 5acf7e98f1 (3 hours ago), 2021-07-20 19:58:58 -0700
    • Engine revision 9b270621e4
    • Dart version 2.14.0 (build 2.14.0-338.0.dev)

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at /Users/roliv/Library/Android/sdk
    • Platform android-30, build-tools 30.0.3
    • Java binary at: /Applications/Android Studio/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.8+10-b944.6916264)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.5, Build version 12E262
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.2)
    • Android Studio at /Applications/Android Studio/Contents
    • 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
    • Java version OpenJDK Runtime Environment (build 11.0.8+10-b944.6916264)

[✓] Connected device (3 available)
    • iPhone 12 Pro Max (mobile) • FB10EA59-14BD-490D-B0BE-7BC05D412B59 • ios
      • com.apple.CoreSimulator.SimRuntime.iOS-14-5 (simulator)
    • macOS (desktop)            • macos                                •
      darwin-x64     • macOS 11.4 20F71 darwin-x64
    • Chrome (web)               • chrome                               •
      web-javascript • Google Chrome 91.0.4472.164

• No issues found!
```

</details>