Repository: flutter
Issue: [iOS] [A11y] `ListView`'s addSemanticIndexes/`IndexedSemantics` don't announce correctly with VoiceOver three-finger tap
Labels: platform-ios,framework,a: accessibility,a: fidelity,f: scrolling,has reproducible steps,P2,found in release: 2.5,found in release: 2.6,team-ios,triaged-ios


## Steps to Reproduce

1. Have a ListView.builder with `addSemanticIndexes: true` (See code sample)
2. Turn on VoiceOver
3. Focus a11y on an item of the ListView & tap with three fingers

**Expected results:**

As documented [here](https://api.flutter.dev/flutter/widgets/IndexedSemantics-class.html), VoiceOver should announce additional info like the index of the focused item.

Apple's document: 

> Three-finger tap: Speak additional information, such as the position within a list or whether text is selected

https://support.apple.com/guide/iphone/learn-voiceover-gestures-iph3e2e2281/ios


https://user-images.githubusercontent.com/37970784/140700862-051595b2-436c-43ac-925c-66d488528e6c.mov

**Actual results:**

It only announces the absolute position of the item like "center of screen, full width of screen".

https://user-images.githubusercontent.com/37970784/140702830-bd389bc2-0882-43b4-975d-86b3309ff93d.mov

<details>
<summary>Code sample</summary>

```dart
ListView.builder(
  itemCount: 50,
  addSemanticIndexes: true,
  itemBuilder: (context, index) {
    return Column(
      children: [
        Container(
          color: Colors.blue,
          child: Text('Hello world'),
        ),
        Container(
          height: 50,
          child: Text('Test index: $index'),
        ),
      ],
    );
  },
)
```

</details>

<details>
<summary>flutter doctor -v</summary>

```
[✓] Flutter (Channel stable, 2.5.3, on macOS 11.6 20G165 darwin-x64, locale en)
    • Flutter version 2.5.3 at
      /Users/nguyenthang/Desktop/ThangDev/FlutterSDK/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 18116933e7 (3 weeks ago), 2021-10-15 10:46:35 -0700
    • Engine revision d3ea636dc5
    • Dart version 2.14.4

[✓] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at /Users/nguyenthang/Library/Android/sdk
    • Platform android-31, build-tools 31.0.0
    • ANDROID_HOME = /Users/nguyenthang/Library/Android/sdk
    • Java binary at: /Applications/Android
      Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 13.1, Build version 13A1030d
    • CocoaPods version 1.11.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2020.3)
    • Android Studio at /Applications/Android Studio.app/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 11.0.10+0-b96-7281165)

[✓] VS Code (version 1.59.0)
    • VS Code at /Users/nguyenthang/Downloads/Visual Studio Code.app/Contents
    • Flutter extension version 3.27.0

[✓] Connected device (2 available)
    • Testflight’s iPad (mobile) • 30f479e7ee4c7fd9518711dd7017eb75383a46db •
      ios            • iOS 14.4.1 18D61
    • Chrome (web)               • chrome                                   •
      web-javascript • Google Chrome 95.0.4638.69

• No issues found!
```

</details>