Repository: flutter
Issue: iOS Keyboard flickers when switching the TextFields which have obscureText set to true
Labels: a: text input,platform-ios,framework,f: material design,has reproducible steps,P2,found in release: 2.2,found in release: 2.5,team-ios,triaged-ios

Switching the TextFields which have `obscureText` set to true causes iOS Keyboard with AutoFill Passwords enabled to flicker. This also affects the widgets due to the change in the keyboard height. There is apparently no issue when AutoFill Passwords is disabled.

https://user-images.githubusercontent.com/12843244/129736966-5efed6d0-0298-4294-af8e-04f4939f917f.mp4

## Steps to Reproduce

1. Open Settings on iOS Simulator or an iPhone.
2. Go to Passwords.
3. Add a new password (if you haven't before).
4. Go to AutoFill Passwords.
5. Enable AutoFill Passwords.
6. Run the Flutter code below:
<details> 
<summary>minimal code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Test',
      debugShowCheckedModeBanner: false,
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Test'),
      ),
      body: SafeArea(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: <Widget>[
            Expanded(
              child: SingleChildScrollView(
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.start,
                  mainAxisSize: MainAxisSize.max,
                  children: <Widget>[
                    TextField(
                      obscureText: true,
                      decoration: InputDecoration(
                        hintText: "Obscure TextField 1",
                      ),
                    ),
                    TextField(
                      obscureText: true,
                      decoration: InputDecoration(
                        hintText: "Obscure TextField 2",
                      ),
                    ),
                    TextField(
                      obscureText: true,
                      decoration: InputDecoration(
                        hintText: "Obscure TextField 3",
                      ),
                    ),
                  ],
                ),
              ),
            ),
            ElevatedButton(
              onPressed: () {
                print('pressed');
              },
              child: Text('Button'),
            ),
          ],
        ),
      ),
    );
  }
}
``` 

</details>

7. Switch between TextFields.
8. Notice that the keyboard flickers and causes the button to flicker as well.

## Details

- I tested it on both Simulator and a real device. I got the same result.
- Flutter version is `Flutter 2.2.3 • channel stable`.
- The issue doesn't occur when I disable AutoFill Passwords.
- No difference between the debug and release modes regarding to the issue.
- This might be related: #64628.