TextFormField - changing obscureText with state has a bug

[callawey]

Steps to Reproduce

1. Run flutter create bug.
2. Update main.dart as below

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Bug'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool _hidden = true;

  void _changeVisibility() {
    setState(() {
      _hidden = !_hidden;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Container(
          width: 300,
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              TextFormField(
                obscureText: _hidden,
                decoration: InputDecoration(
                  contentPadding: const EdgeInsets.all(0),
                  prefixIcon: Icon(Icons.lock_outline),
                  border: const OutlineInputBorder(),
                  labelText: "labelText",
                  suffixIcon: GestureDetector(
                    onTap: () {
                      _changeVisibility();
                    },
                    child: Icon(_hidden ? Icons.visibility_off : Icons.visibility),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

3. run application on your android device, type something to textfield, click eye icon to change obscureText to false. Now you can see your entry. hide them again. delete a few characters. write something else. Click again to see your entries. Now you cant see first a few characters, they are still obscure.

Expected results:
when its state changes, it must show all

bug-2020-12-22-224631.mp4

or hide all of the characters.

[√] Flutter (Channel beta, 1.24.0-10.2.pre, on Microsoft Windows [Version 10.0.19042.685], locale en-GB)
    • Flutter version 1.24.0-10.2.pre at D:\Flutter
    • Framework revision 022b333a08 (5 weeks ago), 2020-11-18 11:35:09 -0800
    • Engine revision 07c1eed46b
    • Dart version 2.12.0 (build 2.12.0-29.10.beta)

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at D:\Android\SDK
    • Platform android-30, build-tools 29.0.2
    • 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.

[√] 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, 64-bit edition (version 1.52.1)
    • VS Code at C:\Program Files\Microsoft VS Code
    • Flutter extension version 3.17.0

[√] Connected device (1 available)
    • SM G960U1 (mobile) • 3054524d4a313498 • android-arm64 • Android 10 (API 29)

• No issues found!

[LongCatIsLooong]

Duplicate of #71207. FYI it's already fixed on master.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.