Windows: Dead keys are not producing accented characters

[tgucio]

Steps to Reproduce

1. Run flutter create text_field_test.

2. Update main.dart as follows

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData( primarySwatch: Colors.blue),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextFormField(),
            const TextField(),
          ],
        ),
      ),
    );
  }
}

3. Set the keyboard layout to English United States - International

4. Type anything in the text field

Expected results: Characters with accents typed using two key strokes ("'" key plus letter, "`" key plus letter, "^" key plus letter) can be entered

Actual results: Accent characters are entered (repeated twice), followed by letters

> flutter analyze
Analyzing text_field_test...                                            
No issues found! (ran in 5.1s)

> flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel master, 2.4.0-1.0.pre.151, on Microsoft Windows [Version 10.0.19042.1083], locale en-US)
[X] Android toolchain - develop for Android devices
    X Unable to locate Android SDK.
      Install Android Studio from: https://developer.android.com/studio/index.html
      On first launch it will assist you in installing the Android SDK components.
      (or visit https://flutter.dev/docs/get-started/install/windows#android-setup for detailed instructions).
      If the Android SDK has been installed to a custom location, please use
      `flutter config --android-sdk` to update to that location.

[√] Chrome - develop for the web
[√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.9.4)
[!] Android Studio (not installed)
[√] VS Code, 64-bit edition (version 1.58.0)
[√] Connected device (2 available)

! Doctor found issues in 2 categories.

[tgucio]

cc @gspencergoog @dkwingsmt

[tgucio]

Possibly a regression after flutter/engine#26757
Another note: while capturing the screen recording, I also hit an issue where every keystroke would produce two charaters.

[tomm1e]

Just experienced that same issue with keys producing twice on the master build for Windows. Pressing CTRL once will activate that issue, pressing CTRL again will stop it, but not always. It even persists after closing the executable. Press CTRL once, close the app, open it again, its still producing double characters until you press CTRL again to stop it.

[tomm1e]

Double character issue is coming from: flutter/engine#27064

[knopp]

I can confirm dead keys are broken on windows. But as far as I can tell doesn't seem to be a regression after flutter/engine#26757.

[knopp]

The problem with dead keys is that when redispatching the event in KeyboardKeyHandler::RedispatchEvent the dead key buffer is already empty. So the first WM_CHAR message has proper value, but that is not used because it's handled by OnKey, second WM_CHAR (synthetized by RedispatchEvent) has character as if no dead key was pressed.

Unfortunately as far as I can tell there is no access to dead key state (i.e. there are numerous threads about people complaining about ToUnicode modifying dead key state (there is now a flag for it in ToUnicodeEx to not do that, but that doesn't help here).

I don't think the event handling as it works today can handle dead keys properly.

/cc @dkwingsmt

[knopp]

Possible solution for this would be to

1. Do not redispatch dead keys (this breaks dead key state)
2. Preserve character from the original WM_CHAR message (which has the proper accent) and use that instead in the synthetized WM_CHAR message.

[knopp]

Well, I tried it and even with this in place it's still not possible to reproduce the exact windows behavior. For example
' (acute) followed by f (which is "illegal" combination) generate string 'f with following sequence of messages:
WM_DEADCHAR: ' (actute)
WM_CHAR: ' (actute)
WM_CHAR: f

If we try to redispatch the first WM_CHAR event ('), it will alter the global dead key state. Unfortunately it seems that SendInput as it is used now is simply too crude for this.

[knopp]

There is one more option that might work and should handle the case above:

1. Only redispatch events when OnKey is called from WM_KEYDOWN/WM_KEYUP. (this will avoid redispatching dead key down events. it will redispatch dead key up events, but that doesn't seem to affect dead key state fortunately).
2. In WM_CHAR call OnText either uncondtionally, or if no KeyboardKeyHandlerDelegate delegate handles the event (would be more work). This way we wouldn't disturb dead key state and it should be possible to have exact same behavior as other window apps.

[knopp]

This is a very quick and dirty demonstration of the approach above: knopp/engine@4083753

It gets dead keys working, but it is hacky, doesn't handle surrogates and also doesn't handle the case when dead key doesn't combine with character. i.e. ' (acute) + f should result in 'f (there are two WM_CHAR messages in this case: WM_DEADCHAR: ' (acute), WM_CHAR: ' (acute), WM_CHAR: f)

[TahaTesser]

Hi @tgucio
Thanks for filing the issue, I can confirm the regression.

flutter doctor -v

[✓] Flutter (Channel master, 2.4.0-1.0.pre.189, on Microsoft Windows [Version 10.0.22000.65], locale en-US)
    • Flutter version 2.4.0-1.0.pre.189 at C:\Users\Taha\Code\flutter_master
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision d6a844360a (2 hours ago), 2021-07-12 05:06:04 -0400
    • Engine revision 7851449b4d
    • Dart version 2.14.0 (build 2.14.0-301.0.dev)

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at C:\Users\Taha\Code\android-sdk
    • Platform android-30, build-tools 30.0.3
    • ANDROID_SDK_ROOT = C:\Users\Taha\Code\android-sdk
    • Java binary at: C:\Users\Taha\Code\android-studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 11.0.8+10-b944.6842174)
    • All Android licenses accepted.

[✓] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[✓] Visual Studio - develop for Windows (Visual Studio Community 2019 16.10.3)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
    • Visual Studio Community 2019 version 16.10.31424.327
    • Windows 10 SDK version 10.0.19041.0

[✓] Android Studio (version 4.2.0)
    • Android Studio at C:\Users\Taha\Code\android-studio
    • 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-b944.6842174)

[✓] VS Code (version 1.58.0)
    • VS Code at C:\Users\Taha\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.24.0

[✓] Connected device (5 available)
    • IN2011 (mobile)         • c9d8ee0c • android-arm64   • Android 11 (API 30)
    • Windows (desktop)       • windows  • windows-x64     • Microsoft Windows [Version 10.0.22000.65]
    • Windows (UWP) (desktop) • winuwp   • windows-uwp-x64 •
    • Chrome (web)            • chrome   • web-javascript  • Google Chrome 91.0.4472.124
    • Edge (web)              • edge     • web-javascript  • Microsoft Edge 91.0.864.67

• No issues found!