web: Implement basic IME

[kjarosh]

Basically, if we ignore composing input events, we can just accept the final composed text as key presses (not as text composition).

This is the first step towards implementing IME on web.

IME mechanics are not being used, no preview is available, but inserting text with IME should work.

• Progresses Can't enter Korean/Chinese/Japanese in textboxes (IME support) #1778

[danielhjacobs]

This isn't working on Android when I try to type in an EditText with the virtual keyboard.

[danielhjacobs]

A virtual keyboard like the one on Android isComposing.

[danielhjacobs]

To explain the current logic, which I guess may need comments.

1. If you type a single character using the virtual keyboard, that character fires a keydown and then keyup event into the focused EditText.
2. If you backspace or delete a single character using the virtual keyboard, that backspace/delete fires a keydown and then keyup event into the focused EditText.
3. If you paste a string using the virtual keyboard, each character in that string in sequence fires a keydown and then keyup event into the focused EditText.

[danielhjacobs]

I'm guessing the IME logic can be used to support this more properly, but landing this PR without IME support would regress the virtual keyboard.

[danielhjacobs]

Maybe we can do exactly this but also keydown and keyup the event.data character(s) on compositionend.

[kjarosh]

@danielhjacobs can you check if the current code works properly? It does for me

[danielhjacobs]

Tried with GBoard and it worked perfectly. With Samsung Keyboard it's unfortunately a different story, see recording.

Screen_Recording_20250329_171917_Chrome.online-video-cutter.com.mp4

[kjarosh]

It seems that this keyboard uses IME for inputting all text. Without implementing IME on web we cannot have both IME preview and IME input working :/ This PR breaks IME preview, but fixes IME input.

[kjarosh]

Okay, as IME on web is a mess, I've decided to implement basic IME mechanics (including preediting) on web.

@danielhjacobs @n0samu You can test it out here: https://kjarosh.github.io/ruffle/pr19896/