Fcitx5 IME with preedit composition not working in Firefox

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Is there an existing issue for this?

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Does the issue happen when logged in?

Yes

Does the issue happen when logged out?

Yes

Does the issue happen in incognito mode when logged in?

Yes

Does the issue happen in incognito mode when logged out?

Yes

Account name

bczhc

Account config

No response

Current Behavior

Can't start typing using Fcitx5 input method, with "preedit" composition mode. It's OK when preedit is disabled.

Expected Behavior

Type normally.

Steps To Reproduce

1. Set up Fcitx5 IME and add an input method that needs composing (like Chinese or Japanese). In this case I'm using fcitx5-rime with schema "朙月拼音".
2. Go to Monkeytype with Firefox.
3. Select the input language to the desired one. I use "Chinese Simplified".
4. Toggle the input method (the shortcut I'm using is Super+space) and start typing.
5. Observed: The composition view won't show, and it's still typing alphabets.

Environment

• OS: Arch Linux rolling; kernel 6.11.0-modified-dirty.
• Browser: Firefox
• Browser Version: 131.0

Anything else?

Screenrecord demonstration:

https://github.com/user-attachments/assets/8b26971d-7133-47b1-9d98-d29c0ae217d1

Start by reproducing the issue in Firefox on Arch Linux with Fcitx5 and a composing input method such as fcitx5-rime using the steps in the report. Trace Monkeytype's typing and composition input handling, then verify that preedit composition works without breaking ordinary typing.