We've found the cause for this crash, it should be fixed in a future beta.
Workaround: in the stable channel only, install ibus-anthy for the Japanese (Rōmaji) OSK, or ibus-hangul for the Korean OSK. (Those package names are valid for at least Arch and Debian, other distros might vary.) If you want to type in Japanese or Korean, you'll probably need those packages installed anyway.
This workaround will not work in the beta channel. A proper fix for that is in progress.
when I switch back to the Steam Deck Stable client (1705108172) from desktop mode, OSK works fine
I suspect that the Steam Deck already has ibus-anthy and/or ibus-hangul installed as part of SteamOS. If that's the case, then its situation is equivalent to the workaround I described.
This should be fixed in latest Steam beta client on Steam Deck.
Closing per the last comment.
Nothing extracted yet.
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In addition to "Japanese (Rōmaji)", "Korean" also causes OSK to crash.
I have confirmed this issue on Arch Linux, occurring in both Beta channel (1705720677) and Stable channel (1705108172).
This problem also occurs with the SteamOS / current Steam Deck Beta client (1705720677), but strangely enough, when I switch back to the Steam Deck Stable client (1705108172) from desktop mode, OSK works fine. (Are there different builds even with the same version number? I don't know)