I also tested on Xubuntu 22.10, which I happen to have access to. The reason is that I there (unlike on GNOME) can disable ibus completely.
Using the Mozilla tool you linked to, and with ibus disabled, I get:
compositionstart: compositionend:
in the event log (even if the û character is shown as expected).
Then I enabled ibus, and got this in the event log:
compositionstart: compositionupdate: ^ compositionupdate: û compositionend: û
So ibus is indeed involved somehow, but not (for me) in a way that results in an issue.
I also tested on Xubuntu 22.10, which I happen to have access to. The reason is that I there (unlike on GNOME) can disable ibus completely.
Using the Mozilla tool you linked to, and with ibus disabled, I get:
compositionstart:
compositionend:
in the event log (even if the û character is shown as expected).
Then I enabled ibus, and got this in the event log:
compositionstart:
compositionupdate: ^
compositionupdate: û
compositionend: û
So ibus is indeed involved somehow, but not (for me) in a way that results in an issue.