Typing CJK punctuation such as `；` or `，` with fcitx5 in the Linux
AppImage sent each character twice: WebKitGTK commits it as
insertFromComposition + compositionend without compositionstart, so
xterm's CompositionHelper finalized the previous composition's stale
position and emitted a suffix. ImeCommitGate was meant to drop that second
emission, but compositionStart() disabled it for the rest of the session
and cancelled its rebase. The gate now works per commit: compositionstart
stands down only for its own composition, every keydown 229 starts a fresh
commit, and the textarea is rebased after each commit.
A keydown the IME did not take (Enter, an arrow, a letter outside the IME)
ends the open commit, so a key pressed right after a directly committed
punctuation mark is not dropped by the commit's timeout.
Closes#1230
Co-authored-by: devadapter <[email protected]>
