the one thing i've learned after fixing CJK input across 23 open-source PRs: if you don't have a regression fixture for IME composition, it breaks again within six months.
this isn't theoretical. i've watched the same compositionend-on-blur bug re-enter codebases that had no IME test at all. the fix goes in, the bug comes back, nobody notices until a Japanese or Chinese user files a report.
here's how to actually lock it down.
why IME input is different from regular keyboard input
when a user types Japanese on a standard western keyboard, they're not pressing letter keys one-to-one. they type romaji (the romanized phonetic form), and the operating system's IME intercepts those keystrokes to build a candidate string. the user sees a temporary underlined "composition string" and then confirms it -- either by pressing Enter/Space or by selecting a candidate.
