@sinpowei thanks for making this, and extra marks for adding the documentation in the manual :smile:.
The way I read the Scintilla documentation, how the IME defaults is platform dependent, so setting neither may be different to setting one or the other, so the current behaviour of not setting any ime interaction should be preserved as the default.
So if you follow @b4n's suggestion you need three options "none", "inline" or "windowed" or you can follow my suggestion of setting only inline in an if and retain the boolean setting.