Backspacing over the last radical of a Cangjie composition empties the IME's
marked text without reaching compositionend, and the vendored xterm
CompositionHelper only dropped the overlay's `active` class there. The box
stayed painted with whatever glyph it last held (#11951).
Clear on the state rather than on the key, as native terminals do: an empty
`compositionupdate` now hides the overlay instead of only ever showing it, and
a key the IME swallows re-derives the preedit from the textarea once it settles
so a composition emptied with no composition event at all is cancelled too.
Neil
17ef6ccce6