Make the filtering and user input render asynchronously, such that there
is no latency penalty for user typing.
A single keystroke with hundreds of thousands of selectable items blocks
the script for significant time. Especially filtering by single letter
which would produce a lot of matches. This could be fixed by delaying
filtering by the time user "stops" typing, but this adds artificial
delay on top of the filtering delay itself. Instead we start the
filtering always, but have a deadline of 5 ms, at which point the
coroutine is paused and script is allowed to process user events again.
If user typed or closed the dialog, we simply abort the filtering and
start new one. Instead of waiting for full filtering to finish when the
result is already stale.
The first slice runs synchronously, so anything but huge item lists is
still filtered within the keypress handler exactly as before.
Fixes: #18268
Kacper Michajłow
e61d819bf6