undo by ctrl-z is too sticky

[robx]

If you press cmd-z on firefox/mac (but likely same for ctrl-z elsewhere), undo starts, and keeps going. The idea seems to be for this to go on until you lift the keys, however that only seems to work if you lift cmd first, otherwise it keeps erasing all the way to the start.

[robx]

The issue is fixed for ctrl-z, but persists for cmd-z, because for whatever reason the keyup event is lost.