Comment 28 for bug 263435

What I suggest is that we identify a small handful of crucial keystrokes that we
never give the plugin. Perhaps only a single keystroke that means "Exit plugin".
It could be something uncommon. This way if the plugin swallows all keystrokes
there's still an escape hatch.

All other keystrokes get sent to the plugin, which can either use them or send
them back to us.