- Don't leak 'command' events to web pages. They're only needed for XUL.
- Don't generate 'mousedown' and 'mouseup' when simlating clicks for browser UI
elements. They seem to sometimes trigger some buttons' actions twice. It also
caused background tabs to be selected before closed when clicking on their
close button, which caused the selected tab to change. However, an exception
had to be made for tabs. 'mousedown' seems to be the only relevant event
there.