Original Description:
Click on the button and hold down the mouse while keeping still. If the mousedown event doesn't appear in the text box, move the mouse slightly to trigger the event. Test movie is here. The behaviour on the button square is this:
This bug can be seen in Safari 1 and Safari 2. KeyDown events can also get held up, though it is harder to identify the exact cicumstances that cause the bad behaviour (though moving the mouse does seem to cause all the 'blocked' keydown events to fire off).