Wayne Pan

PPK, of quriksmode.org fame, has been busy updating his events compatibility table and has uncovered some interesting bugs in the mousemove event.

When doing my new mousemove test I found a bug in IE5-7 that I was previously unaware of. When the user moves the mouse over the element, the mousemove event fires many times, as it should. However, when the user stops moving the mouse, IE5-7 continues firing the event every once in a while. This stops only when the mouse leaves the target element entirely.This is obviously a bug: when the mouse does not move the mousemove event should not fire.  

On top of that he’s stating that the bug has been copied by the Safari and Opera team. I think it has less to do with the browser implementations with more with operating system oddities.

For example, OS X Firefox 2 mousemove events are triggered when Firefox is brought back into focus (using cmd-tab, no mouse movement). However, on Windows XP sp2 Firefox does not display the same behavior! You can say this is a bug for Firefox’s mousemove event implementation or you can push that on the operating system for sending a mousemove event to firefox itself. Six in one hand, half dozen in the other.

Also interestingly, Safari 3 on OS X, the mousemove events are fired every time a key is depressed and then released! On Windows XPsp2 IE6, the mousemove events are triggered anytime the screen is redrawn. You can all sorts of fun yourself with PPK’s test page.

The bottom line is that if you happen to attach logic to the mousemove event just make sure it’s safe to call multiple times.