On this site, left menu, when you hold your mouse button down on a menu item it seems the menu(s) underneath shift upward.
I tried to clear any floats, thinking this was the culprit. But I don’t seem to see anything that would be causing it.
Solutions?
The behavior you’re describing would be associated with a:active. I don’t see any problem in Firefox. Is the problem browser specific?
It happened in all browsers.
Funny thing was I thought it was caused by the a:active as well. What I wound up doing was removing the ‘height: 22px’ in the CSS and it doesn’t do it anymore in any of the browsers.
The height that you deleted was apparently overriding the default (lesser) height when :active.
Glad you found a fix. 
It was the ul li a:active, a:hover, and a:visited that I removed it from.
Yes, they must have been overriding the height assigned to a:link. Happens sometimes when you least expect it.
Cheers!