using jQuery('#xtree\\:mydiv').scrollWidth instead of jQuery('#xtree\\:mydiv').width goes, but there is a strange appening.
I put my onmouseover in the rich:panel and the onmouseover is called 2 times instead of 1 ... and... when move the mouse directly from one treeitem to other, onmouseover is called!!!
This appens because the div is not recognized like an entire element, but it is the space where its children aren't (for this reason the browser start out and over few times).
I solved it using "stop" and "clearQueue" but it seems a jQuery's problem, not richfaces's.