Treeview 'spillage' problem

Hi guys,
I think this is a problem of my own making but I can't track it down and was wondering if any one else had stumbled upon it. My treeview menu spills out over the page 'wrapper' instead of invoking the vertical scrollbar. This doesn't happen in ie6 and as thats my default browser I didn't catch the problem sooner but it does in every other browser I've tried.

thanks in advance

Tony