If you scroll down, the menu slides to fullsize but the offset of the display menu entries does not dynamically fit to the new height of the menu... so you can't click it, it disappears immediately...
I have the same problem. It's happening if you scroll down a large page. If you hover over a menu entry with subentries in the header navigation, you can't select an entry in the list because it gets hidden if you want to select an entry with the mouse.
Did you meet the same issue on the demo site?