Not being a browser developer, I can't say for sure, but it does look that way. I assume that the browser must redraw the position:fixed div as the page scrolls - and it seems to forget in this case (there are nested divs inside the fixed div.) The elements in the div never reappear unless you scroll back up. Sort of defeats the purpose of :fixed.
It's a bit frustrating - one of the natural uses of position:fixed would be for menus. But it's not nice to have the navigation vanish altogether when you scroll! Fortunately, this isn't my site. It's a sample site for an open architecture community system that I was checking out.
Early Gecko engine browsers used to let a fixed div scroll for a few pixels before redrawing in the original position - making the whole div jitter around in a very disturbing fashion.