Why Facebook topbar with Position:fixed not doing repaint

I'm optimizing my scrolling performance. I use position:fixed for my topnav and I noticed that some of the performance issue when I scroll up and down. I use Show paint rectangles and it's clearly shown that the nav is being repainted all the time. However, when I look at Facebook.com their navbar is using position:fixed as well but I don't see their navbar being repainted when I scroll up and down.