Comment 54 for bug 568988

I've actually noticed that this "backfill" behavior itself seems to cause glitches on my Intel netbook. When I use the "vacuum" animation for menu creation, new menus grab bits of the window behind them and drag those bits along, until the real content finishes appearing. That's exactly what Xorg deliberately does: it copies the contents of underlying windows into new windows.
I'll test it with the backclear patch now.