[mythtv-users] Black square in top-right corner

[Ronald Frazier]

Every now and then when I load up x-windows, I get a small black
square in the top right corner. It almost looks like a text mode mouse
cursor:
http://www.ronfrazier.net/mythtv/blacksquare.jpg

If I alt-tab between apps, or if I use Alt-F[1-4] to switch between
desktops, the black square will flicker away for an instant and then
reappear. If I close and restart xwindows it will usually disappear,
though occasionally I have to do it a couple times before it goes
away.

Any ideas what it could be?

-- 
Ron