I'd been seeing redraw problems in GTK+ 2 applications - sometimes, for example, the pixmaps on GNOME 2 apps' buttons wouldn't appear, or Nautilus file icons would get drawn in the wrong positions. Anyway, having been informed (by Owen Taylor) that this was a known XFree86 bug with Matrox cards, I've now installed the experimental X 4.2.0 Debian packages, and will see if this fixes the problem. (I haven't seen it yet since restarting X, anyway.)