From: https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=237842 gnome-session is pretty dumb, and tries to grab the X server to draw its iris effect for the log out dialogue. Because it grabs the server, compiz can't actually draw the iris. The "fix" is to completely disable the iris when using a compositing manager (or at least compiz), but then there's no effect showing the logout dialogue. It would be nice if compiz could "fade" the whole desktop to gray when a desktop modal dialogue shows up.
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