https://bugs.freedesktop.org/show_bug.cgi?id=34427
--- Comment #10 from Michel Dänzer michel@daenzer.net 2011-02-23 08:38:46 PST --- I guess it's most likely an xserver issue. E.g. commit 6dd775f57d2f94f0ddaee324aeec33b9b66ed5bc ('composite: Don't backfill non-bg-None windows') looks possibly relevant.
Sounds like bug 27596 btw (though AFAICT that was reported before the change above was made).