Comment 9 for bug 771308

same problem. had it while testing the beta in virtualbox (I remember seeing someone posting a workaround for it then, but I can't remember it now).

I still encounter it on a clean install on my laptop. seems fairly random when it turns up, but effects the top bar and applications as described by the other posters. for me, choosing different themes in the Appearance dialogue didn't fix it, I just restart (or, I'm sure, anything that restarts the xserver would fix it).