Openbox and Mythfrontend: video displayed in background

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https://bbs.archlinux.org – I switched to Openbox from KDE. Now when I play video with Mythfrontend and change to another desktop I see some pixels changing colour. The pixels were originally black or at least nearly black. This didn't happen before with KDE. I've set Mythfrontend to use VDPAU for video playback. I don't run xcompmgr right now, though I did earlier but closed it because of buggy shadows (Opera Browser had some trouble).Mythtv Frontend: 0.25.3-1-g3800761-dirty (I think it's aur-package, I'm not going to update to 0.26 yet, because my server is running 0.25)Nvidia drivers: 304.60-1 (HowTos)