&gt; And vsync across cards. That is quite a feat.<p>Well, what it does is, that the compositor (DWM) keeps a (small) backlog of presented window framebuffers (think GL SwapBuffers, that add to the queue) and whenever a screen&#x27;s VSync comes along it blits the most recent picture presented to it. Which has the effect that if you have a window spanning multiple GPUs&#x27; outputs they may not all show the same frame, because the display refresh frequencies <i>will</i> beat (even if you set the same refresh frequency on all outputs, since between different GPUs they&#x27;re not driven by the same clock, you&#x27;ll get a little bit of clock skew; and video clocks are usually not of the low drift type, because for video it doesn&#x27;t matter if you&#x27;re off a few ppm). If you need synchronized video sync vendors are happy to sell you genlock-able cards for $$$.