Comment by atombender

So when the OS needs to refresh a portion of the screen (e.g. everything behind a top window that was closed), what happens?

My guess is it asks each application that overlapped those areas to redraw only those areas (in case the app is able to be smart about redrawing incrementally), and also clips the following redraw so that any draw operations issued by the app can be "culled". If an app isn't smart and just redraws everything, the clipping can still eliminate a lot of the draw calls.