[barrkel]

It isn't. Normally the application needs to be aware of dirty rectangles; it fetches them from the window compositor, and it needs to limit its drawing calculations based on the dirty rectangle, in order to get the full benefit.

(Dirty rectangles are still perfectly common in desktop apps for when you e.g. drag a window back on screen after overlapping the edge, or if you scroll a window.)

[badsectoracula]

You are thinking about regions, dirty rectangles were a common thing in games to avoid redrawing the entire thing and rarely seen outside of them.