Comment by gblargg

The key seems to have been recognizing the utility of the region concept and making it fundamental to the QuickDraw API (and the clever representation that made finding the main rectangular portions easy). This insulated QuickDraw from the complexity of windowing system operations. Once you go implementing region operations you probably find that it's fairly efficient to work out the major rectangular regions so you can use normal graphics operations on them, leaving small areas that can just be done inefficiently as a bunch of tiny rectangles. All this work for clipped graphics was applicable to far more than just redrawing obscured window content, so it could justify more engineering time to polishing it. Given how easy they were to use, more things could leverage the optimization (e.g. using them to redraw only the dirty region when a window was uncovered).