I will note that Flash also had some really terrible redraw logic; if you had a largely static image with something animating in the upper left and lower right corners, the dirty rectangle would cover the whole thing.
Around the Flash Player 10 days, I remember seeing a presentation from someone at Adobe that showed off their debug feature that allows developers to see dirty rectangles. The demo showed that the renderer would use up to two dirty rectangles when redrawing. In your example with two corners, it wouldn't be one giant rectangle that filled the full stage. It would be two smaller rectangles, as one might expect. However, once something was moving in a third corner, one of the two rectangles would need to expand.
Again, this was probably 3-5 years ago, and older versions of Flash Player may have been less optimized. This may be one of the many optimizations they made to Flash Player after Apple decided it wasn't fast enough for iPhones.
