This is interesting! I had implemented a similar system a while back for a mobile OS company. It was used for optimisation of the rendering of the UI scenegraph

This is interesting! I had implemented a similar system a while back for a mobile OS company. It was used for optimisation of the rendering of the UI scenegraph. The rectangles were cut(taking into account alpha blending) before feeding it to the renderer to improve performance. This was all post-layout though. Didn't realise that it can be used for layout as well!