[renox]

> Note that X Windows, despite having a window server, did not use "retained mode" rendering and still required the app to respond to do every repaint such as if an occluded window was moved to the top.

My memory are quite fuzzy but I think that this is incorrect here, what you're describing was only the default behaviour: you could program your application to tell the X Window server to use a 'backing store' for your window and it would memorize your window to re-render it by himself.

[zigzigzag]

There is/was an extension that did that at some point, but it was never used (on Linux) due to poor implementation and a desire to target low RAM machines. Mac's backing store implementation was quite heavily optimised - macOS can do on the fly memory compression and the window server uses shared memory to obtain the window bitmaps.