> The "single buffer with invalidation" model of Win16 (I cannot remember how it works in X)

Same way, they both come from Macintosh (which, if i remember the apocrypha correctly, was Bill Atkinson's idea based on what he thought Xerox Smalltalk was doing even if it turned out it wasn't working like that).

eh, there is nothing a gpu can do here within the concept of composition that a cpu could not also do. the gpu simply has buffers that it compsits, the cpu can do that as well. with the benefit of less complexity leading to not needing to worry about driver crashes. on sane architectures its all the same ram anyway