Do any operating systems expose mixed buffering (for real screen rendering) to applications? I know Windows used to allow full-screen apps to implement single buffering, but I don't know if it could seamlessly transition into double buffering.
I think that it is up to the toolkit you use. For example with Wayland you could work with a single buffer and commit it, or work with two buffers and swap them. Moreover you can do things on the client side, say:
3 comments
[ 2.2 ms ] story [ 14.1 ms ] threaddrawing-buffer (single) [user copies to] -> commit-buffer ---------------------------------- [system copies to] -> screen
alternatively
drawing-buffer (double) [user swaps to] -> commit-buffer ---------------------------------- [system copies to] -> screen
etc.