I am trying to revive an old cross-platform graphics library that I wrote for my students over 20 years ago. After Apple pulled the plug on Carbon, I reimplemented the Mac side on top of SDL2, which was working about 10 years ago. This library is built around a persistent-screen model rather than a persistent-object model: pixels once drawn stay drawn until erased. Obviously, this model won’t work with double buffering. So I have rebuilt around this answer in order to get single buffering.