Even with modern graphics cards there's a global framebuffer for the entire desktop. It's updated every frame by the operating system

Is there though?

https://docs.kernel.org/gpu/amdgpu/display/mpo-overview.html

Plane independent page flips - No need to be tied to global compositor page-flip present rate, reduced latency, independent timing.

There's at least a buffer for scanout but that doesn't necessarily have to be larger than one line and I don't believe it's end use accessible.

As an example. Consider running a game at one refresh rate in windowed fullscreen (or whatever it's called) mode with your compositor at a different refresh rate. With your desktop spanning two monitors, one operating at 60 Hz the other at 50 Hz. And throw in a media player with a window that's split across those two monitors rendering a 29.97 fps (ie NTSC) video stream via hardware accelerated decoding.