Before Windows Vista, Windows apps normally didn't have framebuffers. There was one framebuffer for the whole screen, and apps drew into it. That's why dragging another window over a non-responsive window left a "slime trail" - the non-responsive window wasn't redrawing parts of itself when the window covering them moved away.

> many modern computers are using unified memory.

Not really true. Even machines with integrated graphics in Windows aren't truly using a fully shared memory pool. Usually the hardware reserves a chunk of the system memory for the iGPU.