Mixed refresh rates do not work because X technically is not doing multi monitor. Both monitors are rendered from the same “screen” that uses one refresh rate. If it’s running at 144hz, the 60 fps screen gets frame pacing issues. If it runs at 60, then the 144hz monitor is slow and gets frame pacing issues, and from most anecdotes and videos I’ve seen, it’s usually the latter and a pain to fix. If you wanted perfect frame pacing on both, you’d have to have the X11 screen set to 8640hz, which I don’t even think can render on modern systems. Wayland, on the other hand, just has multi monitor support built in and actively used. Each display has its own screen and renders at its preferred refresh rate, giving perfect frame rates and frame times for both.
Mixed refresh rates do not work because X technically is not doing multi monitor. Both monitors are rendered from the same “screen” that uses one refresh rate. If it’s running at 144hz, the 60 fps screen gets frame pacing issues. If it runs at 60, then the 144hz monitor is slow and gets frame pacing issues, and from most anecdotes and videos I’ve seen, it’s usually the latter and a pain to fix. If you wanted perfect frame pacing on both, you’d have to have the X11 screen set to 8640hz, which I don’t even think can render on modern systems. Wayland, on the other hand, just has multi monitor support built in and actively used. Each display has its own screen and renders at its preferred refresh rate, giving perfect frame rates and frame times for both.