[bitwize]

My understanding was that the Xlib protocol's asynchrony was hardly wonderful, and made syncing with vblank and pixel-perfect frames difficult, which motivates its abandonment in favor of the synchronous, local-host-only Wayland protocol.

[dllthomas]

Certainly, the X protocol's asynchronous nature isn't without some downsides (though I think you could address the vblank sync without discarding it). However, I maintain that Xlib itself was a synchronous interface in front of an asynchronous protocol - which gives us the worst of both worlds and motivated Xcb.