This also means low-performing clients can make the whole Desktop stutter/freeze and it is one of the many reasons the Wayland architecture is beyond idiotic. High responsiveness is obviously far more important than avoiding the occasional artifact.
Clients (apps) should still be async in Wayland; it's just the window manager that's tightly integrated. Wayland compositors should probably use some kind of fair queuing to prevent misbehaving apps from spamming the event loop but they probably don't.
Example: Windows can only be resized when the client finished drawing the new resized window. Otherwise you would get "visual artifacts". So typical operations dealing with window management can not be async when the insistence on the nonsensical "every frame is perfect" mantra is upheld.
Graceful degradation is a thing. If you don't get a response from a client that you are redrawing, you may just put a translucent shader on the window, show the user the changing borders immediately, and then resume whenever it's ready.
