Comment 9 for bug 1751593

It's two different things imho. _NET_WM_SYNC is not to avoid black box flickering of the drop shadows on X11 (as these are outside the client toplevel window), it's to synchronize frames redraw and avoid resizing faster than the client can cope.

For reference, see Owen's description of the the extended synchronisation version of _NET_WM_SYNC used by gtk+ and mutter.

It could be a Xwayland bug, possibly, but so far I haven't really found hard evidence of that.

Besides, on X11/Xwayland with server side decorations, mutter draws the drop shadows underneath the windows.