Shadow of Xwayland windows using `_NET_WM_SYNC` not properly synchronized to frame while resizing
This is what it looks like with current mutter:
This is what it looks like with !942 (merged):
As you can see, while the above MR fixes the drop shadow intermittently flashing up all black, the Thunderbird window still tends to lag behind the shadow, leaving a black gap when the window is enlarged.
It should be possible to make this work at least as well as with Xorg, which looks like this (notice how there's never a black gap, and the buttons at the bottom right are always positioned consistently with the right window edge):