Xwayland: resizing windows is glitchy

In many wayland compositors, resizing Xwayland windows leads to visual artifacts like black bars where client side drop shadows are or the right or the bottom xwindow edge "bouncing".

It's not clear what causes the black bars in GTK apps that have client side drop shadows. This issue is not reproducible with Xorg. This glitch can be observed both in Mutter and KWin.

The second issue where the right/bottom edge bounces is caused by wl_surface and Xsync being out of sync. For example, a new buffer can be attached to the wl_surface and N milliseconds later, the wayland compositor is notified about that Xsync has been acked so it can update the window geometry; or vice versa, i.e. update the geometry first, then N milliseconds later receive a new buffer.

It's worth noting that the glitches can't be observed in weston. I suspect that it's due to weston recomputing window position when a new buffer is attached to the wl_surface.

edit: I've created this issue upstream because I believe this issue is not specific to any concrete Wayland compositor.