It always takes one update cycle for the copy to become visible on the host windowing system, so waiting for the target MSC resulted in 1 cycle delay.
We re-use the idle list for copies which were executed but need their completion event sent.
Fixes black seams when resizing the "Builder" sub-window of
GDK_BACKEND=x11 gtk4-demoon Xwayland (see https://gitlab.gnome.org/GNOME/mutter/-/issues/1290#note_873557).
Unfortunately, this cannot completely fix the seams with apps which queue up multiple frames in advance, since there's always at least one queued frame corresponding to the old window size. But it should at least help a little in that case as well.
/cc @romangg