Comment 12 for bug 1107850

It looks like _NET_WM_SYNC_REQUEST with background None would be a way to postpone client and window manager drawing until the ConfigureNotify has been received.
http://standards.freedesktop.org/wm-spec/1.5/ar01s06.html#id2761266
However, the support doesn't seem to be there in GTK or in window managers.
It seems it is used mostly for window-manager-initiated resize drags, possibly because most apps don't have background None.

gtk_window_show() does not increment configure_request_count when it resizes the
window, and so gtk_window_configure_event() bails out early, replying to the
_NET_WM_SYNC_REQUEST before the drawing has taken place.

kwin 4.8.5 does not wait (any significant time at least) for the
_NET_WM_SYNC_REQUEST reply before drawing the frame. It only sends the
message for the resize before show when compositing is enabled.

metacity 2.30.3 and 2.34.8 do not send a _NET_WM_SYNC_REQUEST for the resize
before show, with or without compositing enabled. They do send these when
dragging the frame border to resize.

I haven't tested compiz or mutter which I would expect to use
_NET_WM_SYNC_REQUEST at least for resize drags. Bug 378293 seems to suggest compiz
is using _NET_WM_SYNC_REQUEST as an indicator of when it expects to have all Damage events for a window show.