X11 clients expect a ConfigureNotify after a ConfigureRequest. If the compositor/window manager chooses not to honor the request (e.g. due to the window being maximized), XWayland will not send a "real" ConfigureNotify event and the window manager is expected to send a synthetic event instead. Otherwise, the X11 client is left waiting and may not repaint its window properly.
For comparison, see Openbox's client_configure() or Weston's
weston_wm_window_send_configure_notify().
This is needed to fix https://github.com/labwc/labwc/issues/785.