Xfwm4 compositor unredirects fullscreen override-redirect windows such as the one used as the fade-out (greyed-out?) effect in xfce4-session, and that causes a unmap/map event which clears the fadeout window. The following patch processes all pending gtk events and then clears the fade-out window so that it repaints itself (forcing a redraw)) and gets actually visible to the user.
Created attachment 606 Clear fade-out override redirect window to force a refresh
Created attachment 607 Take 2 (Removes undeed glist)
Ok, this patch is now obsoleted by a fix committed in xfwm4 (revision 22163.)