gdk:offscreen-window-embedder (window)

window -- a gdk:window object
embedder -- a gdk:window object that window gets embedded in

The embedding gdk:window object, or nil if window is not an embedded offscreen window. The gdk:offscreen-window-embedder function gets the window that window is embedded in. The (setf gdk:offscreen-window-embedder) function sets window to be embedded in embedder.

To fully embed an offscreen window, in addition to calling this function, it is also necessary to handle the "pick-embedded-child" signal on embedder and the "to-embedder" and "from-embedder" signals on window.