gdk:selection-owner-get-for-display (display selection)

display -- a gdk:display object
selection -- a string indentifying a selection

If there is a selection owner for this window, and it is a window known to the current process, the gdk:window object that owns the selection, otherwise nil.

Determine the owner of the given selection. Note that the return value may be owned by a different process if a foreign window was previously created for that window, but a new foreign window will never be created by this call.