GdkWindow: Check for offscreen windows in set_transient_for ()
Fixes a crash when opening a GtkComboBox contained in a GtkOffscreenWindow. gtkmenu.c
doesn't know much about offscreen windows, so sets the popup window as transient to the offscreen window.
The second commit is a best effort fix for positioning of popup menus anchored to widgets in GtkOffscreenWindow.
Fixes gimp#6711 (closed)
Edited by Luca Bacci