(glib:define-gboxed-opaque toplevel-layout "GdkToplevelLayout"
  :type-initializer "gdk_toplevel_layout_get_type"
  :alloc (%toplevel-layout-new))  

Toplevel surfaces are sovereign windows that can be presented to the user in various states (maximized, on all workspaces, etc). The gdk:toplevel-layout structure contains information that is necessary to do so, and is passed to the gdk:toplevel-present function.