GtkEventControllerMotion does not translate coordinates to widget coordinates
GtkEventControllerMotion, as backported to 3.24, does not translate coordinates to the proper coordinate space of the widget used to create the controller. This is problematic when you have children widgets which have windows as the x,y may be relative to their window without a way to detect this.
I think for the 3.24 backport, we might want to translate coordinates in gtk_event_controller_motion_handle_event(). I suspect this wasn't necessary for gtk4 given that GdkWindow/surface is only toplevel.