Gtk.Widget.prototype.drag_dest_set_proxy

function drag_dest_set_proxy(proxy_window:Gdk.Window, protocol:Gdk.DragProtocol, use_coordinates:Boolean):void {
    // Gjs wrapper for gtk_drag_dest_set_proxy()
}

Sets this widget as a proxy for drops to another window.

proxy_window

the window to which to forward drag events

protocol

the drag protocol which the proxy_window accepts (You can use gdk_drag_get_protocol() to determine this)

use_coordinates

If true, send the same coordinates to the destination, because it is an embedded subwindow.