Gtk.Widget.prototype.drag_source_set

function drag_source_set(start_button_mask:Gdk.ModifierType, targets:[Gtk.TargetEntry], actions:Gdk.DragAction):void {
    // Gjs wrapper for gtk_drag_source_set()
}

Sets up a widget so that GTK+ will start a drag operation when the user clicks and drags on the widget. The widget must have a window.

start_button_mask: the bitmask of buttons that can start the drag
targets: the table of targets that the drag will support, may be null
actions: the bitmask of possible actions for a drag from this widget