Gtk.WindowGroup

const Gtk = imports.gi.Gtk;

let window_group = new Gtk.WindowGroup();
  

Gtk.WindowGroup objects are referenced by each window in the group, so once you have added all windows to a Gtk.WindowGroup, you can drop the initial reference to the window group with GObject.unref. If the windows in the window group are subsequently destroyed, then they will be removed from the window group and drop their references on the window group; when all window have been removed, the window group will be freed.