Gtk.Window.new

function new(type:Gtk.WindowType):Gtk.Widget {
    // Gjs wrapper for gtk_window_new()
}

Creates a new Gtk.Window, which is a toplevel window that can contain other widgets. Nearly always, the type of the window should be #GTK_WINDOW_TOPLEVEL. If you're implementing something like a popup menu from scratch (which is a bad idea, just use Gtk.Menu), you might use #GTK_WINDOW_POPUP. #GTK_WINDOW_POPUP is not for dialogs, though in some other toolkits dialogs are called "popups". In GTK+, #GTK_WINDOW_POPUP means a pop-up menu or pop-up tooltip. On X11, popup windows are not controlled by the <link linkend="gtk-X11-arch">window manager</link>.

If you simply want an undecorated window (no window borders), use Gtk.Window.prototype.set_decorated, don't use #GTK_WINDOW_POPUP.