const Gtk = imports.gi.Gtk;
const GObject = imports.gi.GObject;

Gtk.init(null, 0);

let dialog = new Gtk.Dialog({title: "Whatever"});
dialog.add_button(Gtk.STOCK_OK, Gtk.ResponseType.ACCEPT);

let store = new Gtk.ListStore();
store.set_column_types([GObject.TYPE_STRING]);

for (let i = 0; i < 500; i++ ) {
    store.set(store.append(), [0], [i]);
}

let combo = Gtk.ComboBox.new_with_model_and_entry(store);
combo.set_entry_text_column(0);
dialog.get_content_area().add(combo);
combo.show_all();

dialog.run();
