This was the one place where we relied on
the menubar fallback in GtkApplicationWindow.
So turn it on explicitly.
main_window = gtk_application_window_new (GTK_APPLICATION (app));
gtk_window_set_icon_name (GTK_WINDOW (main_window), "text-editor");
gtk_window_set_default_size (GTK_WINDOW (main_window), 400, 600);
+ gtk_application_window_set_show_menubar (GTK_APPLICATION_WINDOW (main_window), TRUE);
update_title (GTK_WINDOW (main_window));
box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 0);