// of the submenus correctly.
VerticalToolbarButtonWidget overflowButton = new VerticalToolbarButtonWidget();
overflowSubmenu.hackInsertWidget(overflowButton, beforeIndex);
// Return the item, initially proxying the toplevel button.
Item item = new Item(toplevelButton, overflowButton, new ToolbarButtonUiProxy(toplevelButton));
items.add(beforeIndex, item);
return item;
}