sclass JPopDownButton > JButton {
  S menuPosition = "left"; // or "center" or "right"
  
  *() {
    super(unicode_downPointingTriangle());
    main addActionListener(this, r openMenu);
  }
  
  *(IVF1<JPopupMenu> *fillMenu) { this(); }
  
  event fillingMenu(JPopupMenu menu);
  
  swappable void fillMenu(JPopupMenu menu) {
    fillingMenu(menu);
  }
  
  void openMenu {
    new JPopupMenu menu;
    int emptyCount = menu.getComponentCount();
    fillMenu(menu);
    if (menu.getComponentCount() == emptyCount) ret;
    
    int x = 0;
    if (eq(menuPosition, "center")) x = (getWidth()-getPreferredWidth(menu))/2;
    else if (eq(menuPosition, "right")) x = getWidth()-getPreferredWidth(menu);
    menu.show(this, x, getHeight());
  }
}