// menuParams: text, Runnable, ...
// also: position := 'left/'center/'right
static JButton jPopDownButton(S text, fO... menuParams) {
  ret jbutton((empty(text) ? "" : text + " ") + unicode_downPointingTriangle(), r {
    new JPopupMenu menu;
    int emptyCount = menu.getComponentCount();
    S position = cast optPar_ignoreOddLength position(menuParams); // TODO: this fails when you include a separator without a following null
    fillJPopupMenu(menu, paramsWithout(menuParams, 'position));
    if (menu.getComponentCount() != emptyCount) {
      JButton btn = heldInstance(JButton);
      int x = 0;
      if (eq(position, 'center)) x = (btn.getWidth()-getPreferredWidth(menu))/2;
      else if (eq(position, 'right)) x = btn.getWidth()-getPreferredWidth(menu);
      menu.show(btn, x, btn.getHeight());
    }
  });
}