code: protected void configurePopup() {
          setLayout(new BoxLayout(this, BoxLayout.Y_AXIS));
          setBorderPainted(true);
          setBorder(LIST_BORDER);
          setOpaque(false);
          add(scroller);
          setDoubleBuffered(true);
          setFocusable(false);
      }
docs: Configures the popup portion of the combo box. This method is called when
  the UI class is created.