addNonNull(v);
addNonNull(v, JScrollPane.VIEWPORT);
  public void setViewport(JViewport v)  {    if (viewport == v)      return;        JViewport old = viewport;    removeNonNull(old);    if (old != null)      old.removeChangeListener(scrollListener);    viewport = v;    if (v != null)      v.addChangeListener(scrollListener);    addNonNull(v);    revalidate();    repaint();    firePropertyChange("viewport", old, v);    sync();  }