addNonNull(v);
addNonNull(v, JScrollPane.ROW_HEADER);
  public void setRowHeader(JViewport v)  {    if (rowHeader == v)      return;        JViewport old = rowHeader;    removeNonNull(old);    rowHeader = v;    addNonNull(v);    firePropertyChange("rowHeader", old, v);    sync();  }