code: private void addCustomGridded(AbstractButton button) {
          builder.getLayout().appendRow(this.rowSpec);
          builder.getLayout().addGroupedRow(builder.getRow());
          button.putClientProperty("jgoodies.isNarrow", Boolean.TRUE);
          builder.add(button);
          builder.nextRow();
      }
docs: Handle the custom RowSpec.
  
  @param button