code: public Packer fillboth(final double wtx, final double wty) {
          gc.fill = GridBagConstraints.BOTH;
          gc.weightx = wtx;
          gc.weighty = wty;
          setConstraints(comp, gc);
          return this;
      }
docs: Add fill=BOTH, weighty=1, weightx=1 to the constraints for the current
  component.