// layouter: voidfunc(Container) or func(Container) -> Dimension
// (return value is preferred size)
static JPanel customLayoutPanel(fO layouter) {
  ret jpanel(new AbstractLayoutManager {
    public void layoutContainer(Container parent) {
      Dimension size = cast callF(layouter, parent);
      if (size != null) preferredSize = size;
    }
  });
}