// layouter: voidfunc(Container) or func(Container) -> Dimension
// (return value is preferred size)
static JPanel customLayoutPanel_trackWidth(fO layouter) {
  sclass P extends JPanel implements Scrollable {
    *() { super(new AbstractLayoutManager {
      public void layoutContainer(Container parent) {
        Dimension size = cast callF(layouter, parent);
        if (size != null) preferredSize = size;
      }
    });
    
    @Override
    public Dimension getPreferredScrollableViewportSize() { ret getPreferredSize(); }
    @Override
    public int getScrollableUnitIncrement(Rectangle visibleRect, int orientation, int direction) { return 1; }

    @Override
    public int getScrollableBlockIncrement(Rectangle visibleRect, int orientation, int direction) { return 10; }

    @Override
    public bool getScrollableTracksViewportWidth() { true; }

    @Override
    public boolean getScrollableTracksViewportHeight() { false; }
  }
  ret new P;
}