// layouter: voidfunc(Container) or func(Container) -> Dimension
// (return value is preferred size)
static JPanel customLayoutPanel_trackWidth(fO layouter) {
  class P extends JPanel implements Scrollable {
    *() { super(layoutManagerFromFunction(layouter)); }
    
    @Override
    public Dimension getPreferredScrollableViewportSize() { ret getPreferredSize(); }
    @Override
    public int getScrollableUnitIncrement(Rectangle visibleRect, int orientation, int direction) { return 20; }

    @Override
    public int getScrollableBlockIncrement(Rectangle visibleRect, int orientation, int direction) {
      ret (direction == SwingConstants.HORIZONTAL ? visibleRect.width : visibleRect.height)*5/6;
    }

    @Override
    public bool getScrollableTracksViewportWidth() { true; }

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

Began life as a copy of #1008604