JLayerで隣接する別コンポーネント上に縁を描画

class BreadcrumbLayerUI extends LayerUI<JPanel> {
  private Shape shape;
  @Override public void paint(Graphics g, JComponent c) {
    super.paint(g, c);
    if (shape != null) {
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                          RenderingHints.VALUE_ANTIALIAS_ON);

      Rectangle r = new Rectangle(c.getWidth(), c.getHeight());
      Area area = new Area(r);
      area.subtract(new Area(shape));
      g2.setClip(area);

      g2.setPaint(new Color(0x55_66_66_66, true));
      g2.setStroke(new BasicStroke(3f));
      g2.draw(shape);
      g2.setStroke(new BasicStroke(2f));
      g2.draw(shape);

      g2.setStroke(new BasicStroke(1f));
      g2.setClip(r);
      g2.setPaint(Color.WHITE);
      g2.draw(shape);

      g2.dispose();
    }
  }

  @Override public void installUI(JComponent c) {
    super.installUI(c);
    if (c instanceof JLayer) {
      ((JLayer) c).setLayerEventMask(AWTEvent.MOUSE_EVENT_MASK
                                   | AWTEvent.MOUSE_MOTION_EVENT_MASK);
    }
  }

  @Override public void uninstallUI(JComponent c) {
    if (c instanceof JLayer) {
      ((JLayer) c).setLayerEventMask(0);
    }
    super.uninstallUI(c);
  }

  private void update(MouseEvent e, JLayer<? extends JPanel> l) {
    int id = e.getID();
    Shape s = null;
    if (id == MouseEvent.MOUSE_ENTERED || id == MouseEvent.MOUSE_MOVED) {
      Component c = e.getComponent();
      if (c instanceof AbstractButton) {
        AbstractButton b = (AbstractButton) c;
        if (b.getIcon() instanceof ToggleButtonBarCellIcon) {
          ToggleButtonBarCellIcon icon = (ToggleButtonBarCellIcon) b.getIcon();
          Rectangle r = c.getBounds();
          AffineTransform at = AffineTransform.getTranslateInstance(r.x, r.y);
          s = at.createTransformedShape(icon.area);
        }
      }
    }
    if (!Objects.equals(s, shape)) {
      shape = s;
      l.getView().repaint();
    }
  }

  @Override protected void processMouseEvent(
      MouseEvent e, JLayer<? extends JPanel> l) {
    update(e, l);
  }

  @Override protected void processMouseMotionEvent(
      MouseEvent e, JLayer<? extends JPanel> l) {
    update(e, l);
  }
}