JScrollPaneの範囲外へのマウスドラッグによるスクロールで半透明の楕円を描画する

class OverscrollEdgeEffectLayerUI extends LayerUI<JScrollPane> {
  private final Color color = new Color(0xAA_AA_EE_FF, true);
  private final Point mousePt = new Point();
  private final Timer animator = new Timer(20, null);
  private final Ellipse2D oval = new Ellipse2D.Double();
  private double ovalHeight;
  private int delta;

  @Override public void paint(Graphics g, JComponent c) {
    super.paint(g, c);
    if (c instanceof JLayer && ovalHeight > 0d) {
      JScrollPane scroll = (JScrollPane) ((JLayer<?>) c).getView();
      Rectangle r = scroll.getViewport().getViewRect();
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setPaint(color);
      if (oval.getY() < 0) {
        oval.setFrame(oval.getX(), -ovalHeight, oval.getWidth(), ovalHeight * 2d);
      } else { // if (r.height < oval.getY() + oval.getHeight()) {
        oval.setFrame(oval.getX(), r.getHeight() - ovalHeight, oval.getWidth(), ovalHeight * 2d);
      }
      g2.fill(oval);
      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);
  }

  @Override protected void processMouseEvent(MouseEvent e, JLayer<? extends JScrollPane> l) {
    if (e.getComponent() instanceof JViewport) {
      int id = e.getID();
      if (id == MouseEvent.MOUSE_PRESSED) {
        mousePt.setLocation(e.getPoint());
      } else if (ovalHeight > 0d && id == MouseEvent.MOUSE_RELEASED) {
        ovalShrinking(l);
      }
    }
  }

  @Override protected void processMouseMotionEvent(MouseEvent e, JLayer<? extends JScrollPane> l) {
    Component c = e.getComponent();
    if (c instanceof JViewport && e.getID() == MouseEvent.MOUSE_DRAGGED && !animator.isRunning()) {
      JViewport viewport = l.getView().getViewport();
      Dimension d = viewport.getView().getSize();
      Rectangle r = viewport.getViewRect();
      Point p = SwingUtilities.convertPoint(c, e.getPoint(), l.getView());
      double ow = Math.max(p.getX(), r.getWidth() - p.getX());
      double ox = p.getX() - ow;
      int dy = e.getPoint().y - mousePt.y;
      if (isDragReversed(dy)) {
        // The y-axis drag direction has been reversed
        ovalShrinking(l);
      } else if (r.y == 0 && dy >= 0) {
        // top edge
        ovalHeight = Math.min(r.getHeight() / 8d, p.getY() / 8d);
        oval.setFrame(ox, -ovalHeight, ow * 2.2, ovalHeight * 2d);
      } else if (d.height == r.y + r.height && dy <= 0) {
        // bottom edge
        ovalHeight = Math.min(r.getHeight() / 8d, (r.getHeight() - p.getY()) / 8d);
        oval.setFrame(ox, r.getHeight() - ovalHeight, ow * 2.2, ovalHeight * 2d);
      }
      mousePt.setLocation(e.getPoint());
      delta = dy;
      l.repaint();
    }
  }

  private boolean isDragReversed(int dy) {
    boolean b1 = delta > 0 && dy < 0;
    boolean b2 = delta < 0 && dy > 0;
    return b1 || b2;
  }

  private void ovalShrinking(JLayer<? extends JScrollPane> l) {
    if (ovalHeight > 0d && !animator.isRunning()) {
      ActionListener handler = e -> {
        if (ovalHeight > 0d && animator.isRunning()) {
          ovalHeight = Math.max(ovalHeight * .67 - .5, 0d);
          l.repaint();
        } else {
          animator.stop();
          for (ActionListener a : animator.getActionListeners()) {
            animator.removeActionListener(a);
          }
        }
      };
      animator.addActionListener(handler);
      animator.start();
    }
  }
}