JScrollPane内にある複数Componentを配置したJPanelをJLayerを使ってドラッグスクロール

class DragScrollLayerUI extends LayerUI<JScrollPane> {
  private final Point pp = new Point();

  @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) {
    Component c = e.getComponent();
    if (c instanceof JScrollBar || c instanceof JSlider) {
      return;
    }
    if (e.getID() == MouseEvent.MOUSE_PRESSED) {
      JViewport vport = l.getView().getViewport();
      Point cp = SwingUtilities.convertPoint(c, e.getPoint(), vport);
      pp.setLocation(cp);
    }
  }

  @Override protected void processMouseMotionEvent(
      MouseEvent e, JLayer<? extends JScrollPane> l) {
    Component c = e.getComponent();
    if (c instanceof JScrollBar || c instanceof JSlider) {
      return;
    }
    if (e.getID() == MouseEvent.MOUSE_DRAGGED) {
      JViewport vport = l.getView().getViewport();
      JComponent cmp = (JComponent) vport.getView();
      Point cp = SwingUtilities.convertPoint(c, e.getPoint(), vport);
      Point vp = vport.getViewPosition();
      vp.translate(pp.x - cp.x, pp.y - cp.y);
      cmp.scrollRectToVisible(new Rectangle(vp, vport.getSize()));
      pp.setLocation(cp);
    }
  }
}