JFrameのタイトルバーに追加したコンポーネントをドラッグ可能にする

class TitleBarDragLayerUI extends LayerUI<JComponent> {
  private final Point startPt = 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 JComponent> l) {
    if (e.getID() == MouseEvent.MOUSE_PRESSED
        && SwingUtilities.isLeftMouseButton(e)) {
      startPt.setLocation(e.getPoint());
    }
  }

  @Override protected void processMouseMotionEvent(
        MouseEvent e, JLayer<? extends JComponent> l) {
    Component c = SwingUtilities.getRoot(e.getComponent());
    if (e.getID() == MouseEvent.MOUSE_DRAGGED
        && c instanceof Window && SwingUtilities.isLeftMouseButton(e)) {
      Point pt = c.getLocation();
      c.setLocation(pt.x - startPt.x + e.getX(), pt.y - startPt.y + e.getY());
    }
  }
}