JPopupMenuにマウスドラッグで位置変更を可能にするヘッダを追加する

class PopupHeaderMouseListener extends MouseAdapter {
  private final Point startPt = new Point();

  @Override public void mousePressed(MouseEvent e) {
    if (SwingUtilities.isLeftMouseButton(e)) {
      Component c = e.getComponent();
      Container popup = SwingUtilities.getAncestorOfClass(
          JPopupMenu.class, c);
      SwingUtilities.convertMouseEvent(c, e, popup);
      startPt.setLocation(e.getPoint());
    }
  }

  @Override public void mouseDragged(MouseEvent e) {
    Component c = e.getComponent();
    if (SwingUtilities.isLeftMouseButton(e)) {
      Window w = SwingUtilities.getWindowAncestor(c);
      if (w != null && w.getType() == Window.Type.POPUP) {
        // Popup$HeavyWeightWindow
        Point pt = e.getLocationOnScreen();
        w.setLocation(pt.x - startPt.x, pt.y - startPt.y);
      } else {
        // Popup$LightWeightWindow
        Container popup = SwingUtilities.getAncestorOfClass(
            JPopupMenu.class, c);
        Point pt = popup.getLocationOnScreen();
        popup.setLocation(
            pt.x - startPt.x + e.getX(), pt.y - startPt.y + e.getY());
      }
    }
  }
}