sclass WindowDragger extends MouseAdapter {
  MouseEvent mouseDownEvent;
  JComponent src;
  Pt rel;
  Window window;
  
  *(MouseEvent *mouseDownEvent) {
    if (mouseDownEvent == null) ret;
    src = optCast JComponent(mouseDownEvent.getComponent());
    if (src == null) ret;
    window = getWindow(src);
    if (window == null) ret;
    
    rel = pt(
      mouseDownEvent.getXOnScreen()-window.getX(),
      mouseDownEvent.getYOnScreen()-window.getY());
      
    addMouseAndMotionListener(src, this);
  }

  public void mouseDragged(MouseEvent e) {
    updatePosition(e);
  }
  
  public void mouseReleased(MouseEvent e) {
    if (e.getButton() != mouseDownEvent.getButton()) ret;
    updatePosition(e);
    removeMouseAdapter(src, this);
    dragDone();
  }
  
  void updatePosition(MouseEvent e) {
    window.setLocation(
      e.getXOnScreen()-rel.x,
      e.getYOnScreen()-rel.y);
  }
  
  void dragDone {}
}

Began life as a copy of #1018716