svoid imageSurfaceOnLeftClick(ImageSurface is, VF1<Pt> onClick) {
  if (is == null || onClick == null) ret;
  swing {
    MouseAdapter ma = new MouseAdapter {
      public void mouseClicked(MouseEvent e) {
        callF(onClick, is.pointFromEvent(e));
      }
    };
    is.addMouseListener(ma);
  }
}