// onHover is called with null value when mouse moves outside of image surface
// onHover receives image coordinates
svoid imageSurfaceOnHover(ImageSurface is, VF1<Pt> onHover) {
  if (is == null || onHover == null) ret;
  swing {
    MouseAdapter ma = new MouseAdapter {
      public void mouseMoved(MouseEvent e) { pick(e); }
      public void mouseEntered(MouseEvent e) { pick(e); }
      public void mouseExited(MouseEvent e) { pick(null); }
      
      void pick(MouseEvent e) pcall {
        callF(onHover, e == null ? (Pt) null : is.pointFromEvent(e));
      }
    };
    is.addMouseMotionListener(ma);
    is.addMouseListener(ma);
  }
}

svoid imageSurfaceOnHover(ImageSurface is, IVF1<Pt> onHover) {
  imageSurfaceOnHover(is, toVF1(onHover));
}

Began life as a copy of #1024564