sclass ImageSurfaceDragger extends MouseAdapter {
  ImageSurface is;
  int dx, dy;

  *(ImageSurface *is) {
    if (containsInstance(is.tools, ImageSurfaceDragger)) ret;
    ifclass ImageSurfaceSelector
      for (ImageSurfaceSelector s : collectInstances(is.tools, ImageSurfaceSelector)) {
        is.removeMouseListener(s);
        is.removeMouseMotionListener(this);
        is.tools.remove(s);
      }
    endif
    is.tools.add(this);
    is.addMouseListener(this);
    is.addMouseMotionListener(this);
  }

  public void mousePressed(MouseEvent e) {
    if (e.getButton() == MouseEvent.BUTTON1) {
      Pt p = is.pointFromEvent(e);
      dx = p.x-is.getX();
      dy = p.y-is.getY();
    }
  }

  public void mouseDragged(MouseEvent e) {
    JScrollPane sp = enclosingScrollPane(is);
    if (sp == null) ret;
    Pt p = is.pointFromEvent(e);
    sp.getHorizontalScrollBar().setValue(p.x-dx);
    sp.getVerticalScrollBar().setValue(p.y-dy);
  }

  public void mouseReleased(MouseEvent e) {
    mouseDragged(e);
  }
}