sclass ImageSurfaceDragger extends ImageSurfaceMouseHandler {
  sbool debug = true;
  Point start, startScroll;
  int dx, dy;

  *() {}
  *(ImageSurface is) {
    if (containsInstance(is.tools, ImageSurfaceDragger)) ret;
    this.register(is);
  }

  public void mousePressed(MouseEvent e) {
    JScrollPane sp = enclosingScrollPane(is);
    if (sp == null) ret;
    if (e.getButton() == MouseEvent.BUTTON1) {
      start = e.getLocationOnScreen();
      startScroll = new Point(
        scrollBarValue(sp.getHorizontalScrollBar()),
        scrollBarValue(sp.getVerticalScrollBar()));
    }
  }

  public void mouseDragged(MouseEvent e) {
    if (start == null) ret;
    JScrollPane sp = enclosingScrollPane(is);
    if (sp == null) ret;
    Point p = e.getLocationOnScreen();
    //if (debug) print("ImageSurfaceDragger: drag " + p.x + "/" + p.y);
    setScrollBarValue(sp.getHorizontalScrollBar(), -(p.x-start.x)+startScroll.x);
    setScrollBarValue(sp.getVerticalScrollBar(), -(p.y-start.y)+startScroll.y);
  }

  public void mouseReleased(MouseEvent e) {
    mouseDragged(e);
    start = null;
  }
}