// combines mousePressed, mouseDragged and mouseReleased
// left button only
// f : voidfunc(Pt) - point can be null, then mouse was released
static void imageSurfaceOnMouseDrag(final ImageSurface is, final O f) {
  swing {
    MouseAdapter ma = new MouseAdapter {
      bool in;
      
      public void mouseDragged(MouseEvent e) {
        if (!in) ret;
        pcallF(f, is.pointFromEvent(e));
      }
      
      public void mousePressed(MouseEvent e) {
        if (e.getButton() != MouseEvent.BUTTON1) ret;
        in = true;
        pcallF(f, is.pointFromEvent(e));
      }
  
      public void mouseReleased(MouseEvent e) {
        if (e.getButton() != MouseEvent.BUTTON1) ret;
        if (!in) ret;
        in = false;
        pcallF(f, (Pt) null);
      }
    };
    is.addMouseMotionListener(ma);
    is.addMouseListener(ma);
  }
}

Began life as a copy of #1005948