anchor.diff:      move_button.onClick.listen((_) => _enableMove());
       save_button.onClick.listen((_) => _disableMove());
       var mouseMoveListener;
  -
       background.onMouseDown.listen((_) {
         if(mouseMoveListener != null){
           mouseMoveListener.cancel();
  
positive.code:     background.style.background = computedStyle.background;
      background.classes.add('active');
      _orig_position = computedStyle.backgroundPosition;
    }
  
    void _disableMove() {
      save_button.hidden = !(move_button.hidden = false);
      background.classes.remove('active');
      if(element.getComputedStyle().backgroundPosition != _orig_position){
        _change_stream_controller.add(this);
      }
  
    }
  
negative.code:       mouseMoveListener = null;
      });
  
    }
  
    void _enableMove() {
  