anchor.diff:        newTop = windowHeight - barHeightPx;
       }
       barTop = newTop;
  -    contentTop = -(barTop*contentHeight/windowHeight).round();
  +    contentTop = -(windowHeight == 0 ? 0 :barTop*contentHeight/windowHeight).round();
       redrawInvoked = true;
       redraw();
     }
  -  
  +
     mouseUp(ev) {
       ssMouseMove.cancel();
       ssMouseUp.cancel();
  
positive.code:       redrawInvoked = false;
      }
    }
    
    onWheel(ev,step) {
      if (ev is SyntheticMouseEvent) {
        ev.nativeEvent.preventDefault();
  
negative.code:       }, children);
      };
    }
  
    componentWillMount() {
      barTop = 0;
      contentTop = 0;
  