     }  
     
     /**
-     *  Called when the viewport's verticalScrollPosition changes, does nothing by default. 
+     *  Called when the viewport's <code>verticalScrollPosition</code> value changes. Does nothing by default. 
      *  
      *  @langversion 3.0
      *  @playerversion Flash 10

    
    
   /**
    *  Called when the viewport's width or height changes, does nothing by default.
    *  
    *  @langversion 3.0
    *  @playerversion Flash 10

    //---------------------------------
     
    /**
     *  Handles a click on the increment or decrement button of the scroll bar. 
     *  This should cause a stepping operation, which is repeated if held down.
     *  The delay before repetition begins and the delay between repeated events
     *  are determined by the <code>repeatDelay</code> and 
     *  <code>repeatInterval</code> styles of the underlying Button objects.
     * 
     *  @see spark.components.Button
     *
     *  @langversion 3.0
     *  @playerversion Flash 10
     *  @playerversion AIR 1.5
     *  @productversion Flex 4  
     */
    protected function button_buttonDownHandler(event:Event):void
    {
