         // Scroll event.delta "steps".  If the VSB is up, scroll vertically,
         // if -only- the HSB is up then scroll horizontally.
          
+        // TODO: The problem is that viewport.validateNow() doesn’t necessarily 
+        // finish the job, see http://bugs.adobe.com/jira/browse/SDK-25740.   
+        // Since some imprecision in mouse-wheel scrolling is tolerable this is
+        // ok for now.  For 4.next we should add Scroller API for (reliably) 
+        // scrolling in different increments and refactor code like this to 
+        // depend on it.  Also applies to VScroller and HScroller mouse
+        // handlers.
+        
         if (verticalScrollBar && verticalScrollBar.visible)
         {
             navigationUnit = (event.delta < 0) ? NavigationUnit.DOWN : NavigationUnit.UP;

            {
                var vspDelta:Number = vp.getVerticalScrollPositionDelta(navigationUnit);
                if (!isNaN(vspDelta))
                    vp.verticalScrollPosition += vspDelta;
            }
            event.preventDefault();
        }

            {
                var hspDelta:Number = vp.getHorizontalScrollPositionDelta(navigationUnit);
                if (!isNaN(hspDelta))
                {
                    vp.horizontalScrollPosition += hspDelta;
                    if (vp is IInvalidating)
                        IInvalidating(vp).validateNow();
                }
            }
            event.preventDefault();
        }            
