computeVerticalScrollOffset
Compute the vertical offset of the vertical scrollbar's thumb within the horizontal range. This value is used to compute the position of the thumb within the scrollbar's track.
The range is expressed in arbitrary units that must be the same as the units used by computeVerticalScrollRange and computeVerticalScrollExtent.
The default offset is the scroll offset of this view.
Return
the vertical offset of the scrollbar's thumb