computeVerticalScrollExtent
Compute the vertical extent of the vertical scrollbar's thumb within the vertical range. This value is used to compute the length 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 computeVerticalScrollOffset.
The default extent is the drawing height of this view.
Return
the vertical extent of the scrollbar's thumb