computeHorizontalScrollOffset
Compute the horizontal offset of the horizontal 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 computeHorizontalScrollRange and computeHorizontalScrollExtent.
The default offset is the scroll offset of this view.
Return
the horizontal offset of the scrollbar's thumb