<oScrollBar>:update() --> self

        update() is a method of the ScrollBar class that is used for changing
        the thumb position on the screen.  update() uses the values of the
        following instance variables to correctly show the thumb in its
        current context in addition to providing maximum flexibility in the
        manner a scroll bar thumb appears on the screen:  colorSpec, offset,
        orient, stable, start, style, and thumbPos.