I created this scrollbar back in the days of SmartClips, Flash’s precursor to Components (which actually made this obsolete a few months after creating it). I was successful in mimicking the Windows scrollbar exactly to use it in various Flash applications.
Try adding and deleting text to see the scrollbar adjust itself. Also try using the scrollbar as you normally would; drag it, click the arrows, click the negative space between the arrows and the dragable part, etc. Notice the delays when you click and hold the arrows ant the negative space.
The SmartClip was used throughout the site I did for O.A.R.