You cannot overscroll with NSScroller. Nor do they have a transparent background. And the intent of the original request is to get closer to the modern design. NSScroller is just what we’re stuck working with.
To get back to that original request, I think fading it out is a little optimistic. It might be more realistic to slide the scrollbar away. Follow the link in my last reply, those declares can be used to know when to show the scrollbar, and a timer to decide when to hide it. Windows and Linux support will be a little more tricky, as you won’t be able to show the scrollbar until a scroll actually starts.