This is not transparent as it is elsewhere in OS X, would that be simply a browser showing this page. Your project, like Massimo’s, shows a grey scrollbar. Is is cute, but IMHO not conformant to OS X native scrollbar. In fact, it resembles very much Massimo’s project with the modifications I posted.
It is transparent when using OSX10.9 or higher. And it is a native scrollbar (check out the declares - no MBS plugins needed).
The scrollbar also widens up when clicking with when running OSX10.9 - OSX10.10 does not have this behaviour.
Changed the project so the transparency is clearly shown.
I did a quick adaptation of my XDC NSScrollView example to control a Listbox instead of a container control.
It’s about as close to native look and feel as you’ll get with the Listbox. Keep in mind that the Listbox does all vertical scrolling by rows, not pixels, so there won’t be a smooth scroll up and down.