Heh, yeah the context menu on the scroll bars is my go-to tell to discover whether it's a real scroll bar widget or an imitation one. (Gotta write an article on that too one day)
Think of how hard it would be to re-implement a dropdown, for instance: You would need to implement keyboard navigation (arrow keys), jump-to-entry (using the letter-keys), srolling with the mouse wheel, flipping through entries while the dropdown menu is closed and probably a thousand other things that I never used but some people do.
In the end I think I cheated and drew the list box on the parent with an option to go up or down depending on it's relative position.
Certainly was complicated.