Interesting tidbit: There is even a context menu on scroll bars - no idea who would use this, but it is there.
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.
It is for accessibility. Alternative input methods, like emulating the mouse by keyboard for instance, might not have mouse-drag functionality, but will have a right-click function.
The link I posted above links to another one with more historical context. I'd never clicked that the icon was meant to look like a space bar.
https://blogs.msdn.microsoft.com/jensenh/2006/06/08/you-wind...