> You don't need an extra package, but it seems you have to disable
 > scroll-bar-mode.  (IMO that's a bug, isn't it?)

If you use fringes you can try the attached functions.  I once tried to
detect lateral down-mouse-1's on the scroll-bar but gave up soon.  IMHO
Emacs should provide both vertical and horizontal dividers, at least
optionally.  Hence I agree, it's a bug, and - patches welcome.