I was just looking for the same thing on forum and solution described here works for me:
[GTK3: Changing the overlay/momentium scroll bar]
(changing gtk-primary-button-warps-slider = true in active theme)
For the reference, I also had additional problem in FF and few other software where it's easy to miss scrollbar on maximized window, it's like there is 1px right scrollbar padding that doesn't react when you click on it. Also seems a bit better now, but not completely.