There is no need to modify anything in style sheets, although you’re of course free to do so, if you like taking risks.
The solution that I found is much simpler, because it just tells GTK to stop hiding the scroll bars when the mouse is out of sight. Scroll bars will then always be visible, if there is more information than would fit in the window, and the content will be resized automatically, so that there’s no conflict when the scroll bar suddenly appears where you like to clcik.
All you need to achieve this is to add the line
gtk-overlay-scrolling=false
to the ~/.config/gtk-3.0/settings.ini file.
And that’s a file that you need to change anyway, if you see that clicking above or below the thumb does not page up or down, but warps to the place where you clicked, which was a change I never asked for, some years ago.