In standard Windows apps, if you click the scroll-bar arrows, you get "smooth" scrolling, e.g. one line at a time, and if you click in the scroll bar just below or above the arrows, you scroll one page at a time.
Do you want the scroll-bar arrows to scroll one row of thumbnails at a time, rather than a whole page of thumbnails?
Note that you can scroll one row of thumbnails at a time using the up- and down-cursor keys.
(In general, I find the user-interface controls of Lightroom pretty fussy to use and wish they were more standard in their behavior.)