When scroll-step is 1 and if first displayed line overlapped and you go up one line or fisrt non-displayed line at bottom overlapped and you go down one line moving cause displaing engine redisplay windows content and center point as done by 'C-l'. So take it greater.

;; -*- mode: lisp; coding: utf-8 -*-
;;
;; For loading order see README.

;; Set frame width and heith. 150x46 for 13pt 1280x800, 150x60 for 13pt 1280x1024.
;; (setq my-frame-width 150)
;; (setq my-frame-height 60)