bug#42653: 28.0.50; scroll-margin is sometimes ignored with hl-line/display-line-numbers-mode

[Kevin Liu]

This has been a bug since at least 24.x, and I think Eli even fixed
something like it once before:
https://lists.gnu.org/archive/html/bug-gnu-emacs/2012-11/msg00838.html

This problem is a little different because it involves scroll-margin.
When scrolling up, the cursor will seem to ignore it until it hits the
top, and then jumps back down to the margin.

To quickly reproduce this, run emacs -Q and then eval

(progn
 (global-hl-line-mode)
 (setq scroll-margin 5)
 (setq scroll-conservatively 101)
 (view-hello-file)
 (end-of-buffer))

Then (previous-line) until you see the problem.

Further reference:
- 
https://emacs.stackexchange.com/questions/48340/line-numbers-break-scroll-margin
- https://github.com/syl20bnr/spacemacs/issues/8224