bug#37848: 27.0.50; hl-line and line-numbers out of sync

[Eli Zaretskii <eliz@gnu.org>]

> From: Andrey Orst <andreyorst@gmail.com>
> Date: Mon, 21 Oct 2019 11:27:58 +0300
> 
> emacs -q --eval "(progn
>                    (hl-line-mode t)
>                    (set-face-attribute 'line-number-current-line nil :foreground \"#FF0000\")
>                    (display-line-numbers-mode t)
>                    (split-window-right)
>                    (switch-to-buffer \"*Messages*\"))"
> 
> This will open two windows, one with *scratch* and other with *Messages*
> buffer. Try pasting some long text into the *scratch* buffer, and
> scrolling with mouse wheel. The current line number and hl-line
> highlighting should be in sync.
> 
> Then focus window with *Messages* buffer, and try scrolling in *scratch*
> buffer again with mouse wheel. You'll notice that after scrolling the view 
> beyond the point, current line number remains correct, while hl-line highlighter
> doesn't move:
> 
> image.png
> 
> It looks like, while Emacs always keeps the point in the view, hl-line
> acts like editors that alow point to leave the view, but only when
> buffer is not in focus.

The problem is with hl-line mode: it doesn't move its overlay when
point moves due to redisplay keeping it in the viewport.

The line-number display has nothing to do with this, it works
correctly in this case.

Thanks.