bug#65990: 29.1; show-paren-mode should not highlight line-prefix area

[Eli Zaretskii <eliz@gnu.org>]

> From: awrhygty@outlook.com
> Date: Fri, 15 Sep 2023 18:26:12 +0900
> 
> 
> Evaluate the form below,
> the blank area on the left of the open parensis is highlighted,
> while the blank area on the left of the close parensis is not.
> Then type 'C-l C-l' to make the close parensis placed at the window
> start, the area on the left of the close parensis is highlighted.
> 
> (let ((buf (generate-new-buffer "*temp*")))
>   (switch-to-buffer buf)
>   (insert (propertize "(x\n)" 'line-prefix '(space :width 8))))

Normally, none of the line-prefix should be highlighted.  And that is
what happens when the show-paren face is anywhere but at the first
visible character in the window.  When the face _is_ on the first
visible character in the window, it is not clear whether the face
should be considered as starting before the window or not; the current
display code considers it to start before the window-start.

I tend to close this bug as wontfix, since it seems to happen in rare
situations, and fixing it will likely be messy.  If you can show that
it happens in more than just these rare cases, or if you have any
suggestions for how to fix that or even can just point to the code
which causes this, I might change my mind.

Thanks.