bug#63518: 28.2; shr.el seems to break inline latex (mathjax) in html

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 15 May 2023 13:21:51 +0200
> From: mousebot <mousebot@riseup.net>
> 
> The fediverse client I maintain, mastodon.el, uses shr-render-region to render individual posts. Some instances, e.g. https://mathstodon.xyz, allow users to post inline latex using mathjax notation.
> 
> When shr.el renders inline latex, it often breaks it as it fills the text. It inserts a newline in between the two characters that open an inline latex block: `\(` or `\[`. Using normal fill commands to fill text (fill-region, fill paragraph) do not split latex in this way, from what I could gather.
> 
> When digging around and debugging a little, I found that in shr-find-fill-point, the check (shr-char-kinsoku-eol-p (following-char)) in the when condition returns t when point is in between \ and ( or [, meaning that shr-find-fill-point considers that position to be a breakable point. Commenting that single check seems to largely prevent the undesired splitting. (Behaviour confirmed by my checks and also by another mastodon.el user.)
> 
> I don't really understand the significance of the checks that shr-find-fill-point runs, nor whether they can be temporarily deactivated or worked around in some other way.

That function looks for a suitable place to break the line in two.

The question is whether we can reliably determine that we are inside
inline latex, so that we augment the conditions for a break point.
Turning that off unconditionally is not an option.  Do you happen to
know about some criteria to be applied to distinguish this special
case?

Thanks.