Compatibility issue between RevealMath plugin used with revealjs

[cderv]

Take this simple example in test.md

---
title: "title"
---

$x + \sum{y}$

When I convert to revealjs format using

pandoc -t revealjs -s --mathjax -o test.html test.md

this will create a HTML document throwing this issue in the browser console

math.js:1 Uncaught TypeError: Cannot read properties of undefined (reading 'Config')
    at math.js:1
    at HTMLScriptElement.o (math.js:1)

This is from the revealMath plugin inserted by Pandoc when mathjax is used. https://github.com/jgm/pandoc/blob/c256ef34b32308f00b64ca9cb01f1bacd5994802/data/templates/default.revealjs#L308-L310

This plugin is compatible with mathjax 2 and not 3. The error is thrown because MathJax.Hub.Config() does not exist anymore in Mathjax 3. http://docs.mathjax.org/en/latest/upgrading/v2.html#configuration-changes but Pandoc uses 3 by default : https://github.com/jgm/pandoc/blob/25a86fc06fd5d7e07251ca9bd758cc42c4234d70/src/Text/Pandoc/Options.hs#L369

So Math in revealjs slide made with Pandoc does not work well unless the default Mathjax URL is changed.

revealMath plugin is not compatible with mathjax 3 in last released version, there was an issue about that https://github.com/hakimel/reveal.js/issues/2559#issuecomment-629389857, but as you can see in the issue, improvement has been been made to support Mathjax 2, MathJax 3 and Katex.

So next version of revealjs will change the way the plugin is working. Default will still be Mathjax 2 so the compatibility issue will remain. But it can be configured to version 3 or even with Katex, which could help bring support to Pandoc's --katex flag.

Changes for next reveal.js version were done here: https://github.com/hakimel/reveal.js/commit/0ea419300162bfc78f294c22a12046028e6605a7

[jgm]

Are there any changes that make sense now? Or is this more a reminder to modify things when the new release of reveal.js is out?

[cderv]

The change that makes sense now would be to use Mathjax 2 by default with reveals format, if this is possible to tweak the option value for this format only.

Probably https://cdn.jsdelivr.net/npm/mathjax@2/MathJax.js?config=TeX-MML-AM_CHTML or the one advide in the doc http://docs.mathjax.org/en/v2.7-latest/start.html

Then, to support Mathjax 3 and Katex, some change in template and probably other logic in the writer will be needed I think.

[jgm]

Not easy to do this -- at the point where we insert the default mathjax URL, we don't yet necessarily know what the output format will be. And we don't want to override an explicitly given URL.

[cderv]

I am not sure to see how to fix without changing the default for revealjs.

I don't know when the next revealjs version will be out, but if soon, it could worth the wait and adapt the template to use the plugin compatible with Mathjax 3 then. I am just not sure how to adapt the template if someone provide a mathjax url for version 2. Maybe a documentation note saying that --mathjax with revealjs only work with version 3 unless a custom template is used - not great though 😓

There is time to see how to adapt in Pandoc. In our tool, we are overring the default mathjax url to solve this issue.

[jgm]

It's easy enough for users to manually specify the URL with --mathjax, if they know that they need to do this. Hopefully they'll find this issue if they search.

[cderv]

FWIW new reveal.js version is out https://github.com/hakimel/reveal.js/releases/tag/4.2.0

Supporting Mathjax 3 by default to go with the default mathjax-url would mean modifying the Pandoc template in this way

diff --git a/data/templates/default.revealjs b/data/templates/default.revealjs
index 203983522..d4f839719 100644
--- a/data/templates/default.revealjs
+++ b/data/templates/default.revealjs
@@ -303,7 +303,7 @@ $endif$
         // reveal.js plugins
         plugins: [
 $if(mathjax)$
-          RevealMath,
+          RevealMath.MathJax3,
 $endif$
           RevealNotes,
           RevealSearch,

but if someone wanted to use Mathjax 2 using mathjax-url, it would need to be RevealMath or RevealMath.Mathjax2. Same if someone would change the revealjs-url to using an older version.

Also, --katex could be now supported with RevealMath.KaTeX with new version 4.2.0 of reveal.js

Anyway, as I said before, I do not know how Pandoc could handle the conditional between mathjax-url value and the plugin value to use.

Writing this here in case anyone is looking for such information.