MJ4-beta.4: \_ in math mode yields error, not underscore character. Is that expected?

[AlexEdgcomb]

Issue Summary

\( \_ \) yields Math input error in MJ4-beta.4, but not with MJ4-alpha.1 nor with MJ2.

Steps to Reproduce:

1. Go to https://codepen.io/alexedgcomb/pen/wvZPpQQ

Observed: Math input error

Expected: An underscore character, like in MJ4-alpha.1 and MJ2:

Technical details:

• MathJax Version: 4.0.0-beta.4
• Client OS: MacOS 13.4
• Browser: Firefox 124.0.1

I am NOT using a MathJax configuration and loading MathJax via:

<script src="https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.4/tex-mml-svg.js"></script>

Supporting information:

Same LaTeX in:

• MJ4-alpha.1: https://codepen.io/alexedgcomb/pen/LYvOeaL
• MJ2: https://codepen.io/alexedgcomb/pen/ZEZavVY

I suspect the behavior is expected, but I wanted to verify, in case of accidental regression.

Thank you!

[dpvc]

This is a duplicate of #3120, which was caused by the new code that adds the LaTeX that produced each MathML element in an attribute on that element. A fix will be in beta.5, which we are finishing up.

[AlexEdgcomb]

Thank you!