superscript and subscript rendering in MathJax 4

[snorky22]

Issue Summary

MathJax 4 beta 7 fails to render superscript and subscript when there is no opening brace

Steps to Reproduce:

• typesetting \Phi^\rm{ id}_{L} results in the following error message: Missing open brace for superscript
• typesetting \mathcal{L}_\rm{ ce} results in the following error message: Missing open brace for subscript

These two equations render fine in MathJax 3

Technical details:

• MathJax Version: 4 beta 7
• Client OS: Mac OS X 15.5
• Browser: Chrome 137.0.7151.104

I am using the following MathJax configuration:

MathJax = {
                    output: {
                        font: 'mathjax-{{ font }}',
                        displayOverflow: 'linebreak',
                        linebreaks: {
                            inline: true,
                            width: '100%',
                        }
                    }
                };

and loading MathJax via

<script id="MathJax-script" defer
                src="https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7/tex-mml-chtml.js"></script>

[dpvc]

The expression \Phi^\rm{ id}_{L} is not actually valid LaTeX, and produces a similar error message. Version 3 processed this expression incorrectly, and that is corrected in v4.

Note that \rm is not a macro that takes an argument, but is rather a switch that turns on the Roman (upright) font. That is, you should do {\rm id} not \rm{id}. Note that \rm{id}x would render all three characters in the Roman font. The \rm macro is a plain TeX macro; the LaTeX version would be \mathrm{id}, and \mathrm does take an argument, unlike \rm.

So you should use either \Phi^{\rm id}_{L} or \Phi^\mathrm{id}_{L} for your first expression. Similarly for subscripts.

In conclusion, the v4 error message is the correct behavior.