`MathDocument.convert()` does not associate a `MathItem` with the `MathDocument`

Created by: rhansen

Issue Summary

When I follow the documentation to convert a math string to HTML or SVG, no MathItem is associated with the MathDocument (as far as I can tell). In particular, MathJax.startup.document.getMathItemsWithin() does not return anything for the element or its containers.

Is this intentional? If so, how do I access the MathItem?

Steps to Reproduce:

https://jsfiddle.net/9rLhjskz/

<script src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-svg.js"></script>
<script>
  const elt = MathJax.tex2svg('e^{i\\pi}=-1');
  document.body.append(elt);
  const [mathItem] = MathJax.startup.document.getMathItemsWithin(document.body);
  if (mathItem == null) alert('no MathItem!');
</script>

I get the same result if I instead use MathDocument.convert() directly:

<script src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-svg.js"></script>
<script>
  const elt = MathJax.startup.document.convert('e^{i\\pi}=-1');
  document.body.append(elt);
  const [mathItem] = MathJax.startup.document.getMathItemsWithin(document.body);
  if (mathItem == null) alert('no MathItem!');
</script>

Technical details:

• MathJax Version: 3.2.0
• Client OS: Ubuntu 20.04
• Browser: Firefox 95.0.1