Assignment Builder loads MathJax twice; second load drops ignoreHtmlClass

Summary

The Assignment Builder loads MathJax twice, from two different CDNs, and the second load replaces the Runestone MathJax configuration with the builder's own. The surviving configuration drops ignoreHtmlClass entirely, so tex2jax_ignore / ignore-math stop being honored for the rest of the page's life.

This is not currently causing a user-visible bug, but it is a latent hazard — it silently removes a protection that #1248 now depends on. Filing it so it's on the record rather than rediscovered later.

How it happens

1. components/rsptx/templates/common/static_assets_min.html:26-75 sets window.MathJax to the Runestone config and loads MathJax from https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js. This config sets ignoreHtmlClass: "tex2jax_ignore|ignore-math" and processHtmlClass: "process-math", and its pageReady resolves runestoneMathReady.

2. MathJaxWrapper (assignment_builder/src/components/routes/AssignmentBuilder/MathJaxWrapper.tsx:72) renders <MathJaxContext config={mathJaxConfig}>. better-react-mathjax@2.1.0 does this unconditionally on first render — it never checks whether MathJax is already present:

   // node_modules/better-react-mathjax/esm/MathJaxContext/MathJaxContext.js
   function l(t, o) {
     n && (window.MathJax = n);            // n = the config object passed in
     var e = document.createElement("script");
     e.src = f;                             // f = DEFAULT_V3_SRC when `src` prop is unset
     ...
     document.getElementsByTagName("head")[0].appendChild(e);
   }
   

   with DEFAULT_V3_SRC = "https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.js".

So the live window.MathJax — a fully initialized MathJax object — is overwritten with a plain config literal, and a second copy of MathJax is fetched from a different CDN and initialized over the top of the first.

MathJaxWrapper is mounted in four places: ExercisePreview.tsx, StatementPreview.tsx, RunestonePreview.tsx, RunestoneGraderPreview.tsx. A module-level promise guard in the library means the script is only appended once, so this happens once per page load, not four times.

Observed effects

Measured in Chromium against the real CDNs, replaying the exact sequence with both real configs:

Three consequences:

1. ignoreHtmlClass is gone. Any element marked tex2jax_ignore or ignore-math is no longer skipped. Confirmed: after the second load, a tex2jax_ignore div containing \newcommand{\zz}{\$} gets its \$ rewritten to <span>$</span> by processEscapes.

2. There is a window where window.MathJax is a plain object with no typesetPromise. runestonebase.js:591-619 (queueMathJax) guards on MathJax.typesetPromise before enqueuing, but AutoQueue.dequeue re-reads the global inside an await, so a component that queues just before the clobber can hit MathJax.typesetPromise is not a function and have its typeset promise silently rejected.

3. Two MathJax versions. mathjax@3 (floating) from jsdelivr vs. pinned 3.2.2 from cdnjs, plus a second CDN dependency and a second full MathJax download on every builder page load.

Why it matters for #1248

The fix in #1248 keeps the course's latex_macros pristine by putting them in a tex2jax_ignore div, so MathJax's page-load pass doesn't rewrite the \$ in \newcommand{\dollar}{\$}.

That fix is safe today, and I verified it end-to-end with the double load in place: the carrier div survives intact and all macros resolve. It holds for two reasons that are both incidental:

• the only page-wide typeset pass runs under the Runestone config, which still honors tex2jax_ignore; and
• the builder's config sets startup: { typeset: false }, so the second MathJax never does a page-wide pass.

If either of those changes — someone adds a MathJax.typesetPromise([document.body]), or flips startup.typeset — the tex2jax_ignore guard is inert and #1248 comes straight back.

Suggested direction

Pick one owner for MathJax on the page. Either:

• Don't let better-react-mathjax load MathJax. It already exposes src; there is no supported "use the existing instance" mode, so the cleanest version is probably to drop MathJaxContext/<MathJax> in favor of calling the already-loaded global directly, since renderRunestoneComponent → runestonebase.queueMathJax is doing the real typesetting for previews anyway. The <MathJax> wrappers mostly typeset an empty span on mount.
• Or have MathJaxWrapper's config extend the Runestone config rather than replace it, so at minimum ignoreHtmlClass and processHtmlClass survive, and point src at the same jsdelivr URL to avoid the second download.

The first is the real fix; the second is a cheap stopgap.

Repro pages used for the measurements above are throwaway; happy to attach them if useful.

Start with components/rsptx/templates/common/static_assets_min.html:26-75 and assignment_builder/src/components/routes/AssignmentBuilder/MathJaxWrapper.tsx:72, then trace its four preview mounts and runestonebase.js:591-619. Reproduce the load sequence in Chromium and choose one MathJax owner; done means no second CDN load or configuration clobber, while ignoreHtmlClass remains honored and preview typesetting still works.