The following MathJax macro breaks the standalone WYSIWYG editor if the "Load JavaScript Skin Extensions" configuration is enabled:

{{mathjax}}
\begin{equation}
N_{i_r j k+1} = \prod_{0<= i <= k} \left(1 +r_{j i} \left(t_{i+1}-t_i\right) \right)
\end{equation}
{{/mathjax}}

The problem seems to be that when the MathJax macro is rendered some CSS is injected in the HEAD of the document from the iframe used by the editor. This CSS contains unescaped '<' less than symbol, which is valid AFAIK, so this much be a CKEditor bug.

<style id="MJX-CHTML-styles">
...
mjx-c.mjx-c3C::before {
  padding: 0.54em 0.778em 0.04em 0;
  content: "<";
}
...
</style>

I reported XWIKI-21803 on the platform side.