MathJax filter does not work with with ajax

The problem is that we only load MathJax on pages that need it, and only apply it to content that needs it.

This is a problem when you are doing things with Ajax. If the inital page does not contain any mathematics, and then ajax tries to load some content that does contain mathematics, it is not rendered.

Note that, to make this work, you already need code like

if (typeof MathJax !== 'undefined') {

   MathJax.Hub.Queue(["Typeset", MathJax.Hub, this.validationdiv.getDOMNode()]);

}

in your ajax code, to get the maths rendered in content after you have added it to the DOM.

Example steps to reproduce, using STACK, which is where I am hitting this:

1. Install qtype_stack and all dependencies.
2. Create a question with no TeX in the questiontext. (E.g. "What is one and one?")
3. Preview the question, type something into the answer box.
4. When the response validation runs, the maths in the new content is not rendered.

This does not happend if there is an equation like

\[1+1\]

in the question text.