The Mathjax filter does not respect if it is enabled or disabled on the current context. It will always apply.

To reproduce (for example on 5.0 moodle on sandbox.moodledemo.net):

• Create a course
• Enable edit mode
• Add a "page" activity for example
• Add some example content to the content field, for example:

  mathjax example content: inline \( \frac{5}{3} \) and display style: $$ \frac{5}{3} $$

• Save and view, formulas should be rendered by mathjax
• Then go to "Filters" and disable Mathjax filter
• Open page again, here's the bug: Formulas should not be rendered, but they are

I digged a bit further and the reason is that core_filters\text_filter::setup is ALWAYS being called, no matter if the filter actually is active in the context. In the overwritten method filter_mathjaxloader\text_filter::setup however the mathjax javascript is already being loaded and run.

Actually I'm not completely sure if this is a mathjax issue and the setup method is being implemented in a wrong way or if the setup method should not be called for all filters if they are disabled on a context.