/usr/share/SuperCollider/HelpSource/MathJax/MathJax.js is in supercollider-common 1:3.6.3~repack-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/SuperCollider/HelpSource/MathJax/../../../javascript/mathjax/MathJax.js