A GreaseMonkey/TamperMonkey version of the `math-with-slack` script.

script.user.js

// ==UserScript==
// @name        MathJax on Slack
// @namespace   https://github.com/fsavje/math-with-slack
// @description Renders math on Slack using MathJax. Based on github:fsavje/math-with-slack.
// @include     https://*.slack.com/messages/*
// @version     1
// @grant       none
// ==/UserScript==

// See: https://github.com/fsavje/math-with-slack
window.addEventListener('load', function() {
  var mathjax_config = document.createElement("script");
  mathjax_config.type = "text/x-mathjax-config";
  mathjax_config.text = `
    MathJax.Hub.Config({
      messageStyle: "none",
      extensions: ["tex2jax.js"],
      jax: ["input/TeX", "output/HTML-CSS"],
      tex2jax: {
        skipTags: ["script","noscript","style","textarea","pre","code"],
        inlineMath: [ ['\$','\$'] ],
        displayMath: [ ['\$\$','\$\$'] ],
        processEscapes: true
      },
      TeX: {
        extensions: ["noErrors.js", "noUndefined.js"]
      }
    });
  `;

  var mathjax_script = document.createElement("script");
  mathjax_script.type = "text/javascript";
  mathjax_script.src = "https://cdn.mathjax.org/mathjax/latest/MathJax.js";

  document.getElementsByTagName("head")[0].appendChild(mathjax_config);
  document.getElementsByTagName("head")[0].appendChild(mathjax_script);

  var render = function(records, observer) {
        MathJax.Hub.Queue(["Typeset", MathJax.Hub]);
      },
      target = document.querySelector('#msgs_div'),
      observer = new MutationObserver(render),
      config = { attributes: false, childList: true, characterData: true, subtree: true };
  observer.observe(target, config);
});

In my fork I added a line

ignoreClass: "ql-editor",

which fixes problems with the editing behavior. The issue is the same as for the desktop client, as described here.