Overloading "Show Source" as overlay etc

From https://groups.google.com/d/msg/mathjax-users/Qg-tpH6CQGQ/0HCElAu3DmgJ

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Add the following script to your page somewhere BEFORE the script that
loads MathJax.js:

    <script type="text/x-mathjax-config"> 
    MathJax.Hub.Register.StartupHook("MathMenu Ready",function () { 
      MathJax.Menu.ShowSource.Window = function () {};  // don't open the   

Show Math window MathJax.Menu.ShowSource.Text = function (text,event) { ... your code here ... }; }); </script>

Note that I have disabled the routine that opens the window used by
the current ShowSource.Text function, so you won't get an unwanted
window.

Add your code in the place indicated. One reason that MathJax uses a
separate window is that it is then easy to "select all" and copy the
contents of the window. That is not so easy to do with a div. You
might consider setting the selection to the contents of the div so
that is is pre-selected when the source is shown.

Davide