Although this question seems to be new to M.MO, @MartinSleziak points out several related posts on Meta.MSE.
I was curious about the diagrammatic magic of @YaakovBaruch's recent comment, and so clicked on the relevant diagram and viewed its TeX source.
This source turned out to use a macro
\x which, I assume, was defined earlier in the comment. In this case, I think that I can find the definitions in the parent post Tiling rectangle with trominoes — an invariant; but let's imagine that I couldn't. (Actually, @MartinSleziak pointed out in chat that I was viewing the TeX source for the wrong diagram, so the resolution in this case was even easier than I thought! But the main question still stands.)
Is there any way to see the entire MathJax source for a comment, as there is for a post?