When I edit this answer, it takes ages to render the MathJax, quite unlike on other pages, where it's a matter of a second or two. When it does render, the big eqnarray* equation is on a single line extending beyond the right margin, ignoring the explicit line breaks.
This happens in both Firefox 60.0.2 and Safara 11.1.1 under Mac OS 10.13.5. It doesn't happen when I copy the answer to the edit window of another page, so it might be that it's caused not by something in the answer but by something else on the page. It also doesn't happen when the page is simply displayed, only when I edit the answer.
P.S.: I just encountered this problem again when editing my answer to this question. As in the previous case, there's a very long delay in rendering (initially upon opening the edit view it doesn't render at all for up a minute before I make a change), this happens only when editing, and when it does render, the table is messed up and all in two lines with the second one extending way beyond the margin.
P.P.S.: Now, after I made and saved an edit, it's working normally again.