I am using
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0 with a 1536×731-pixel browser window.
This is a pretty common display bug that can be triggered in many ways: silly Unicode tricks, long code blocks, broken MathJax, missing MathJax delimiters, changing the font size in Chrome or in Firefox, etc. It was first reported a long time ago, and a server-side kluge was added to "fix" it (occasionally breaking code embedded in comments or very long URLs as a side effect), but it didn't really solve the problem completely.
The overly long formula in Brian J's comment is the immediate cause here. You can notice that the "server-side kluge" (insertion of zero-width characters) had some effect there, breaking two of the math operators. But it didn't save the layout of the page, since MathJax then rendered the formula according to its own linebreak logic.
- Don't put overly long formulas into comments. Split them in several if necessary: $\arctan \sec$ $\arctan \sec$ $\arctan \sec$...
- If someone else does the above and breaks the page layout, flag the comment and ask a moderator to edit. (Assuming it's a valuable comment, otherwise deleting is easier. The user, if they are still around, can delete on their own but cannot edit beyond the 5-minute period.)