The old HTML-CSS renderer was configured to allow automatic line breaks in long formulas:
<script type="text/x-mathjax-config">
MathJax.Hub.Config({"HTML-CSS": [...], linebreaks: { automatic:true }, [...]
[...]
});
</script>
However, the new CommonHTML renderer is not configured as such, and so long formulas overflow the width of the post. I guess it's an oversight (that would be rather easy to fix). The MathJaX docs recommends these options for automatic line breaking:
<script type="text/x-mathjax-config">
MathJax.Hub.Config({
CommonHTML: { linebreaks: { automatic: true } },
"HTML-CSS": { linebreaks: { automatic: true } },
SVG: { linebreaks: { automatic: true } }
});
</script>
(This was previously reported in the MathJaX 2.6 beta thread, but since it seems to be a configuration error on SE's end rather than a bug in MathJaX, I thought it would be best to report it in its own thread.)