bbox of arrow is not displayed at expected position.

Created by: w32zhong

Issue Summary

When using bbox around rightarrow under a limit, it has unexpected vertical alignment.

Steps to Reproduce:

1. Paste and render TeX

\lim _ {x \bbox[red]{\rightarrow} 0}

at demo page: https://mathjax.github.io/MathJax-demos-web/input-tex2svg.html

You will see a bad alignment like this:

Notice, changing \rightarrow to \to would not have this issue.