[commonHTML, 2.6.0] asciimath input: "vec" arrow position shifted

Created by: beeplin

just open the live demo window on the mathjax homepage and try:

`vec x`
$\vec x $

the two return different results, the first of which (the asciimath one) has a wrong arrow position. it's a calculation error for the padding-left of the ".mjx-over" span:

<span ... class="mjx-over" style="...padding-left:0.201em...">