<!DOCTYPE html>
<p>Horizontal operators or non-stretchy operators inside an mrow element should not stretch.</p>
<math>
  <mrow>
    <mspace height="100px" width="10px" mathbackground="red"/>
    <mo lspace="0px" rspace="0px">_</mo> <!-- stretchy horizontal operator -->
    <mo lspace="0px" rspace="0px" stretchy="false">|</mo> <!-- non-stretchy vertical operator -->
  </mrow>
</math>