<!DOCTYPE html>
<p>Vertical or non-stretchy operators inside an munderover element should not stretch.</p>
<math style="font: 20px Ahem">
  <munderover>
    <mspace width="100px" height="10px" mathbackground="red"/>
    <mo lspace="0px" rspace="0px">|</mo> <!-- stretchy vertical operator -->
    <mo lspace="0px" rspace="0px" stretchy="false" accent="false">_</mo> <!-- non-stretchy horizontal operator -->
  </munderover>
</math>