<!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"/>
    <mtext>|</mtext>
    <mtext>_</mtext>
  </munderover>
</math>