<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Check that UnderBar is visible when stretched</title>
    <style type="text/css">
      html { background-color: white; font-size: 50px }
      mi { color: white; }
      mo { color: black; }
    </style>
  </head>
<body>
  <math xmlns="http://www.w3.org/1998/Math/MathML">
    <munder>
      <mi>long</mi>
      <mo>&#x332;</mo>
    </munder>
  </math>
</body>
</html>