support linebreaks in munder

It would be nice if something like the following would work.

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <munder>
    <mrow class="MJX-TeXAtom-OP MJX-fixedlimits">
      <munder>
        <mrow>
          <mi>f</mi>
          <mo stretchy="false">(</mo>
          <mi>t</mi>
          <mo>+</mo>
          <mn>1</mn>
          <mo stretchy="false">)</mo>
        </mrow>
        <mo>&#x23DF;<!-- ⏟ --></mo>
      </munder>
    </mrow>
    <mrow class="MJX-TeXAtom-ORD">
      <mtext>distance at&#xA0;</mtext>
      <mo linebreak="newline"></mo>
      <mtext>time&#xA0;</mtext>
      <mi>t</mi>
      <mo>+</mo>
      <mn>1</mn>
    </mrow>
  </munder>
</math>