[CommonHTML] Linebreaks in roots cause root to be wrong width

If line breaking is enabled for

<div style="border:1px solid black; width:30px">
<math xmlns="http://www.w3.org/1998/Math/MathML"> 
  <msqrt mathbackground="red">
    <mi>x</mi>
    <mo>+</mo>
    <mi>x</mi>
  </msqrt>
  <mo>=</mo> 
  <mi>x</mi>
</math>
</div>

the width of the square root is incorrectly set to 100%, making it too wide.