Some mfracs unexpectedly closed up to preceding equals sign

Created by: jmitsdarfer

Issue Summary

I was surprised to come across the equals sign and mfrac closed up in this case:

<mfrac> <mrow> <mo>∂</mo> <mi>p</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo>,</mo> <mi>t</mi> <mo stretchy="false">)</mo> </mrow> </mrow> <mrow> <mo>∂</mo> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <mfrac> <mrow> <mo>∂</mo> </mrow> <mrow> <mo>∂</mo> <mi>x</mi> </mrow> </mfrac> <mi>D</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow>

I expected this:

I tested it without the mfrac to the right of the equals, and it spaced as expected, i.e.,

<mfrac> <mrow> <mo>∂</mo> <mi>p</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo>,</mo> <mi>t</mi> <mo stretchy="false">)</mo> </mrow> </mrow> <mrow> <mo>∂</mo> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <mo>∂</mo> <mi>x</mi> <mi>D</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow>

Then, I also tested mi tags rather than mo tags in the mfrac and this also spaced as expected, i.e.,

<mfrac> <mrow> <mo>∂</mo> <mi>p</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo>,</mo> <mi>t</mi> <mo stretchy="false">)</mo> </mrow> </mrow> <mrow> <mo>∂</mo> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <mfrac> <mrow> <mi>A</mi> </mrow> <mrow> <mi>B</mi> <mi>x</mi> </mrow> </mfrac> <mi>D</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow>

Technical details:

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