Some mfracs unexpectedly closed up to preceding equals sign

[jmitsdarfer]

Issue Summary

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

<mfrac> <mrow> <mo>&#x2202;</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>&#x2202;</mo> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <mfrac> <mrow> <mo>&#x2202;</mo> </mrow> <mrow> <mo>&#x2202;</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>&#x2202;</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>&#x2202;</mo> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <mo>&#x2202;</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>&#x2202;</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>&#x2202;</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:

MathJax Version: master - latest commit: MathJax.js v2.7.9
Client OS: Windows
Browser: Version 94.0.4606.61 (Official Build) (64-bit)
Renderer: HTML-CSS

[dpvc]

This looks like it may be a bug in v2. I suspect that it is related to the fact that an mfrac whose numerator is an mo is considered to be an embellished operator in MathML (see the MathML spec), and so MathJax is not getting the correct TeX class for the fraction. It works correctly in v3, so I'll have to look into the v2 code to see why it is not getting the right class. Because this is based on the fraction as an embellished operator, that is why when you change to <mi>A</mi> (no longer an operator), it works.

In the meantime, you can add a blank <mi></mi> to the numerator and that should take care of the spacing.

[dpvc]

OK, here is a configuration script that resolves the issue:

<script type="text/x-mathjax-config">
MathJax.Hub.Register.StartupHook("mml Jax Ready", function () {
  MathJax.ElementJax.mml.mfrac.Augment({
    setTeXclass: function (prev) {
      this.getPrevClass(prev);
      for (var i = 0, m = this.data.length; i < m; i++) {
        if (this.data[i]) this.data[i].setTeXclass();
      }
      return this;
    }
  });
});
</script>

[jmitsdarfer]

@dpvc Thanks for trying to suggest a configuration update to try. In this case, I fear it has made it worse. Here are my examples before the configuration update:

The first example is the problem one, where the equals is closed up on the right. The other examples looked as expected with the equals spaced on the left and right.

And here are these same examples but with the new configuration update:

Now, all three examples have unexpected spacing for the equals sign, i.e., closed up on left and spaced on right. I'm wondering if you see something obvious in the coding provided for the configuration update or if there's something else going on--maybe related to one of the other configuration updates we have in place. If you need to see any further details, just let me know. Thank you!

[dpvc]

Sorry, I left off the return this; that should have ended the updated setTeXclass() method. I've added it in the code example above. See if that resolves the problem.

[jmitsdarfer]

Yep, that fixed it. Thank you!