minsize/maxsize in horizontal operators

[fred-wang]

Currently the core spec only use them for vertical stretching: https://mathml-refresh.github.io/mathml-core/#algorithm-for-stretching-operators-along-the-block-axis
MathML3 says it should https://www.w3.org/TR/MathML3/chapter3.html#id.3.2.5.8.4

WebKit/Blink don't not support minsize/maxsize for horizontal operators. Gecko seems to have code to handle that case but I'm not able to make it work.

Testcase:

<math>
  <mn>1</mn>
  <mo>+</mo>
  <munder>
    <mn>2222222222222</mn>
    <mo maxsize="2em">→</mo>
  </munder>
  <mo>+</mo>
  <munder>
    <mn>3</mn>
    <mo minsize="10em">→</mo>
  </munder>
  <mo>+</mo>
  <mn>4<mn>
</math>

[fourpoints]

Related? https://bugzilla.mozilla.org/show_bug.cgi?id=1494915

A use case for this is when writing chemical equations to ensure the arrows are of desired width, where overset text is common.

[fred-wang]

Related? https://bugzilla.mozilla.org/show_bug.cgi?id=1494915

Seems to be the same bug, yes

[NSoiffer]

The current version of the spec continues to specify rules involving minsize/maxsize for vertical stretching only, which is out of sync with MathML 3.

@fred-wang: If this is the plan for level 1, "needs spec update" label should be removed and (likely) a level 2 label should be added along with a "needs polyfill" label.

I added a "chemistry" label because of the use case noted by @fourpoints.

[fred-wang]

Moved to label 2, since none of the browsers implement or have plan to implement it per initial comment.