Arrows x21c4 and x21c6 do not stretch

[jmitsdarfer]

We have many arrows in our character set, and they all seem to stretch as expected except for x21c4 and x21c6. Here are some examples using the two problem characters:

<m:mi mathvariant="normal">A</m:mi><m:mo>+</m:mo><m:mi
mathvariant="normal">S</m:mi><m:munderover><m:mrow><m:mo>⇄</m:mo>
</m:mrow><m:mrow><m:mi>k</m:mi></m:mrow><m:mrow><m:mtext>dehydration</m:mtext>
</m:mrow></m:munderover><m:mi>AS</m:mi>

<m:mi mathvariant="normal">A</m:mi><m:mo>+</m:mo><m:mi
mathvariant="normal">S</m:mi><m:munderover><m:mrow><m:mo>⇆</m:mo>
</m:mrow><m:mrow><m:mi>k</m:mi></m:mrow><m:mrow><m:mtext>dehydration</m:mtext>
</m:mrow></m:munderover><m:mi>AS</m:mi>

I'd expect the ones above to stretch just like, e.g., this one does:

<m:mi mathvariant="normal">A</m:mi><m:mo>+</m:mo><m:mi
mathvariant="normal">S</m:mi><m:munderover><m:mrow><m:mo>⇌</m:mo>
</m:mrow><m:mrow><m:mi>k</m:mi></m:mrow><m:mrow><m:mtext>dehydration</m:mtext>
</m:mrow></m:munderover><m:mi>AS</m:mi>

Please see attached for screenshots. Is this a known bug?

Thanks!

Technical details:

MathJax Version: master - latest commit: MathJax.js v2.7.5
Client OS: Windows
Browser: Chrome Version 91.0.4472.114 (Official Build) (64-bit)

[dpvc]

Stretched characters like these arrows are made from multi-character assemblies, and in order to do that, we need to have the proper glyphs to make that possible (the two end pieces and the repeated middle piece). While the MathJax fonts have the ones needed for the arrow with hooks (U+21CC), which uses U+296B, U+296C, and the equal sign. Unfortunately, U+21C4 and U+21C6 would require a left arrow under a long dash (and above one) and similarly for the right arrow., but there don't seem to be any unicode positions for those, so fonts don't include them. It would be possible to use the Private Use Area to store such glyphs, but the MathJax fonts don't have them, and not even the STIX fonts seem to have them. So MathJax can't make the stretchy character for these arrows.

Note that the MathJax fonts also don't contain the glyphs needed for U+21CC, but since you are using the STIX fonts, which do, that first arrow is able to be stretchy.

[jmitsdarfer]

OK that makes sense. Sounds like we'll just need to accept it as is then. Thank you for the explanation!

[dpvc]

It is possible to do something more complicated to get this output, however. It displays correctly, but will not work well with assistive technology, and is a terrible hack.

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mi mathvariant="normal">A</mi>
  <mo>+</mo>
  <mi mathvariant="normal">S</mi>
  <munderover>
    <mrow class="MJX-TeXAtom-REL">
      <mstyle scriptlevel="1">
        <mover>
          <mo>&#x2190;</mo>
          <mpadded height="-.1em" voffset="-.1em">
            <mo>&#x2192;</mo>
          </mpadded>
        </mover>
      </mstyle>
    </mrow>
    <mrow>
      <mi>k</mi>
    </mrow>
    <mrow>
      <mtext>dehydration</mtext>
    </mrow>
  </munderover>
  <mi>AS</mi>
</math>

which produces:

in HTML-CSS output.

[jmitsdarfer]

OK, yeah, that is QUITE the workaround! :) Thanks, though--I appreciate you tossing the idea out there. There are just 2 arrows out of our whole set that aren't stretching as we'd expect. I feel like we could just live with this as is, but I'll mention the workaround idea just in case anyone wants to consider it. Thank you!

[dpvc]

Version 4 (in alpha release, and preparing the full release) has a new default font that includes the needed characters to make these two stretchy. You might consider updating to v4 when it is officially released.

[jmitsdarfer]

Thank you for mentioning this, Davide!