output/chtml component sometimes renders msubsup's superscript as a subscript if the real subscript is void/empty

[EricDunsworth]

Issue Summary

MathML's <msubsup> element is meant to contain three child elements that represent base, subscript and superscript. If the subscript is void and the superscript is filled-in, the superscript is still supposed to be treated as a superscript.

MathJax 3.2.0's output/chtml component seems to be prone to misrepresenting the msubsup element's superscript as a subscript if the real subscript is void or empty. But not always :S.

It seems to be dependant on nearby elements and which elements are used inside msubsup. In the reproduction steps below, I was able to provide examples that always render properly, always render improperly and one that renders differently depending on whether it precedes or follows other equations.

The input/mml and output/svg components seem to be unaffected. The accessibility extension always identifies superscripts correctly and the SVG renderer always renders them as such.

Steps to Reproduce:

JSBin examples:

• Cauchy-Schwarz Inequality, never works, always works
• Never works, always works, Cauchy-Schwarz Inequality

Expected outcome:
All superscripts are rendered as superscripts (top-aligned).

Actual outcome:

• The "Never works" example's superscript is rendered as a subscript (bottom-aligned).
• The "Always works" example's superscript is rendered as a superscript (top-aligned).
• The "2" superscript in the Cauchy-Schwarz Inequality appears as a subscript if that equation comes first.
• The "2" superscript in the Cauchy-Schwarz Inequality appears as a superscript if that equation comes last.

Technical details:

• MathJax Version: 3.2.0 (latest commit: 7146ffa)
• Client OS: Windows 10
• Browser: Edge 92.0.902.55, Firefox 92.0b9, Internet Explorer 11

Supporting information:

• MathJax 2.7.x is unaffected.
• Firefox's native MathML renderer is unaffected (although the issue does occur in that browser if MathJax 3.2.0 is used).
• This issue was first spotted in MathML polyfill: Update MathJax to ^3.2.0 wet-boew/wet-boew#9143:
  • I tried updating MathJax 2.7.4 to 3.2.0's mml-chtml preset.
  • @GormFrank noticed that it changed all of the superscripts in WET's MathML examples into subscripts.
  • WET's MathML examples seem to be based on the complex1 examples from the W3C's MathML test suite.

[dpvc]

Thanks for the report. It turns out that the order in which the CSS is generated affected which rule was used (in a way that I didn't expect). I have created a pull request to resolve the issue. In the meantime, you can use

<style>
  mjx-script > * {
    display: inline-block ! important
  }
  mjx-script > mjx-spacer {
    display: block ! important
  }
</style>

to remove the problem for now.