Combining Right Arrow Above U+20D7 display issue

[olegip]

Issue Summary:

The Right Arrow Above U+20D7 character is not displayed as expected. Specifically, the arrow is centered and has a fixed width, so when displayed over a string of characters, it is not stretched to cover the full width of the expression below.

Here is the screenshot illustrating the problem
https://www.screencast.com/t/BfrEZ5G0

Steps to Reproduce:

Render following MathML in the browser

<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent="true"><mrow><mi mathvariant="italic">ABCDE</mi></mrow><mo>⃗</mo></mover></math>

Observed Behavior:

The arrow is centered and has a fixed width, not stretching to cover the entire width of the expression "ABCDE."

Expected Behavior:

The arrow should stretch to cover the full width of the expression "ABCDE."

Alternative Approach:

When replacing U+20D7 with Rightwards Arrow U+2192 the formula displayed as expected.

<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent="true"><mrow><mi mathvariant="italic">ABCDE</mi></mrow><mo>→</mo></mover></math>

Technical details:

• MathJax Version: 3.2 and beta 4
• Client OS: Any
• Browser: Any

Supporting information:

Note that MathML source equation is displayed in MS word correctly. But when copying this equasion and then pasting it into notepad the U+20D7 char is being replaced with U+2192.
When looking in the document.xml however, it is still the U+20D7 that is used in the equation.

Thank you,
Oleg

[dpvc]

Whether a character stretches is determined by the MathML "operator dictionary", which is described in Appendix C of the MathML specification. The character U+2192 (right arrow) is defined as a stretchy accent (it gets stretchy="true" and accent="true" attributes by default), but U+20D7 (combining over right arrow) is not listed in the table, so gets no special attributes. In particular, it is not stretchy by default. It may be that Word is substituting U+2192 for U+20D7 and so is considering it as stretchy, but the MathML specification doesn't indicate that U+20D7 should be stretchy by default. Or they may have augmented the operator table to include other characters that aren't part of the MathML recommendations.

There are two possible solutions. First, you cold use

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mover accent="true">
    <mrow>
      <mi mathvariant="italic">ABCDE</mi>
    </mrow>
    <mo stretchy="true">&#x20D7;</mo>
  </mover>
</math>

or you could use the configuration

MathJax = {
  startup: {
    ready() {
      const {TEXCLASS} = MathJax._.core.MmlTree.MmlNode;
      const {OPTABLE, MO} = MathJax._.core.MmlTree.OperatorDictionary;
      OPTABLE.infix['\u20D7'] = MO.WIDEACCENT;
      MathJax.startup.defaultReady();
    }
  }
}

to add U+20D7 to the operator table with the defaults for the stretchy and accent attributes.

[olegip]

Thanks @dpvc
Adding stretchy="true fixes the display issue.