\ncong and \overrightarrow not typeset properly

[jano-m]

Issue Summary

Mathjax typesets \ncong and \overrightarrow{ab} differently from Latex.
\ncong: in Latex whole \cong is crossed out, while in Mathjax only = is crossed out (which has different meaning)
\overrightarrow{ab}: the arrow above is short, equivalent to \vec{ab}, Latex would make the arrow span from left to right boundary of the (imaginary) box containing ab
Note: if the argument of the command gets longer, it will start working properly

Mathjax:

Latex:

Technical details:

• MathJax Version: 3.0.5
• Client OS: Windows 10
• Browser: Chrome 84.0.4147.125

[dpvc]

Thanks for the report.

The first issue (with \ncong) is due to a bad mapping of the glyphs in the original MathJax TeX fonts: U+2247 was inadvertently placed at U+2246, and \ncong was set to U+2246 (see issue #1994). The font data for version 3 maps the glyph to U+2247 correctly, but we neglected to redefine \ncong to map to the correct location. I have made a pull request to fix the issue. In the meanwhile, you could add

<script>
MathJax = {
  startup: {
    ready() {
      if (MathJax.version === '3.0.5') {
        const MapHandler = MathJax._.input.tex.MapHandler.MapHandler;
        const Symbol = MathJax._.input.tex.Symbol.Symbol;
        MapHandler.getMap('AMSsymbols-mathchar0m0').add('ncong', new Symbol('nconfg', '\u2247'));
        MathJax.startup.defaultReady();
      }
    }
  }
};
</script>

before the script that loads the MathJax component you are using in order to fix the errant definition.

---

The second issue, with \overrightarrow is more significant. MathJax will remap some accent characters to other ones (mostly for MathML input where people use the wrong characters frequently). In particular, the right arrow U+2192 is remapped to the vector arrow U+20D7, so that <mover><mi>x</mi><mo>&#x2192;</mo></mover> will produce the output you would get from \vec x. When the accent is stretchy, U+20D7 is converted back to U+2192, but it turns out that MathJax was applying the remapping again and it ended up U+20D7 after all. I have submitted a pull request to fix that, but it is not so easy to patch it. The straight-forward solution is to remove the remapping of the U+2192 so that neither remapping occurs. That can be done with

<script>
MathJax = {
  startup: {
    ready() {
      if (MathJax.version === '3.0.5') {
        MathJax.startup.defaultReady();
        delete MathJax.startup.output.font.remapChars.accent[0x2192];
      }
    }
  }
};
</script>

Of course, you can combine these two into one configuration:

<script>
MathJax = {
  startup: {
    ready() {
      if (MathJax.version === '3.0.5') {
        const MapHandler = MathJax._.input.tex.MapHandler.MapHandler;
        const Symbol = MathJax._.input.tex.Symbol.Symbol;
        MapHandler.getMap('AMSsymbols-mathchar0m0').add('ncong', new Symbol('nconfg', '\u2247'));
        MathJax.startup.defaultReady();
        delete MathJax.startup.output.font.remapChars.accent[0x2192];
      }
    }
  }
};
</script>

[jano-m]

Thank you!

[jano-m]

Actually, I have just noticed. The proposed fix will break command \vec.
For example, \vec{a}

• in Mathjax (with the above code):
  
• in Latex:
  

[dpvc]

OK, here is an updated configuration that takes care of that as well.

<script>
MathJax = {
  startup: {
    ready() {
      if (MathJax.version === '3.0.5') {
        const MapHandler = MathJax._.input.tex.MapHandler.MapHandler;
        MapHandler.getMap('AMSsymbols-mathchar0m0').lookup('ncong')._char = '\u2247';
        MapHandler.getMap('macros').lookup('vec')._args = ['20D7'];
        MathJax.startup.defaultReady();
        delete MathJax.startup.output.font.remapChars.accent[0x2192];
      }
    }
  }
};
</script>

It is slightly simpler than the earlier one as well.

[dpvc]

Fixed in v3.1 released today.