SVG Renderer: Vertical misalignment of `\rightleftarrows` when used with `\underset` and `\stackrel`

[hbghlyj]

Issue Summary

In MathJax v4.1.0 using the SVG renderer, the \rightleftarrows symbol is rendered significantly lower than its expected baseline when nested between \stackrel and \underset. This issue does not appear when using the CHTML renderer, where the arrows remain correctly centered between the top and bottom scripts.

Steps to Reproduce:

1. Go to the MathJax Live Demo.
2. Select the SVG output renderer.
3. Enter the following LaTeX code:

$L \underset{\rho}{\stackrel{\alpha}{\rightleftarrows}} M \underset{\sigma}{\stackrel{\beta}{\rightleftarrows}} N$

4. Observe that the arrows are shifted downwards, overlapping or sitting too close to the bottom script. The symbol is pushed downwards in the SVG output, creating an uneven vertical distribution.

5. Switch to the CHTML renderer and observe that the alignment is correct.

Technical details:

• MathJax Version: 4.1.0
• Client OS: Windows 11
• Browser: Chrome v145.0.7632.76
• Renderer: SVG (Issue present) vs. CHTML (Issue absent)

[dpvc]

Thanks for the report. This turns out to be due to a bug in the processing of inline line-breaks in SVG output. There is a potential line break before the right-left arrows, but instead of putting the break outside the \underset as it should, the break is being put inside it as a forced break, which is what is moving the the arrows and their oversets vertically down.

Adding

MathJax = {
  loader: {
    'output/svg': {
      ready() {
        const {SvgWrapper} = MathJax._.output.svg.Wrapper;
        Object.assign(SvgWrapper.prototype, {
          _toEmbellishedSVG_: SvgWrapper.prototype.toEmbellishedSVG,
          toEmbellishedSVG(parents) {
            if (parents.length <= 1 || !this.node.isEmbellished || this.node.parent.isEmbellished) {
              return false;
            }
            return this._toEmbellishedSVG_(parents);
          }
        });
      }
    }
  }
};

to your MathJax configuration is one way to work around the issue.

Alternatively, you can use the more compact

$L \overunderset{\alpha}{\rho}{\rightleftarrows} M \overunderset{\beta}{\sigma}{\rightleftarrows} N$

which also avoids the problem.

I will make a PR to fix the issue.