\tag{...} in $$ math blocks renders as a one-character-wide vertical column

[gustavdelius]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Other

Body

Summary
A display equation containing \tag{...} is squeezed into a vertical strip about one character wide in Chromium-based browsers and Firefox. Removing \tag fixes it.

Reproduction

Broken (\tag used):

$$ \partial_t \mathcal{Q} + \partial_x \mathcal{J} = 0 \tag{2.1} $$

Source:

$$
\partial_t \mathcal{Q} + \partial_x \mathcal{J} = 0 \tag{2.1}
$$

Expected: the equation renders normally with the label (2.1) on the right, as in LaTeX/MathJax.

Actual: the equation is crushed into a narrow vertical column and is unreadable.

Likely cause: the generated MathML for \tag appears to use <mlabeledtr>, which MathML Core removed, so browsers with native MathML don't lay it out as a table row.

Workaround: end the equation with \qquad\text{(2.1)} instead of \tag{2.1}:

$$ \partial_t \mathcal{Q} + \partial_x \mathcal{J} = 0 \qquad\text{(2.1)} $$

Possibly related: github/markup#1584 mentions \tag formatting problems in long formulas.

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[fcappellesso]

Thanks for the clear report — I can reproduce it, and a few extra observations may help:

• In Safari, the equation with \tag{2.1} is not squeezed, but the label is placed on the left instead of the right.
• \tag*{(2.1)} breaks in the same way as \tag{...}.
• A \begin{equation} block is shown without any number, and a "preamble" is not possible: \newcommand with arguments is rejected ('#' can not be used here).

Your report made me curious, so I tried a workaround. I'm a student in mathematics, new to GitHub but using LaTeX every day, and I wanted proper numbered equations in my READMEs. Equations are written in ordinary .tex files with a shared preamble (\begin{equation}, align, \label, section-style numbering). A GitHub Actions workflow compiles them with real LaTeX into SVG, and the README shows them in light and dark versions, so they follow the reader's theme.

I used your equation as the first example (with credit to you and a link to this discussion): https://github.com/fcappellesso/latex-equations-on-github

I know this does not fix the bug itself: the equations are images, not copyable LaTeX, and references in the surrounding text are still typed by hand. It is only a temporary option for anyone who needs to display numbered equations correctly right now. I'm also looking forward to a real fix on GitHub's side. Thank you for reporting it!