Underline does not display in fraction. Mathjax bug?

In this answer the underline below $k$ in the exponent in the definition of $\binom nk$ cannot be seen. It took \displaystyle for it to appear.

I can reproduce the issue here as well. Compare this: $\binom nk = \frac{n^\underline{k}}{k!}$, with this: $$\binom nk = \frac{n^\underline{k}}{k!}$$

The code is the same,

\binom nk = \frac{n^\underline{k}}{k!}


but in the first instance the underline is not appearing. For reference, this is what I'm seeing:

My browser is Chromium 55.0.2883.87 (64-bit), and other users reported the same issue.

• I'm running Chrome 55.0.2883.87 on Windows 10. It might be an issue with the MathJax math renderer (MathJax context menu ⇒ Math Settings ⇒ Math Renderer). With HTML-CSS I don't see the underline in the inline equation. With Common HTML and Preview HTML I see the underline, though it's positioned horribly. With SVG I see it, faintly as it is somewhat poorly positioned. – user642796 Jan 8 '17 at 9:21
• I'm running MacOS X 10.11.5 and Firefox 50.1.0 and using HTML-CSS. Everything looks fine here. – robjohn Jan 8 '17 at 9:57
• I am running Arch Linux (64bit) with Firefox 50.1.0, HTML-CSS, and it looks the same. But I had to use zoom to actually see that the underline is missing on the inline formula. – Asaf Karagila Jan 8 '17 at 10:31
• Interestingly, without zoom if I look hard enough (or if I zoom using the browser feature), there is an underline. If I change the render to zoom by 300%, though, the underline is gone. Odd. – Asaf Karagila Jan 8 '17 at 10:36
• @arjafi I can confirm. With those two renderers the underline is visible but it's horribly positioned. So it looks like a bug. – rubik Jan 9 '17 at 8:51
• It does seem to be an issue with MathJax. I have started an issue tracker for it on GitHub. – Davide Cervone Jan 9 '17 at 15:58

I'm seeing the underline running through the bottom part of the $k$ on Chrome, which makes it essentially invisible at normal resolution. I'm answering just to try out the possible workaround of specifying an increased exponent size:
$\binom nk = \frac{n^{\large{\underline k}}}{k!}$
\binom nk = \frac{n^{\large{\underline k}}}{k!} - still not highly visible but better
$\binom nk = \frac{n^{\Large{\underline k}}}{k!}$
\binom nk = \frac{n^{\Large{\underline k}}}{k!} - the underline is maybe now too separated but definitely visible.