Underline does not display in fraction. Mathjax bug?

Question

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.

Answer 1

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.