Have you encountered a spacing problem with the command \tilde? To show what I mean, let's take a look at $\tilde{f}_{k}$ and $f_k$. Do you notice that the subscript $k$ is awkwardly spaced from $\tilde{f}$ in $\tilde{f}_k$? The same problem occurs if I use \widetilde: $\widetilde{f}_k$. Even with \dot, \hat, and \bar commands, the same problem occurs: $\dot{f}_k$, $\hat{f}_k$, and $\bar{f}_k$.
I know there is a way to re-adjust the spacing by using \!, so $\tilde{f}_{\!k}$ and $\tilde{f}_{\!\!k}$ would become $\tilde{f}_{\!k}$ and $\tilde{f}_{\!\!k}$, respectively. They look a lot better than $\tilde{f}_k$, although the spacing isn't the same as in $f_k$ (which annoys me a little bit).
For some other characters, the spacing is ok. For example, $\tilde{a}_k$ produces $\tilde{a}_k$, which has the same spacing as $a_k$. It seems the problem occurs with tall characters like $d$ and $t$: $\tilde{d}_k$ and $\tilde{t}_k$ (comparing with $d_k$ and $t_k$). The problem is lessened with deep characters like $g$ and $y$: $\tilde{g}_k$ and $\tilde{y}_k$ (comparing with $g_k$ and $y_k$).
Is this a bug? If it is, by whom? By MathJax, by StackExchange, or by my browser? I don't recall seeing this problem in LaTeX.
Edit. I got a response from a MathJax developer. If you are interested, see here.
$f_k\hat f_{\hspace{0pt}k}\hat f_{\hspace{-0.5pt}k}\hat f_{\hspace{-1pt}k}\hat f_{\hspace{-1.5pt}k}$$f_k\hat f_{\hspace{0pt}k}\hat f_{\hspace{-0.5pt}k}\hat f_{\hspace{-1pt}k}\hat f_{\hspace{-1.5pt}k}$ (this does not excuse the problem) $\endgroup$