MathJax is really eager to be applied to posts

Question

When writing a MathJax heavy post I noticed a couple of bugs. For reference, here is an example of how the MathJax should look:

$$ a_{i+1} = \begin{cases} \frac{a_i}{2} & \text{if $a_i \% 2 = 0$ (even)} \\ 3a_i + 1 & \text{if $a_i \% 2 = 1$ (odd)} \end{cases} $$

Begin and end work outside of math mode

Code

a_{i+1} = \begin{cases}
    \frac{a_i}{2}  & \text{if $a_i \% 2 = 0$ (even)} \\
    3a_i + 1 & \text{if $a_i \% 2 = 1$ (odd)}
\end{cases}

Live

a_{i+1} = \begin{cases} \frac{a_i}{2} & \text{if $a_i \% 2 = 0$ (even)} \\ 3a_i + 1 & \text{if $a_i \% 2 = 1$ (odd)} \end{cases}

Preview

An unclosed math-mode delimiter swallows some backslashes

Related: The MathJax newline \\ is broken in tag wikis

Code

$$
a_{i+1} = \begin{cases}
    \frac{a_i}{2}  & \text{if $a_i \% 2 = 0$ (even)} \\
    3a_i + 1 & \text{if $a_i \% 2 = 1$ (odd)}
\end{cases}

Live

$$ a_{i+1} = \begin{cases} \frac{a_i}{2} & \text{if $a_i % 2 = 0$ (even)} \ 3a_i + 1 & \text{if $a_i % 2 = 1$ (odd)} \end{cases}

Preview

Answer 1

Begin and end work outside of math mode

The dollar signs are not the only delimiters that MathJax looks for. Because environments like \begin{align}...\end{align} also introduce math mode (they don't require other delimiters in LaTeX), MathJax also processes any \begin{xyz}...\end{xyz} pairs as math delimiters. So even though you haven't closed your dollar signs, MathJax picks up on the \begin{cases}...\end{cases} and thinks they are math delimiters, so processes the contents. That is why you are seeing part of your expression processed by MathJax.

An unclosed math-mode delimiter swallows some backslashes

It's not MathJax that's doing it, it is the Markdown processing. The \% has been turned into % in the page source, and so the rest of that line (2 = 0$ (even)}) is considered by MathJax to be a comment. Note also that the final \\ has been turned into a single slash. It used to be that the preview code was set up to work the same as MathJax in terms of determining what is math and what isn't, but that seems to have diverged over time. So the preview doesn't realize this is math, and so is performing Markdown substitutions. I suspect the unclosed $$ is confusing the preview.

Answer 2

Although we don't have the new "Live/Preview" editor on math.SE, I have seen the the Live part of the first bug many times when entering a post. I have believed this to be a problem with the browser. Usually, if I wait for a few seconds, the MathJax renders fine in the Live window. If I copy the post (so that I keep all my changes after the last draft save) and refresh my page until the Live window renders immediately, I can paste the copied version of my post back into the Code window and all continues as it should.

I've not noticed the second bug, but when my MathJax is not rendering properly, I look for where the problem is in my code and correct it. Perhaps this bug exists for me, but I've corrected my code too quickly to notice that it is this bug.