\\[] fails with negative height lengths

Created by: DonJBridges

If \\ is given a negative length, for example like in

\begin{align}
x + y = z \\[-1 pt]
1 + 2 = 3
\end{align}

MathJax gives no output. However, negative vertical spacings are extremely useful for fine-tuning the spacing between interwoven formulas, e.g.

\begin{align}
x_{x_{x_{x_x}}} + y &= z \\ %Insert [-6 pt] space here!
2 &= 1 + 1 + 3^{3^{3^3}}
\end{align}

This might also be related to this issue.