\newcommand redefines TeX commands (such as \begin, \mathbf, ...)

Created by: SpectralFlame

Issue Summary

When defining a new command (using \newcommand or \def) on a line before a command such as \begin{...}; in the process of typing that, it will redefine \begin to whatever is in the braces, essentially breaking any environments defined with \begin. This issue is not specific to \begin, though.

Steps to Reproduce:

Starting point

$$
\begin{aligned}
a = 3
\end{aligned}
$$

Type \newcommand on the first line to get

$$
\newcommand

\begin{aligned}
a = 3
\end{aligned}
$$

At this point, \begin is redefined to aligned, even though \begin is already a defined command. Also, the purpose was to define another command entirely, but in the process of writing it, something else got redefined too. Ideally, one would be able to define a new command at the top without having to worry that stuff could break by accident while typing out the command.

Technical details:

MathJax Version: 3.2
Client OS: Linux 5.19.9-arch1-1
Browser: Vivaldi 5.4.2753.51 (Stable channel) stable (64-bit)

I am using the following MathJax configuration: Live demo on https://www.mathjax.org/#demo