Middle delimiter do not recognize in some cases

[den-low]

Issue Summary

The middle delimiter do not recognize inside \left \right tex expression in some cases.

Steps to Reproduce:

Enter tex expression in MathJax demo: \left( \color{red} a \middle| \color{yellow} b \right)
and get an error message: \middle must be within \left and \right
a correct math expression is expected

Technical details:

• MathJax Version: 3.1.2
• Client OS: Windows 7
• Browser: Version 89.0.4389.90 (64-bit)

[dpvc]

Thanks for the report. The implementation is not sophisticated enough, it seems. I will look into it further.

A workaround for now would be to do use

\left( {\color{red} a} \middle| {\color{yellow} b} \right)

where the color is in braces so that the color command ends before the \middle is processed. (It is the presence of the color-change item in the internal parser stack that is confusing the current implementation of \middle. Ending the color before \middle eliminates that.)