Middle delimiter do not recognize in some cases

Created by: 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)