[KevinH]

Have you by any chance disabled client javascripts in Sigil's Preferences?

Under Sigil Preferences -> General Settings - > Security:

Control Use of Javascript by Epubs

check Epubs may use javascript.

Then restart Sigil so the new MainWindow will get that new setting. The current one has already opened and its setting is the old one.

If I uncheck that and open a new Window (macOS allows that without restarting), then try the example, all I get is "a b". If I enable it (and mathjax is javascript) and then open a new window and try the example it works.