I'm not entirely sure why this happens, but this callback was passing (menu, event, undefined, instance_id) - menu, event are normal, from the parent (PopupBaseMenuItem), but whatever I put next is undefined, followed by a valid argument.

Changing it from a callback function to an anonymous function fixes the problem.

This worries me a bit, why this suddenly doesn't work.  Could be cjs, but everything else appears working, I'll bisect when I get a chance.