I've used collapsed topics since moodle version 1.9. I've upgrade successfully to moodle-2.2, 2.4 and 2.5. But after upgrading to 2.6.1 the topics are there but toggles didn't work. I've removed collapsed topics in the previous moodle version and reinstalled it in the upgraded version 2.6.1. Toggles in courses which used collapsed topics in the previous version didn't work, in courses which didn't use them in the previous version they do work.
I attach a screenshot with debug info.