The changes in Moodle 2.3 to the structure of course formats has changed radically and with the implementation of the cookie law in some countries I have decided to rewrite Collapsed Topics for Moodle 2.3. This will take some time and although I have a beta version for it, the AJAX drag and drop does not work well due to changes in core code - but it does work - just!
Therefore I have created CONTRIB-3652 to manage this transition. If you have any thoughts / suggestions / willing to help then please comment here or on the tracker issue.
I am also giving a presentation at iMoot 2012 on the format if you would like to participate .