I am a refugee from the Collapsing Categories plugin, which seems to struggle with WordPress 3.8. This looks almost perfect as a replacement - it works very smoothly indeed!
The "almost" is because I really need the posts titles to show under each subcategory. Then I'd have a top-level category - then (when the top-level category is expanded) several sub-categories within each top-level category - then (when the sub-level category is expanded) a list of the posts within that sub-category.
That way, a reader can find the relevant post in the list, click on its title and be taken straight there.
Would it be possible for you to add this option?
Thanks very much!