Comments are not ordered by date

So we're currently relying on old Document#getComment API for getting the comments. A proper fix is probably to deprecate this API and the related ones, and to introduce a proper decoupled API in a new module xwiki-platform-comment. We would need interface for defining the comment, a component role and its default implem and a script service. Ultimately this API should also contain the code for computing the threads.

Note that a quick custom fix for deployed instances is also possible by editing directly commentsinline.vm and performing the sort operation just after retrieving the comments.