Forum engine with tree-like discussions like on Habré?

Question

CMS

What you want:
List of forum sections, topics, subtopics - like regular forums.
Discussions in threads are tree-like, as in Habré. The text of all comments can be seen immediately, without the need to click somewhere and expand something.
I suspect that this can be configured in some common forums, but nothing is found by googling. I look forward to your advice.
I want the engine to be mature enough and proven.
Paid options are being considered. SaaS option is also suitable, even preferable.

Answer 1

LiveStreet CMS is almost a copy of Habr, try it. True, this is rather an engine for a collective blog than for a forum.

Answer 2

So in any engine with the appropriate functionality, you can set the discussion tree in ... What's complicated is ... Here's how to do it in Wordpress

Answer 3

userecho.com - SAAS service, supports a forum with nested categories, comments are also tree-like.