How to detect to whom a reply is addressed

Good catch. The forum does track this, it just never showed anyone.

Every post stores the post it's replying to. That gets set when you use the Reply button on a specific post (or Quote it) rather than the composer at the bottom of the thread, and it's what drives the "so-and-so replied to your post" notification. It simply never appeared anywhere in the thread itself, so as a reader you had no way to see it. Fair complaint.

I've just added it. Two things, in the next update:

• A reply carries a small link at the top of the post showing who it's answering, with a snippet of what they said. Click it and you jump to that post, even if it's several pages back.
• The post being answered gets an expandable "3 replies" list at the bottom, so you can follow it the other way too.

No nesting, and that's deliberate. Indentation works for Reddit-style branching arguments, but it wrecks a long support thread: you lose the ability to read it top to bottom, and deep indents are miserable on a phone. A flat stream with links pointing both ways is what Discourse landed on, and I think it's the right trade here.

One bit of restraint worth mentioning: if a reply already sits directly under the post it answers, it gets no marker at all. Otherwise a normal two-person back-and-forth picks up a badge on every single post telling you what the layout already made obvious.

For the case you actually asked about, though: if someone posts from the box at the bottom with no @mention and no Reply button, there genuinely is no target recorded. That's less a gap than a convention. On a flat forum a bare reply is addressed to the thread rather than to a person.

Nice bonus: the reply relationships came across in the Discourse import, so this works on the old threads too, not just new ones.