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This one will have a dot.

Edit: Seems to be true, but porpoise's comments is a clear counterexample.

If you are to delete your last comment, the preceding comment will not reacquire its dot.

This is my most recent message. Does it have a dot?

Yes

Yes, but please don't participate in any more discussion on HN anymore otherwise this thread will all seem very confusing.

I think this is right, where "most recent post" is chosen from both comments and submissions. Looking at top of the https://news.ycombinator.com/newest page illustrates this pattern. Almost every timestamp there ends with a dot, except for the cases where the submitter commented on their own submission, in which case that comment has the dot. For example: https://news.ycombinator.com/item?id=25198319.

Seconded; this can be seen on Hemmert's last two comments (both of which are on this post): https://news.ycombinator.com/threads?id=hemmert

This one will not have a dot.

I'm just gonna test this here.

This comment should have a dot. The above one not.

Dot comment?

I think you are correct. That's something I failed to notice when I referenced your post.