The limit seems to be 5 avatars per thread. The OP is always first (apparently). Then it seems to be the posters with the most posts in the thread, in descending order. But in short threads, where there are several posters with only one post, what factors determine which of those appear among the avatars shown?
This is completely trivial, idle curiosity on my part. Just wondering.
If you click on the thing to the left of the 5 avatars that says [## users], that’ll pop up the list of the top 24 participants in the thread. Which is OP first, then the most prolific posters in decreasing order, then finally the one-post posters.
Among the one-post posters they seem to be arranged in chrono order of when they posted.
The row of 5 avatars is simply the first 5 of the longer list of 24 inside the pop-up.
Also notice that this feature doesn’t appear at all until the thread grows to a certain number of posts / posters. Right now with 2 posts and two posters it isn’t visible in this thread. Also note that if it appears at all, this same UI element appears just below the OP and also at the end of the thread.