Looking at the forum, I can see which threads I've looked at easily enough (they're a different colour) and I can see which ones I'm "up to date with" because their background is shaded.
Some time a few weeks ago that second part stopped working properly. When I post something to a thread it now appears immediately (before that I needed to hit "refresh", this is an improvement) but the index entry is no longer shaded, even though I've seen my own post.
Of course if someone else also posted to that thread while I was perpetrating my update, all bets are off.
Update: this does not apply with a new thread, like this one.