TLDR: workaround: sort by activity and scroll to the bottom if this happens to you.
If the last event in a question in post-voting mode is the closure of that topic, then the category overview will still show 1 unread message, even after you have read all the visible replies.
This appears to be because the actual “reply” for the closure is hidden when sorted by votes. I presume this is because a topic closure is not vote-able, but for some reason still counts as a reply “to be read” even though it displays a lock on the category view.
I noticed this today in the Help and support category. After reading the latest message on this topic (A) the main category listing was still showing 1 unread reply. Another topic (B) that was closed at around the same time correctly showed as closed once the final response was viewed. The only way to get topic A to show as read was to change the sort from “Votes” to “Activity” and scroll down to the bottom at which point the previously filtered “
Closed [X hours] ago” message is shown and the unread flag is finally cleared. Sorting topic B by activity shows that it was closed before the final moderator message was posted and so the final message was not filtered in the default “Votes” sort and the unread flag was cleared when it was displayed.
Does this mean that if a user returns to a topic after a long time and sees a late, but highly rated, reply at the top of the page the whole thread will be marked as read even if earlier lower ranking replies haven’t actually been read?