Similar to when a member makes a poorly formatted post, the tries to edit it.
If at the same time a Mod fixes it before the member finishes the Mod fix will be replaced with the member fix when they Save.
I think most times the chances of more than one working on a post / topic at the same time are rare. Though the problem has come up before, wiki edits in particular.
I know databases can lock transactions, but I don’t know how it could be dealt with here.
I have seen it suggested to prevent further topic replies if another is in the process of replying, which I think is a poor idea.
I tried to repro and that is what I found, I could not repro the ‘aggravating’ from OP.
userA and userB have the topic open in the editor, topic category is I.
userA changes topic to category II and posts something
userB receives the reply immediately, but for him topic is still in category I, i.e. the post count and content refresh but the category and title do not.
if userB replies (for him the post is still category I) this does NOT put the topic back to category I. If userB leaves the editor he will see the new category for the topic in the topic list.
I think the category and title change commit button should really trigger the same type of refresh event for category and title that is triggered by the reply button if that makes sense.