No matter which way you decide to go you are going to get howls of protest from one quarter or another.
Speaking for myself and those who think like I do:
- You should nuke that code. On a web interface in particular no one would ever expect that if they've done a reply to a message in an existing thread in topics view, even if they tweaked the subject in some way, that it would not remain a part of the thread in which that reply is being made. I have never had a web forums interface, and that's what I consider the Groups.io web interface to be, ever split a thread, period. The user must make the explicit decision, followed by the effort necessary to hit the "New Topic" button, to create a new thread. If context needs to be borrowed from an existing thread that is done via copy and paste, not by relying on being able to hit "reply" in an existing thread, changing the subject, and then editing down the existing message material to retain the context one wishes to maintain.
- By extension, I firmly believe that this should be extended to how the e-mail interface works, too. An announcement, perhaps repeated announcements, would be needed ahead of the actual implementation. I can see no reason why the same explicit kind of "new thread creation" decision making should not be built-in to the e-mail side of the world just like it is (or we thought it was) on the web side. If you want to create a new thread you should have to create a brand new message addressed to the posting address for a given group, with a new title (even if that title includes a "was: blah blah blah"), and with the intentional transfer of content, if any, from the old thread that served as its genesis. This is precisely how entirely new topics get introduced when that's the intent, so no one can say that this is something they cannot understand if they've been something other than a lurker or a "reply only" participant. It makes the user consider what they're intending to do before they actually do it, and that always tends to be for the good overall (particularly since it can sometimes give people a chance to reconsider something).
With regard to your question regarding manual changes to a subject on an existing thread by an owner or moderator, I believe you are correct that each and every existing message in the thread (and, in this case, the archive since the web interface is the only place I know of that one can retitle existing topics) should have its subject changed. If you were to implement the "ironclad" header threading discussed above, this would allow for mixed titles in a topic to occur, but that's probably not going to happen often nor be a problem. If an owner/moderator is retitling a thread it's because they feel that, to use a metaphor, "the drapes don't match the rug," and the intent is to get a match. This is particularly important when an existing thread is being split. One would want the new title of each thread created from the messages in a single existing one to have the same title as "message one" in each new one. From what I've seen this process occurs already automatically in the case of a thread merge. The thread being merged always has its message subjects changed to the same one as the thread that it's being merged into.
I have made mistakes, but have never made the mistake of claiming I never made one.
~ James G. Bennet