moderated Re: How on earth does Groups.io thread a topic? BV
Clearly, my assertion that "one must hit 'New Topic' to create a newPro: it is consistent with email posting. That's "AND logic" - thread the new message onto an existing message if the citation matches AND the subject matches.
Pro: in the Topics list and particularly in the Topic view, the reader is shown only one instance of the subject line. If changing the subject of a reply didn't create a new thread then Reply might as well not have that field as editable: its content cannot be displayed. Or, the layout of the Topic View must be redesigned.
Con: it may not be least astonishment behavior for the web site, where it can be known that the user clicked "Reply" versus "New Topic".
I think the only reason Reply allows one to alter the Subject text is specifically for the use case of launching a new topic tangent from (and copying some quote from) an existing topic. Maybe the UI needs to be more clear about that purpose and consequence.
FWIW Yahoo Groups operate with "OR logic" - thread onto an existing message if the citation matches OR the subject matches. This has the "over-threading" that you mentioned, particularly since they have no time limits on the threading. In Yahoo Groups classic each reply in their equivalent of Topic View did show its own instance of the subject line. Not so since the Neo redesign - only email readers see any changes to a reply Subject.