Often a reply-all to a thread goes off topic. It's a good idea to change the subject line to indicate that.
It can also be useful to send a new message to a group of email addresses by replying-all to a previous email to them and clearing the body.
Currently, TBird keeps all of these in the original thread.
I propose that changing the subject should start a new thread. This could replace the current behavior, or alternatively be a configuration choice for the user.
I support @Flapjack's proposal for more extensive control, and suggest giving it a thumbs-up too. But this would get some of the functionality and be simpler to implement.
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