[donaldguy]

Very differently. Slack does threading on the basis of replies to a message and hides/collapses those under a thread, showing them only to people who expand and notifying only those who are already involved in the thread (by replying to it or being OP)

Zulip's threading is based, more like email, on a Subject which all messages (can) have. Though Zulip subjects are more likely to be one or two words.

In this way threading is tacked on to Slack whereas it is as at the heart of Zulip

[zAy0LfpBZLC8mAC]

Threading in email works via message-ids and references to preceding messages, nothing to do with subjects. That is, except as a workaround for garbage email clients and services too incompetent to use email correctly.

[donaldguy]

I mean, depends on your MUA in practice. I think by standard for server side threading— https://tools.ietf.org/html/rfc5256 — "base subject" is definitely a valid consideration. I only skimmed though. :-) feel free to extract a spec, I'm curious (but not enough to read 19 pages right now)

[zAy0LfpBZLC8mAC]

If I may quote:

      ORDEREDSUBJECT

         The ORDEREDSUBJECT threading algorithm is also referred to as
         "poor man's threading".  The searched messages are sorted by

In other words: as a workaround for garbage email clients and services too incompetent to use email correctly.

Yes, the REFERENCES algorithm also uses the base subject--but never to override message-id based threading, only for merging together threads that couldn't be joined using message-ids ... or in other words: as a workaround for garbage email clients and services too incompetent to use email correctly.

Well, and for merging together chunks of a thread where you have major gaps in your mailbox, so that reference headers don't overlap enough to make the connection, but the point remains that it's not really a threading mechanism, just a last-ditch attempt to recover a thread where proper threading information is missing, which is fine for corner cases like large gaps in which messages you have in your mailbox, and doesn't change that a sender that doesn't supply reference headers in the first place is incompetent.