The short version is that each message in a chat appears as a different conversation to regular mail users on some popular webmail systems. To be clear, this issue applies to one-to-one chats, not group chats (which has been been well-resolved in my view).
The long version is that by experimentation, I believe that at least GMail and Outlook webmail group conversations based on In-Reply-To and subject matching. So a correctly threaded reply will appear to be a different conversation to a GMail user if the subject has changed. DC changes the subject for each message in a one-to-one chat, but even though it handles the other headers absolutely correctly, a long thread will end up looking like quite a mess to users of many popular ‘MUAs’.
The behaviour of GMail/Outlook/etc. is very odd from my PoV as a ‘mutt’ and ‘K9’ user. Some might even say ‘broken’, but I think this is a practicality vs. principle issue - DC has to be able to communicate smoothly with ‘normies’, that’s it’s unique selling point.
That said, I’m not quite sure what the answer is.
Customisable subject maybe? Makes DC less like a chat-app
Stick with a single subject that’s somehow inferred? Is it the first message in the chat? In which case, it’ll need to be changeable at some point, or conversations with your best friend will bear the subject “re: Hey, I found this cool new chat app” 5 years later! Perhaps some heuristics to decide when a ‘conversation’ is over based on time gaps, and it’s ok to then reset the subject?
I very nearly added this to the “Subject of emails” thread, but that went so far into being group-specific that I worried about being off-topic. FWIW, I quite like the solution to group chat, as that resolves this GMail/Outlook problem very nicely.