I always tried to be quite lenient on that (I hope). Most of the time things got back on topic, I thought. But there are examples where sometimes a separate conversation is worth having…
Back when I used to use Usenet, that sort of thing would be called thread drift. Oftentimes posters don’t even realise that what they’re writing in reply has diverged that far from the topic as written ( ) and it’s a bit of an ‘oops’ moment when called out.
Unlike Usenet, the options availble on Discourse are very good, and I think splitting such posts out to a separate topic is very useful, as this allows people who want to discuss the new issue to carry on doing so, without drowning out or blocking people who’d still want to talk about the original, topical issue.
My own opinion on this is if it’s something that’s grown organically from the OP topic then it should be allowed to stay and run it’s course. Similarly to how @revels put it. I’d forgive certain contributions that the code of conduct would encourage you to flag for similar reasons too. As @DaveJ mentioned below, you lose context if you split in these instances.
In the instance of your example, i think that’s a good idea of where splitting should be used.
Problem with splitting threads is you usually lose the context to them. I know I’ve read some before and thought eh none of this makes any sense, where’s this come from.
I think give it several posts. If it looks like it’s going to fizzle out or meander back to the main topic that’s fine. If it looks like it’s going to become a big topic in its own right and make responding about the first topic difficult split.
Ultimately it’s a judgement call, someone might make a judgement call different to the one you make but hopefully no one is losing sleep over it!
I think what that boils down to is that it’s better to be reactive than proactive when splitting. Better to split when “OK, people are really liking this chat which is totally different to the topic” than to split when it might be a direction that would quickly fizzle out regardless.
When I happen to start an offshoot discussion in a thread that I think deserves it’s own thread, what I’ll do is start that new thread then ask a moderator to merge those off topic posts into the new thread.
I think if you’re splitting into a new thread, you need that new title post to anchor the discussion and provide context where it might lost.
Sometimes people get caught up in discussions and don’t realise they’re going off on a tangent. So just a nice reminder is usually all it takes for them to realise.
Just like in the ‘best bank award’ topic - hopefully that’s finished now too and can get back on topic.