Lucca asks Define “on topic”.
It is hard, perhaps, inpossible to define "on topic".
For example in the virtual cruise on the river Main, someone suggested visiting Heidelberg. So Heidelberg is not on the Main. But that was accomodated by pretending to have a side trip to go on a train from Frankfurt to Mannheim, then onto a sister ship of our cruise ship, along the river to Heidelberg, a walk around Heidelberg, then back overland to Frankfurt. And the trip to Heidelberg was one of the highlights. I think a number of people enjoyed that side trip. I had not thought of it. One place to moor up and visit was suggested by a participant. There were other overland side trips too, one I found in a real world cruise brochure, somone else suggested another, a two-stop one that I devised, and a single one that I devised. That was a very happy thread.
So, that is more a meandering rather than off-topic, though strictly speaking some of it could be designated off-topic. Had there been a suggestion to go to, say, Paris, that would have been off-topic.
I suppose a lot depends on knowing what exactly is "the topic" of a thread before one can go "off" it.