I don't know about jigsaw etiquette, as I am not a jigsaw fan, but if anyone picked up my knitting I would be furious as people's tension is different, and I follow a pattern closely, so don't want someone moving things on without my knowledge.
I do think that in someone else's space guests should respect the fact that that they are guests, and behave accordingly, though. No helping yourself to things without asking, no wandering round uninvited, (and probably no finishing of jigsaws😀). In someone else's space you behave respectfully, IMO.
I say that as someone who has just had one of my (adult) children visiting immediately after the other, and have had three weeks of dealing with with loo rolls being put on backwards (when in use), dogs allowed to sit on sofas, used crockery left on worktops, TVs on in empty rooms and so on. A lot of my disgruntlement is because of my grumpiness (and other family stresses), which I don't deny, but I would do none
of those things in someone else's house.
Mi castillo no es tu castillo! 😎