It's been a while since I fiddled with it, as my Mac OS set up kind of just works at the moment for me (or rather... I've learnt it's general nuances), so I don

It's been a while since I fiddled with it, as my Mac OS set up kind of just works at the moment for me (or rather... I've learnt it's general nuances), so I don't change things unnecessarily, but I think this is related to the setting of having monitors having their own "Spaces". From memory, you can set monitors to have their own "Space" - in which case windows don't span multiple monitors (if a window would span multiple monitors, it won't, part of it just disappears off into the ether), or you can set them to share a "Space" in which case windows can definitely span multiple monitors, but from memory that came with its own quirks. In my set up, I never need/want a window to span multiple monitors anyway, so I've not fiddled with it in recent years. EDIT - here's a general explanation, which I think probably still holds somewhat true - https://www.macobserver.com/tmo/article/os-x-how-to-use-separate-spaces-with-multiple-monitors https://www.macobserver.com/tmo/article/os-x-how-to-use-sepa... I'm guessing (hoping) with things like Sidecar in recent years, at some point fairly soon Apple will generally unify how they approach multiple monitors/displays, but it is a tricky problem to fundamentally solve, especially with multiple displays with differing DPI's, dynamic range support, etc.