On the website via desktop computers.

Drives me nuts as I never use the mini player function and it's a huge and frustrating disruption that I didn't ask for.

EDIT: actually I think I figured it out and it's probably nothing to do with YouTube apart from an unexpected design decision that's probably been around for a while and I never noticed. Turns out if you click and drag a video you're watching anywhere around the screen it launches the mini player. I'm not used to this kind of thing on desktops but I guess it's becoming the norm and could be helpful. I think I've been accidentally clicking and dragging just a little bit sometimes.