Oh. Today's realisation is that Ctrl+W will close out of a Teams meeting, but not the regular Teams app. I think that means a Teams meeting is secretly a window of Edge, but the main app is its own microcosm of chaos. That also explains why the back/forward buttons on my mouse don't work in Teams itself, which *would* have been a very useful carryover from a browser.
I think that firmly answers anyone (cough, Reddit thread) who exclaims "Ctrl+W is meant to close a window, what did you expect!?"