It's not really polite to call it FUD right off the bat, especially when you're probably just misunderstanding. A browser obviously can't get events from a different app.

Moving your mouse over the window, despite not interacting with it, will allow the page to track your cursor, and could provide a surprising amount of information. That's why I wrote "over".

You built a strawman, but it can actually be dangerous to interact with an application on the same screen as a browser!

https://jameshfisher.github.io/cursory-hack/

Subject to the same origin policy.

https://en.wikipedia.org/wiki/Same-origin_policy

That protects you (to some extend) from page A accessing resources from page B.

Same origin policy doesn't do anything to stop you from being tracked, though.

Yeah, of course, I was replying to a post about the concern about a website getting your entire 1password database.