I think OP's point is you can listen for a bunch of signals: If you see the right keydown events, or if you see a contextmenu followed by a rapid change to innerWidth, they probably opened dev tools, and you should delete your evil cookies.
The person who figured this out probably opened them via menu and had dev tools in another window, so the evil folks couldn't detect the resize.
The person who figured this out probably opened them via menu and had dev tools in another window, so the evil folks couldn't detect the resize.