For some reason, this made me think of privacy & security issues: user fingerprinting, user-type detector (devtools open = dev/engineer) etc.

Detecting the difference between window.innerWidth (viewport) and window.outterWidth (window with bar etc) in JS, can be powerful to the browser user via an Extension. However, not so if the site/page uses (e.g a JS Lib).

Personally, i like the viewport vs window separation that the browser provides. I don't want an Element to not budge or care about my sidebar/window dressings, thank you very much.