I have the opposite problem. I typically have many browser windows open at the same time but only two screens, and many sites that I use are designed to assume that everyone has full-screen browsers.
With this setup, many sites work, but a few... a few have a top ad banner, a side banner and a footer of 'cookie acceptance'... then add in a 'subscribe to our email' and a google login prompt.... (Game Wiki's.. I game in smaller windows too -- what good is a multi tasking computer if you don't use it?)