I don't know how they're doing it, but I assume they're using data given to them by your browser, e.g. measuring the difference in height/width between the wind

I don't know how they're doing it, but I assume they're using data given to them by your browser, e.g. measuring the difference in height/width between the window and the viewport. Either that, or (as someone upthread suggested) they're reading a request your browser sends for mapping files, which would again be information you provided. If anything, I guess your browser is the one spying on you, by providing this information. But, realistically, I don't think it counts as spying either way. Hostile behavior on Mailchimp's part, yes. Dumb idea, yes.