By "focus" i mean the current window on your desktop that has the focus, as in, the one that's going to receive the input you give through your keyboard. I've seen plenty of people (and it's happened to me too) fumble and accidentally type or paste things in the wrong window. If that window happens to be your browser on a website that does this kind of tracking, that information goes to some random third party instead of the window you thought it was going to.