> The interaction paradigm you describe (where it's literally impossible to look at one window while actively using another) makes so little sense that we can almost certainly rule it out.

I think the implication is that the window you are looking at is focussed automatically. Which wouldn't work for say, typing up a document to a text editor while reading from a reference doc. That's just the beginning, I use a mouse-to-focus window manager and that happens all the time with all sorts of things.