> "applications need to be able to decide where exactly windows go." There is basically no behavior which literally requires this, and certainly no sane behavior that requires this.

I just had a thought. Whenever I pop-up a child dialog (incl. message boxes) in a desktop UI app, I normally (1) center it over the parent window, or (2) place to the right of the parent window so that many layers of blocking child dialogs common in complex config settings build a "UI train" from left to right. In both cases, I am using "pixel perfect" placement of these windows. I never had any users complain about it. Is this not a valid use case?