Ancient X11 scaling technology

It's not "can you provide the screen DPI to a window?" people bemoan, it's "can you draw one window across two screens with differing DPIs, transparent to the application?"

I'm struggling to imagine why would anyone ever want to do that. It's not even an edge case at this point, it's over the edge.

It happens. You might push a window aside so that it's partially on a second screen and still see activity on the second screen. It should be the expected behavior.

I'm not saying it doesn't happen. Of course it will happen. I'm just saying it isn't something that needs to be handled perfectly, specially when handling it perfectly requires immense design changes at a fundamental level.

Why can't you just display a blurry rectangle until the mouse cursor goes to the other screen and then you switch the primary resolution from one screen to the other?

I feel like trying to be extremely clever to handle this particular problem would lead to a solution that handles far more common situations much worse for everybody...