Turn Off Scaling on Mirrored Internal Mac Display

When mirroring an internal Mac display to a TV, the Mac will e.g. set its resolution to 1080p and scale up the image on the internal display. How do I turn this off and display 1080p 1:1 on the internal display as well, windowboxed?

The usual suspects (Fn-Key-Combos, SwitchResX, yelling) don’t work.

Any ideas? Maybe some Retina display utility?

When mirroring an internal Mac display to a TV, the Mac will e.g. set its resolution to 1080p and scale up the image on the internal display. How do I...