Made another run at testing the screen mirroring over AirPlay.
There are exactly three resolutions possible:
- 2880x1620 (which is also "Best for the AirPlay display")
- 1920x1080 HiDPI (should utilize all UHD pixels at 2:1 HiDPI ratio, right?)
- 1280x720 HiDPI (did not bother to test).
The image is always scaled to UHD on the aTV side (which is normal, as it does not support dynamic resolution switching), as can be seen on the screenshot below (well, you'd need to download it to see that it has image size of 3840 × 2160 pixels at 72 DPI).
Available resolutions can be seen in macOS System Preferences and just to double-check, I confirmed them also with the SwitchResX as seen on the attached screenshots (these were taken on iMac screen).
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