It seems the new upcoming Apple TV 4K (2021) will support HLG over HDMI. It will have a HDMI 2.1 connection and HLG over HDMI is supported since HDMI 2.0b. The old/current Apple TV 4K has a HDMI 2.0a connection.

Wondering what your source for this info is, tom ? I haven't found anything yet to say the new Apple TV 4K 2021 box will support HLG. This was just an assumption. Today I tested with the new Apple TV 4K 2. Gen and unfortunately on none of my TV Sets AVPlayer.availableHDRModes contains .hlg.

I have now tried playing HLS with SDR and HLG and HLG-only. The new Apple TV 4K 2. Gen with a HDMI 2.1 compatible cable successfully played the HLG variants for both playlists. The old Apple TV 4K 1. Gen and the Apple TV 4K 2. Gen with an old HDMI cable played the SDR variant of the mixed playlist and failed to play the HLG-only playlist. This is the desired behaviour. The downside of AVPlayer.availableHDRModes reporting the support incorrectly is that we cannot show our users if they will be able to play content in HDR.

The issue is fixed in iOS 15.2 RC

the issues where resolved with Xcode 13.3.1

There is now an API available in iOS 17: https://developer.apple.com/documentation/uikit/nscollectionlayoutdimension/4173072-uniformacrosssiblings

Thank you for confirming the bug. The feedback ID is FB13942310

This was fixed in iOS 18 beta4.