Over-saturated colors when playing SDR content with HDR ON on external display

[MKReyesH]

• Steam client version: 1726683985
• SteamOS version: 3.6.19
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

Context: Steam Deck LCD connected through display port to an external monitor (which supports HDR).

1. When the Steam Deck switch for HDR is disabled, the colors are correct.
2. When the Steam Deck switch for HDR is enabled, but you are playing non-HDR content (e.g. the Steam OS menu, a game without HDR support, or a game with HDR disabled), the colors are over-saturated.
3. When the Steam Deck switch for HDR is on, and you are playing HDR content (a game with HDR enabled), the colors are correct again.

As an example, I am showing photos (no screenshots) of the three cases using Disco Elysium:

Changing the SDR content brightness slider does not solve this problem, as it only changes the brightness, but not the color saturation.

[matte-schwartz]

I grabbed drm_info for these three images from my OLED Deck while docked to my HDR monitor, which shows the same behavior reported here.

Steam HDR off/Game HDR off:
disco_elysium_hdr_off.log

Steam HDR on/Game HDR off:
disco_elysium_hdr_off_game.log

Steam HDR on/Game HDR on:
disco_elysium_hdr_on_game.log

@misyltoad is the middle image expected behavior when gamescope is using PQ inv_EOTF and BT2020_RGB rather than sRGB EOTF and Default?