Your system information

• Steam client version: 1766529085
• SteamOS version: SteamOS Holo 3.9
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

Additional system details (for reference):

• OS build: 20251218.1000
• Kernel version: 6.16.12-valve7-1-neptune-616-g499ac884388d
• BIOS version: RC73XA.314
• Device hostname: ROG-Xbox-Ally-X
• OS variant: steamdeck

Please describe your issue in as much detail as possible:

When using an Apple Studio Display connected to an ASUS ROG Ally X running SteamOS, the display functions correctly in Desktop Mode at its native 5K resolution. However, in Game Mode, the display fails to work correctly when configured at the native 5K resolution.

Expected behavior:
The Apple Studio Display should function correctly in Game Mode at its native 5K resolution, consistent with its behavior in Desktop Mode.

Actual behavior:
In Game Mode, setting the Apple Studio Display to its native 5K resolution results in a non-functional display (no usable output). When the resolution is manually lowered to 4K in Game Mode, the display immediately starts working correctly.

This strongly suggests a Game Mode limitation or bug related to handling very high–resolution (5K) external displays, while Desktop Mode does not exhibit this issue.

Steps for reproducing this issue:

1. Connect an Apple Studio Display to an ASUS ROG Ally X.
2. Boot into SteamOS and switch to Game Mode.
3. Set the external display resolution to 5120×2880 (5K, native).
4. Observe that the display does not function correctly.
5. Lower the resolution to 3840×2160 (4K).
6. Observe that the display functions normally once set to 4K.