SteamOS Gaming Mode external display freeze at 4K165Hz on Samsung Neo G7 via ASUS Bulwark dock

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Your system information

• Steam client version: 1782861641
• SteamOS version: 3.8.16
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

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Please describe your issue in as much detail as possible:

Using an ASUS ROG Xbox Ally X (Z2 Extreme) with the official Valve SteamOS image, an ASUS ROG Bulwark dock, and a Samsung Neo G7 monitor connected via HDMI.

When switching from Desktop Mode to Gaming Mode while the Samsung Neo G7 is set to 3840×2160 at 165Hz, the external monitor freezes/loses signal during the transition. The Ally itself remains responsive and does not appear to crash.

The expected behavior is that Gaming Mode should transition normally while keeping the external display active.

The issue does not occur in Desktop Mode at 4K165Hz.

Changing the Neo G7 refresh rate to 4K120Hz resolves the issue. Enabling PIP mode on the Neo G7 also allows 4K165Hz to work.

Additional testing:

• Samsung TV at 4K60Hz works correctly.
• HDR works.
• Disabling Adaptive Sync causes the display handshake failure.
• The issue appears to occur specifically when Gaming Mode initializes the external display.

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Steps for reproducing this issue:

1. Connect an ASUS ROG Xbox Ally X running SteamOS 3.8.16 to a Samsung Neo G7 monitor through an ASUS ROG Bulwark dock using HDMI.
2. Set the external display to 3840×2160 at 165Hz.
3. Enter Desktop Mode and confirm the external display works.
4. Switch from Desktop Mode to Gaming Mode.
5. Observe that the external monitor freezes/loses signal while the Ally remains responsive.

Reproduce the Desktop Mode to Gaming Mode transition with the ASUS ROG Xbox Ally X, SteamOS 3.8.16, ASUS ROG Bulwark dock, and Samsung Neo G7 at 3840×2160 and 165Hz. Compare the transition with 4K120Hz, PIP enabled, and the other reported display configurations; done means Gaming Mode keeps the external display active at 4K165Hz.