Your system information

• Steam client version (build number or date): 1782952760
• Distribution (e.g. Ubuntu): CachyOS (Arch Linux)
• Desktop Environment: KDE Plasma 6 (Wayland)
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes
• Steam Logs: Attached (steam-logs.tar.gz)
• GPU: NVIDIA GeForce RTX 5090

Please describe your issue in as much detail as possible:

When Steam is launched normally, games correctly enumerate my monitor's physical display modes, including 3840×2160.

When Steam is launched with the -steamos3 command-line argument, games no longer detect the monitor's physical display modes when KDE Wayland fractional scaling is enabled. Instead, they only enumerate the logical desktop resolution created by KDE's scaling.

My display configuration is:

• Physical resolution: 3840×2160 @ 119.88 Hz
• KDE Plasma Wayland
• Fractional scaling: 250%
• HDR enabled

At 250% scaling, KDE reports a logical desktop size of 1536×864, and when Steam is launched with -steamos3, games only offer 1536×864 as the available resolution instead of the monitor's native 3840×2160 and other physical modes.

Expected Behavior

Launching Steam with -steamos3 should not change how display modes are enumerated. Games should continue to detect the monitor's physical display modes (3840×2160, 2560×1440, 1920×1080, etc.), regardless of KDE fractional scaling.

Actual Behavior

When Steam is launched using:

steam -steamos3

games enumerate the logical Wayland desktop resolution (1536×864) instead of the monitor's physical display modes.

Reproduction Steps

1. Configure KDE Plasma Wayland with:

   • Resolution: 3840×2160
   • Fractional scaling: 250%

2. Launch Steam normally.

3. Verify that games detect 3840×2160.

4. Exit Steam.

5. Launch Steam with:

steam -steamos3

1. Launch any game.
2. Open the game's display settings.

Result

The game only reports the logical desktop resolution (1536×864).

Additional Testing

Changing KDE fractional scaling from 250% to 100% while continuing to use -steamos3 immediately restores proper display mode enumeration. Games once again detect the monitor's native 3840×2160 resolution and other physical display modes.

This appears to indicate that Steam launched with -steamos3 is exposing the Wayland logical desktop size instead of the monitor's physical display modes when fractional scaling is enabled.

Additional Context

I originally discovered this while testing HDR support on Wayland. During that testing I observed that launching Steam with -steamos3 enables correct HDR behavior and resolves a separate Steam Controller issue I was investigating. However, doing so consistently introduces the display mode enumeration issue described above. The resolution issue reproduces across multiple games and does not appear to be game-specific.