Forcing Wayland video driver in classic SDL2 makes 4K HiDPI unavailable in Source games

Your system information

• Steam client version (build number or date): 1745876290
• Distribution (e.g. Ubuntu): Arch + Plasma Wayland
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz
• GPU: AMD Radeon RX 6600 XT

Please describe your issue in as much detail as possible:

I have two displays, one 4K (200% scale) and another 1080p (100% scale). Steam's Scout runtime for native Linux games seems to mistake the 4K display for a 1080p one when running Half-Life 2 or Portal 2 with SDL_VIDEODRIVER=wayland. Switching to the legacy runtime works as intended. Disabling the secondary screen doesn't seem to help.

Steps for reproducing this issue:

1. Use a 4K primary display with scaling set to 200% (in Plasma)
2. Put SDL_VIDEODRIVER=wayland in a Source 1 game's launch parameters (I tested Portal 2 and Half-Life 2)
3. Launch the aforementioned game
4. Observe incorrect 'native' display resolution detection:

Scout, monitor 1:

Scout, monitor 2:

Legacy, monitor 1:

Legacy, monitor 2:

(Sorry in advance if I'm using the wrong issue tracker, please point me to a more appropriate one if necessary)

Reproduce the issue with a 4K display at 200% scaling, a 1080p secondary display, and SDL_VIDEODRIVER=wayland in Portal 2 or Half-Life 2. Compare Scout and legacy runtime behavior; done means the Wayland video driver reports the correct native 4K display resolution.