GPU acceleration disabled despite being enabled in settings

[rhoot]

Your system information

• Steam client version (build number or date): 1694466342
• Distribution (e.g. Ubuntu): Endeavour OS
• Opted into Steam client beta?: No (but I tested beta as well)
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz
• GPU: AMD RX 6900 XT (RADV)

Please describe your issue in as much detail as possible:

Enabling GPU acceleration in Steam settings appears to have no effect what so ever. The steam UI remains exceedingly sluggish in both desktop and big picture modes. Especially at a large window size. Opening a popup that blurs the background (e.g news or achievements) in the Library view makes the FPS drop to low single digits.

I eventually found this in webhelper.txt, which may explain it:

[2023-09-23 21:45:00] Browser - launching child process with: /home/johan/.local/share/Steam/ubuntu12_64/steamwebhelper --type=renderer --disable-seccomp-filter-sandbox --log-file=/home/johan/.local/share/Steam/logs/cef_log.txt --field-trial-handle=823699858905907602,18172577380390560174,131072 --enable-features=CastMediaRouteProvider --disable-features=SameSiteByDefaultCookies --disable-gpu-compositing --enable-blink-features=ResizeObserver,Worklet,AudioWorklet --disable-blink-features=Badging --lang=en-US --log-file=/home/johan/.local/share/Steam/logs/cef_log.txt --product-version=Valve Steam Client --buildid=1694466342 --steamid=0
[2023-09-23 21:45:00] Disabling GPU acceleration due to --disable-gpu-compositing (renderer)
[2023-09-23 21:45:00] Disabling GPU acceleration

I don't know what's causing --disable-gpu-compositing to be passed to the render process however. The GL/Vulkan drivers are installed as both 32-bit and 64-bit:

$ pacman -Q mesa vulkan-radeon lib32-mesa lib32-vulkan-radeon
mesa 1:23.1.7-1
vulkan-radeon 1:23.1.7-1
lib32-mesa 1:23.1.7-1
lib32-vulkan-radeon 1:23.1.7-1

Amdvlk is not installed:

$ pacman -Q amdvlk lib32-amdvlk
error: package 'amdvlk' was not found
error: package 'lib32-amdvlk' was not found

Steps for reproducing this issue:

1. Enable GPU acceleration in Steam's Interface settings.
2. Restart Steam.
3. GPU acceleration is still disabled.