Your system information

• Steam client version (build number or date): 1743554648
• Distribution: Gentoo (LLVM)
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz
• GPU: AMD RX 7800 XT

Please describe your issue in as much detail as possible:

The Steam Deck allows users to use Mangohud (in the right pane menu) in place of Steam's Desktop Mode's FPS overlay (I suspect this might be related but I'm unsure), so I thought to do the same by running Steam through gamescope (which I'm pretty sure the Steam Deck does) with the --mangoapp argument: but this causes Steam to disrespect my mangohud settings in ~/.config/MangoHud/MangoHud.conf

How do I know Steam must be responsible?
Because this doesn't happen when running (DRM-free) GOG games through Vanilla Wine: my settings are respected when I do so. All I wanted was the FPS counter but, with Steam, I'm forced to have a graph and 2 other rows of information that takes up more of my screen than I wanted.

Steps for reproducing this issue:

1. Run Steam through gamescope with the --mangoapp argument and the -e flag

example:

env ENABLE_GAMESCOPE_WSI=1 gamescope -e --nested-refresh=60 -w 3840 -h 2160 -f -R --rt --force-grab-cursor --hide-cursor-delay 3000 --fade-out-duration 200 --prefer-vk-device --adaptive-sync --hdr-itm-enable --mangoapp -- steam -bigpicture

1. Launch a game through Proton