Proton Experimental: VSync causes fps locked under Wayland (XWayland), fixed in GE-Proton

[smithyyang]

Symptoms:
Games running via Proton Experimental under XWayland are locked well below the monitor refresh rate when VSync is enabled in-game. On a 165Hz display this results in 40-50fps, on 60Hz results in ~30fps. Disabling VSync restores normal framerate but causes heavy screen tearing and stutter.
Switching to GE-Proton 10-32 completely resolves the issue with no other changes. The same games also run fine under KDE Plasma Wayland with Proton Experimental, suggesting an interaction between Proton Experimental and XWayland/Hyprland.
When fps is locked, GPU usage remains at ~50%, indicating the GPU is being throttled waiting for vsync intervals rather than running at full capacity. Disabling VSync causes GPU usage to reach ~100% with normal fps, confirming the issue is vsync-related frame pacing.

Steam launch options:
gamemoderun mangohud prime-run %command%

Name of the game: Sherlock Holmes Chapter One
Steam AppID: 1137300
GPU: NVIDIA RTX 4060 Mobile
Video driver version: nvidia-open-dkms 590.48.01
Kernel version: 6.18.9-zen1-2-zen
Proton version: Proton Experimental

Reproduction:
Use Hyprland on Arch Linux with Nvidia GPU
Launch game via Proton Experimental with VSync enabled
Observe fps locked