I didn't know Webgl didn't support mutli-threading, interesting, thanks for the tip. Why not use WebGPU instead? Looks like it's supported on both Chrome and Edge: https://caniuse.com/webgpu
I installed the latest version of the itch app on Windows 10 but butler is no longer located in the apps folder, it's now in broth\butler\versions\15.21.0\
