Found a way to control Steam

[nonoche2]

If you launch Steam with this argument:
-cef-enable-debugging

it opens up the client to a Chromium-based browser through http://localhost:8080, but it's also accessible through websockets. This allows to send commands to Steam's console and have it download Windows games without updating the installed mac games for instance. It makes Steam completely controllable and accessible from a discrete app. I think it's a perfect candidate for Kaon to interact with the Steam client.

[nonoche2]

note that you need to wait for Steam to have fully loaded the library before sending @sSteamCmdForcePlatformType windows to the console otherwise it'll fall back to updating all installed mac games as their windows counterparts. You can inject a javascript in Steam to ensure it's safe to send it