SteamGame - bakkeby/dusk GitHub Wiki
The SteamGame flag is specifically named so to work around issues that affect Steam games in a
non-gnome environment.
The flag is set by default for Steam games if the class hint contains "steam_app_" or if the window has a "STEAM_GAME" property referring to the steam app ID.
There is currently only one feature that the flag provides - and that is to set the client state to NormalState whenever the window receives focus.
This is a workaround for games having a black window if you switch to another tag and back while being in-game. Reportedly this does not work for every situation, however.
Similar issues have been reported for other game platforms, so the flag may be subject to a rename at some point to make it more generic.