Whatever Steam is using to render that CEF to the screen in MacOS, it's not presenting it to the OS as a window, and as such, blithely ignores things like Stage

Whatever Steam is using to render that CEF to the screen in MacOS, it's not presenting it to the OS as a window, and as such, blithely ignores things like Stage Manager, responding only to things they manually reimplemented, like resizing. I've seen better-behaved Mac GTK apps.