Would still be nice if steam added a toggle to hide the mouse cord or when a controller is detected, wonder if they will ever add such a feature
The ability to hide the mouse from the display varies from OS to OS, to API to API, and even from Fullscreen to Boarderless fullscreen. That's why it's a problem. For example, if you're using SDL to write your game, you need to pass SDL_ShowCursor(SDL_DISABLE) to the init routine when creating your context, but if you pass SDL_SetWindowFullscreen(SDL_Window* window, SDL_FULLSCREEN_DESKTOP) to the video initialization method, it gets overriden and the mouse still appears. And if you use OpenGL or DirectX, it's different too.
So it's not a universal fix kind of thing, otherwise people would haved fixed it by now.