That is because you don't have any window manager running. Window manager is important to change focus for each application.
You can try steam-login project I did that aims to do exactly the same as you are trying to do, already with a simple window manager and other workarounds on steam issues: https://github.com/thor27/steam-login#readme
Thanks, I'll definitely give that a try. I still think there's a bug worth addressing here though - it's certainly possible to launch another app and give it mouse and keyboard input without a window manager.
I don't think that it is possible without ANY window manager at all. Some games even have traditional windows for settings and stuff. However, it's possible to write a very simple window manager from scratch that does the minimum to make steam and games to work.
This would actually be an interesting project to hack on. :)
Nothing extracted yet.
To save resources and simplify the launch procedure, I've created a lightDM session for Steam, meaning that I can launch a session without a window manager, and instead run steam -bigpicture as the session.
When launching Amnesia: The Dark Descent, mouse and keyboard input continues to go to Steam, and not to the game launcher, meaning that I can't launch the game proper, nor see what I'm doing in Steam. The same happens when launching Frozen Synapse.
In both cases, if I navigate to the exit button blindly and select it, the game application gets focus and can be played. Once the game exits, the session never logs off since steam.sh is still running.