Launching a game means that the entire Steam Window fades and it will become entirely unusable until the game starts. That is not a separate startup window, thats just an embedded element inside the main one. If you accidentally click outside, the game startup gets canceled.
It is a bit more than mildly frustrating when Im waiting on the Vulkan shaders for BFV for like half an hour to process.
Yeah, I gad a lot of problems with it on NixOS.l, to the point of kde becoming unresponsive during shader processing. I had a much better experience once I installed cfs zen tweaks which iptimizes the ketnel a bit for desktop usage.