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.
This only affects a fraction of games (those that have to process their shaders), and then only on first startup.
Gotcha, thank you for explaining