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.
“Upgrade your pc, lol” should not be a solution to a problem that a company randomly created for no real reason. What OP is doing is right, bitch about it and hopefully Valve will notice and fix it, as they should.