If the default settings for minecraft are "windowed" and I launch the game using the new fast launch feature clicking directly on the icon of the installation and the world I want to launch and then while waiting for the game to launch I switch to another window, when the game will complete launching my mouse will be captured in the boundary of the window because no pause-screen/esc key-screen is used if you are on another window while the game launch with fast launch. An easy fix for that could be to launch the game paused or to launch it paused only if your mouse is on a different window placed on top of minecraft window.

Thank you