Why does ingame always open an additional window?

I have a question: If I play Fortnite and always press a few more keys at the same time, the game stops and displays it in a smaller gray-framed window, and when I let go of the keys, it closes. This annoys a lot, because then I can't move for a moment and I die by it. Can someone help me?

Which keys?

Do you mean Shift?

If you do not say which keys you press no one can help you