What I expected to happen was...:
I used to play Minecraft with F11 like other players. But one day, when I click F11, every screen become dark, and then it returns. I thought it was just coincidence, so I tried to play.

What actually happened was...:
As I entered at up, every screen become dark once. Also, small key delay occurred and Frame late became very low. 

Steps to Reproduce:
Pressing F11.