October 2019
I turned off " virtual super resolution " in radeon settings and it went away.
April 2020
Not all have Radeon
March 2021
I somehow managed to get a workaround by spamming ESC when trying to maximize the game.
After I did it, I managed to go into settings and set it to borderless.
December
also battlefield 5 dosnt like 4k screen - change the resolution from the window display settins to 1920x1080