Windowed mode is 1:1 in a window.
But borderless window should have resolution choice and should stretch desired resolution into maximized borderless window. The game is piratically unplayable on 4k/2160p screens with average GPU. I can't set 2k/1080p full screen, as it will mess up all opened windows and because i need to switch of the game often and i can't use borderless window mode as well, because it's locked to desktop resolution.