Some games exhibit strange behavior under Windows, if the operating system is controlling its scaling. An example is Xonotic, where you cannot use full 4K resolution, if Windows controls its scaling. In order to turn this off, simply open the properties of the game's binary and select the Compatibility tab.
Clicking on the Button Change high DPI settings opens the following Window:
Select the checkbox next to Override high DPI scaling behavior. and select Application. This makes the game do the scaling, which can solve problems for some games.