5 seconds after launching DayZ, the client opens up a borderless game window even though my in-game graphics option is set to Fullscreen.
Clicking inside the Window does nothing to solve this, but minimizing it or refreshing Windows's focus resets it properly into Fullscreen mode instead.
RPT-log entry "ENGINE (W): Creating in minimized window mode"
Tested on experimental version 1.13.153989 but previously experienced in Stable DayZ before 1.12.