After ALT-Tabbing in and out of DayZ several times, when I came back into DayZ the last time, the game window was cut short, not filling the entire screen (there was about a 10% portion of the right-hand side that just showed my underlying desktop). This screwed up my interaction with the in-game UI as well, i.e., the dimensions were "off" -- to interact with an object in my inventory I had to mouse-over and click an area about 10 cm to the right and below where the object actually was on my screen.