I don't know if this can be fixed, but it seems the game is really not very alt-tab friendly in fullscreen. If you happen to alt-tab or even accidently click outside the window because you have a dual monitor set up, the game minimizes and you no longer can go back to the game, the only way is to terminate it.
Edit: Often when I try to click the menu icon, it also minimizes in fullscreen. Dangit, that's the only way to actually see all the units in game, but it seems I'm forced to play in tiny windows or big windows with tiny units.
Edited by Solais, 16 December 2016 - 08:40 PM.