Already suspected both me being too dumb to find the already existing thread and something along the lines of changed code that isn't properly reflected in game yet..

Thanks. ;/

What's the bug about:
When starting the game and going into full screen mode, the game will use the left(most) monitor, even if it is not the main monitor.

How to repro:
Hook up multiple monitors, at least two.
Start Factorio, Version 0.10.12
If not already on fullscreen, go to options ...