@Moon @matrix There are monitors (mine included) that can take 2 inputs and put one input on each half of the screen. That's possibly what's going on here. I remember when I had to do that for my work's shitty laptop they forced me to use, I used synergy and it made a perfect transition between the two OSes.
I used to hit the button that did this accidentally all the time, so perhaps yours supports this and you are accidentally enabling that feature? The Samsung SA850 I have has a button specifically for this mode.