That's X11 panning, which has a similar effect. The X server has one big screen buffer, and the physical screens get mapped into regions of it. You can enable

That's X11 panning, which has a similar effect. The X server has one big screen buffer, and the physical screens get mapped into regions of it. You can enable it by using xrandr with the --panning option, and it will work with any DE. fvwm doesn't change the X11 screen size, it just moves all the windows to give the appearance of moving the screen in the other direction.