Okey your issue sounds like a basic problem of Windows display, when the screen is stretched across two or more monitors of different resolution they can need calibration from time to time
In Windows you calibrate your screen (with the software running) by pressing the [windows key] on your keyboard and using the arrows to move the into the 4 corners ... It's one of those tricks that is easy to show but hard to explain
If you posted an image (which this forum is not set up to allow) it would be much better... When a dual screen needs calibration a part of one display will creep over onto the other monitor, anything from a few pixels is enough depending on how much difference there is in the two resolutions
If you can't work it out I recommend taking a screenshot to show your issue and post it in the Steam forums so people can help you more... Good luck
