Hey wolfdog, I heard of something super similar in the past that was resolved after messing with the computer's resolution of display. If you're using Windows, you can find display settings in the control panel, or sometimes by right-clicking on your desktop and going to "Display Settings." From there find what resolution best fits your window (if it is not already set). You can check online what your display's resolution is here: https://www.infobyip.com/detectscreenresolution.php
After optimizing your resolution, switch your video mode to match it if it is not already the same. From there try taking a screenshot without full screen on.
(https://i.imgur.com/2kkaGbh.png)