I believe that diagonal line is an artifact from the display updating from frame to frame. Display's update, line by line, not all at once. This could happen top to bottom of the screen, or side to side. for a small length of time, that line (or even a single pixel) will either go bright or dark as it is updated depending on the display technology.
so why is it a diagonal line? well your camera captures images line by line as well. In this instance, it seems that your camera is capturing data from a different direction than your screen is updating. Your camera and screen are probably capturing at a similar frame rate, hence the diagonal line is approximately 45degrees.
If your camera is scanning downwards, and your display horizontally, its going to draw a diagonal line as the updating line on your display (which will either be lighter or darker than all the other pixels) intersects with the captured line on the camera over each frame.
This effect is clearest on OLED screens, because their pixels go completely dark when they update. Further, I bet if you were to rotate either screen or camera 90degrees, the effect will disappear.
