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youtube is a pretty shit platform. When the sliderbar is visible, it adds this subtle dark shadow to the bottom half of he video. This means anyone taking a screenshot that leaves the cursor on the video causes the sider bar to appear, and thus the dark shadow on the bottom half of your screenshot. To fix this problem, you either need to use printscreen button, and then crop the image in your program of choice (such as gimp). Or you can add an extension that adds a screenshot button to your video interface
