A thick black line that keeps getting thicker and longer

This is on a 2010 MacBook pro, and a week ago I noticed an inch-long black line right on the dock, which has since grown both in thickness and in length (it might be about 4-5 inches now). I did all the resets such as NVRAM, SMC, and so on and so forth but with no effect. Interestingly, this line doesn't appear when I take a screen-shot using Mac's built-in screen-capture sequence shift-command-3. Wonder what this is and how to get rid of it, if at all possible. This following picture is through a mobile phone camera, with Mac's dock showing at the bottom of the screen. Any solution/work-around?