I have the same issue. Have been for almost a year now. From what I can tell it has to do with the temperature of the laptop/display itself. That's why when you do more demanding tasks like playing games, it tends to disappear. However over long periods of time the threshold of heat increases with the size of the black bar, the bigger the blackbar, the more heat required to make it disappear. Having dark themes, or anything with the color black near the bottom will cause it to appear again, usually if you leave on a plain white screen on YouTube, it will disappear. The fix I used for the entire year I've been having this problem was an application called Cru refresh rate adjuster. By gradually lowering my refresh rate by 5hz everytime the black bar appeared I was able to get another year out of it and the cost of lower refresh rate, and in turn fps of course. Unfortunately there aren't any other 'software' fixes for this aside from manually lowering your refresh rate which will impact gaming performance.
When I took my laptop in, they advised that the only permanent solution would be to get a screen replacement altogether. So if you're still on warranty, you should probably take advantage of it as soon as you can.
As for the cause of the problem, apparently it has something to do with loose connections causing dead pixels, which is why applying heat or heavier PC loads tend to reignite those lost connections temporarily.
I hope this helped.