For the last week, I starting seeing a thin black Horizontal Line on the youtube video when I hover to see the timeline and it disappears when the timeline is hidden. I previously thought this was a cable problem or a monitor problem.
Many people online have been reporting the same on the Google Chrome forum here.

So it’s a problem with the timeline design CSS code mostly. So in order to fix it, we will see some simple solutions.
This problem doesn’t occur on Microsoft Edge browser. Also for some users it goes away when they use the play / pause button.
How to fix thin Horizontal black line on Youtube Video on Chrome
Step 1: On youtube.com, Go to a video that you wanted to watch.
Step 2: Now if you are using a chrome browser, click on the three dots icon at the top right of the browser window to the settings dropdown panel.
Step 3: Now you will see a Zoom option here alter it to 110% or 150% to make the black bar go away. So it is a styling issue on youtube. It appears again if you choose the 125% option.

Step 4: If you are still seeing the black line at 110% then try zooming out instead to 90% until the black line disappears.
Now the small black line on the Youtube video will disappear.

Conclusion :
This is only a temporary fix for now until google releases a permanent fix. Do share this post with your friends facing the same issue. Also google must be made aware of this issue so they can give a permanent fix.

Gowtham V, the founder of howtodoninja.com is an experienced tech writer and a cybersecurity enthusiast with a passion for exploring the latest technological advancements. He has 10 years of experience in writing comprehensive how to guides, tutorials, and reviews on software, hardware, and internet services. With an interest in computer security, he strives to educate users by writing content on how to use technology, and how to also protect their smart devices and personal data from cyber threats. He currently uses a Windows computer, and a Macbook Pro, and tests hundreds of Android phones for writing his reviews and guides.
Can u tell me if there is a permanent fix for it now please? Its been months now. Do they have a permanent fix? Plz respond