uBlock Origin is my extension of choice for this. It makes it really easy to block those distractions, and there are quite a few pre-existing filters to choose from.
I think you missed what the poster is asking for. They want to block a portion of the video itself. For example when you watch the news on TV, there is constant scrolling text on the bottom of the screen with the latest headlines. They want to block stuff like that.
I believe you're right! I can't think of any extension that would be able to modify the picture of a stream itself in real-time. What came to my mind was the kind of 'picture-in-picture' video that some questionable news sites display as you scroll down an article, usually a distracting broadcast which is barely related to the news itself.
