It’s not neurosis; it’s not OCD. It helps establish a line for your eyes to follow when flicking back to the left side of text. It’s just like the ruler or piece of paper that speed-reading and sight-reading teachers instruct students to use.
Notice that you will hardly or never do this on narrow columns of text.
Not everybody highlights text in a functional way. In fact, what I'm highlighting usually has nothing to do with what I'm reading at the moment, and is more of a nervous habit/tic than anything else.
Notice that you will hardly or never do this on narrow columns of text.