I was actually going to also have it display a toast-like "7 minutes [of reading] left" on the top left when you scroll a good amount.
Having a thing across the top is perpendicular and doesn't relate to the vertical structure of a scrolling page. You have change the vertical position of your eyes to see it in a context where vertical position is significant.
Edit: Keep in mind that as I'm reading the story, my eyes are nowhere near the top of the browser. If you want to give me context, the top of the page is not the best place to provide it. It does make for a cool demo though.
I whipped it up pretty quickly so if there are any issues or enhancement ideas feel free to post them on Github.