Scrollbars. Ever heard of them? They’re pretty cool. Click and drag on a scrollbar and you can move content around in a scrollable content pane. I love that shit. Every day I am scrolling on my computer, all day long. But the scrollbars are getting smaller and this is increasingly becoming a problem...
I recently had to talk a designer out of implementing a “webpage progress indicator” that was a thin horizontal bar across the top of the page that filled in as you progressed through the content.
They are bad replicas of school bars. Except you can't use these to scroll the page and they use horizontal progress to express vertical progress. Everything they do could be done more effectively by having a visible scroll bar.
I don't see them as scroll bar replicas; good ones are very narrow but high contrast, to easily offer a
compact visual sense of article length -- rather than page length, an important distinction when there are cited sources, recommended articles, and a footer menu. Different functions.
IMO, they should coexist with a well-designed vertical scroll bar.