I also appreciated the dynamic progression indicator at the top of the page. Some of these journal articles are organized and written such that there is no end in sight - it's hard to intuitively know where you are in the article, if you are nearly the buried results/conclusion, etc. The simply indicator tracks progress through the article without having to glance at the oft-invisible right-side scrollbar.

I haven't had that problem as far as I can remember, I rely on the (very thin) scroll bar that's shown on firefox for android.

I'm diverging into off-topic territory now, but I stopped reading the featured article because of the large static header div. Am I in the minority of mobile users who spends all the time in horizontal mode? It seems that probably 50% of mainstream blog or news sites I come across have some sort of obnoxious static header or footer (or worst, both!) that takes up a huge portion of the view when horizontal. Usually useless shit like a link to the main page, or their social media mention count. I take a screenshot of the worst offenders but I haven't posted them anywhere. The absolute criminal in this respect is tech crunch. You MUST orient to portrait and click the expand article button to read anything. Really unprofessional and disappointing, I avoid their links if I catch it in time.

Sorry, rant over.

Similarly, PDFs of articles in the PeerJ family of journals (e.g. [1]) have a progress bar symbol at the bottom-right of each page, which is also a nice feature.

[1]: https://peerj.com/articles/cs-103.pdf