It's an odd site design. Stories are loaded dynamically by script based on the URL fragment identifier; I'd imagine Reader Mode isn't geared toward that sort of thing.

I also wonder why they decided that hiding the scrollbar was a good idea.

And for the digital graphics people, consider using a modern variant instead: https://mortenhannemose.github.io/lena/

Discussed here a few weeks ago: https://news.ycombinator.com/item?id=46999224