Ah, a very good find! We did test a progress indicator style page turning, but dropped it at some point. It's still present in the code but automatically disabled.
If you want to try it out, you can edit the story HTML and find G=!1 at the very end of the file. (It's in two places, you can change both if you're unsure.) If you change it to G="full" or G="sequential" the indicator will be enabled. It was never finalized so if you do enable it, it's not guaranteed to work correctly. Offered purely as a curiosity.