lesson does not scroll
You can't scroll actually, a lesson is split in pages. What you need to do is to click on the right arrow to move on to the next page (on the web), or just swipe screen left if you are using the app.
The arrow keys don't do anything. There are no scroll bars, the page up and page down keys also do not do anything. So how do I access the other pages of the lesson?
If you are using the browser, there are arrows in the left and right margins of the text.
Strange, are you not seeing these arrows?