Please, consider putting a 'tracking button' in the end of each lesson as well. It's not an issue, but it's more practical than browse to the beggining of the page to click this button before moving to the next lesson.
Sorry for this displacement, I think this is not the right place to suggest this, but nevertheless, better here than nowhere. :)
Known issue, the platform's devs have it in their immense feature and UX todolist. I don't know when this will change, but it surely will at some point.
Meanwhile, I changed the right menu bar to follow your scrolling, so you can access the next lesson from there without scrolling back up.