Preventing Users from Skipping Ahead with Sidebar Navigation On

I recently published a lesson where users can open a sidebar to view the slides. This feature was added because users wanted to be able to revisit specific parts of the lesson without having to start from the beginning. However, it also allows them to jump straight to the last slide, which automatically marks the lesson as complete.

I'm looking for suggestions on how to prevent users from skipping ahead while still letting them view the slide titles. Alternatively, if there's a better way to let users revisit specific points in the lesson without revealing the entire slide deck, I'd love to hear your ideas.

Any advice would be greatly appreciated!