As I watch these tutorials videos they are being marked near the end as completed automatically. It is not me that is marking them as completed. I checked the site preferences and those regarding auto-watching videos and such are unchecked for my account.
The videos mark as watched when it's about 95% done. The site does it automatically I believe. You can mark them as incomplete if you desire. Just helps you track where your at.
Those settings in preferences are
a) automatically start playback on entering a lesson, and b) automatically skip to the next lesson when the current lesson ends.
There is no setting to turn on/off mark complete. However at the top of each video you can mark or unmark the current lesson as complete.
It is a feature to help track your progress. You can use the progress section in your dashboard to quickly see all the course you have watched previously, completed or not.
It is a very useful feature, sometimes we start a project based course and become interested in a topic, lighting for example, and decide to watch a course focused on lighting, intending to return to where we left off in the project.