I noticed that videos gets a checkmark just by starting it. I'd find the checkmark more useful if it was given only after the video has ended. Furthermore, it seems not possible to revert the checkmark, or at least it can't be found easily.
Basically, I want to keep track of what I've finished. In this case it's to come back to the video later if I've not finished it.
Edit: this might be partly a bug, not a feature request. I just noticed that another video did *not* get a checkmark, though I finished it and even clicked to button at the bottom to proceed to the next. The video that was checked was exited early and not using this button.