Now any video you have watched will have your viewing progress displayed as a red bar at the bottom of the thumbnail.
For example: have a look at this playlist from YouTube content creator Northernlion in which he covers indie games. All the videos apart from the latest one based on Planet Coaster have been partly watched by us:
While this update is meant to show watched progress at a glance, videos you’ve watched to completion will also be changed with a completely red bar filling up the bottom of the thumbnail.
This is a very stealthy update from YouTube as we haven’t seen any official word from them about it. We can confirm that we have started seeing it from Friday afternoon on November 12th.
Strangely, it hasn’t seen widespread adoption. While the examples above were from Chrome and the Android app respectively, some users are not seeing the update even using the same, updated software.