Documentation
YouTube REALLY Mark As Watched adds one item to a video thumbnail's More menu. That item updates your YouTube account and checks the result.
Install the extension
Load the current release in desktop Chrome.
Mark a video as watched
Know what happens after you click.
Troubleshooting
Understand missing menus and unconfirmed progress.
What the extension confirms
Success means YouTube returned the selected video in your watch history with a progress value of 100%. A successful network response alone does not count.
Your history and native progress bar persist on YouTube after a reload. The extension keeps no separate watched-video database.
You control the account change.
Nothing is marked automatically. Each Mark as watched click updates the selected video in your currently signed-in YouTube account.