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Add an indicator for watched videos on YouTube. Use GM menus to display history statistics, backup history, and restore/merge history.
Use ALT+LeftClick or ALT+RightClick on a video list item to manually toggle the watched marker. The mouse button is defined in the script and can be changed.
For restoring/merging history, source file can also be a YouTube's history data JSON (downloadable from https://support.google.com/accounts/answer/3024190?hl=en). Or a list of YouTube video URLs (using current time as timestamps).