Retake screenshots this time without subtitles (vice versa)
Free media player with support for Youtube
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rvm
Perhaps a feature that takes in a folder of SM screenshots,reads their filenames for media file name(s) and timestamps, and retakes those screenshots again either with or without the subtitle visible option. In the case of someone spending two hours watching and screenshotting something and in the end realizing they had the undesired setting set.
Beyond that, maybe a screenshot history log that can open the media file at the screenshot time when you click on individual log entries.