I just switched to a brand new computer and have been getting settings back to the way I like them. At first, I thought the light green background this script gives to thumbnails on watched videos (making them easy to see, something I'd taken for granted for years) was something YouTube did by default. The tiny red progress bar that Youtube adds is too small and hard to see, making this script needed.
But, even as someone who had regularly used this script, I didn't recognize it now from the description. I've never used the manual marking feature, automatic is all I ever needed it for. I've been working on a different personal Youtube script that happened to increase the spacing of the light-green background, which I think helps visually - maybe instead, the green should also appear behind the video's title-link in the main script. I think some screenshots similar to these might help to communicate how this script can be useful to others: