This looks great! Very good to see another implementation of marginalia.
In the current UI, though the “comment indicator” looks a bit disconnected from the text on wide-screens.
You are right that it looks a bit disconnected. I basically never have my browser at full-screen (usually it’s at split-screen), so I didn’t consider this very much. This is a presentational (CSS) thing though, not so much code.
Since then, I have added a “highlight”-class to the current paragraph in question, to make it look more connected. The icon itself is still disconnected though, which could be fixed with simple @media-checks on the screen width, I think.