Marginal от ComplexAce

This add-on does NOT require any special sign up or software, it works by storing a paragraph number in the comment and reading it, then places the comments in the relevant location.

This Add-on supports:
- inline/per-paragraph comments
- replies for inline comments (not limited to inline comments)

You can check an example on the top of this story:
https://archiveofourown.org/chapters/210498011?show_comments=true&view_full_work=false#comment_1128797241