I'm not the first one to come up with this idea. I do think Private Comments is the best implementation of the idea though. Primarily because of it's cautious handling of repository metadata, the automatic archiving of comments when the line they comment on changes, the possibility of team sharing, and its optional support for encryption.
But, maybe you want something that works differently.
Here's what I've found:
- Emacs Virtual Comments Same core idea as Private Comments but different implementation.
- Phantom Inline Comments Same core idea as Private Comments but different implementation.
- Org-remark Enables commenting on a file level, rather than line specific comments. It lets you create your own marginalia.
- In Place Annotations
- Codex JetBrains support coming "soon". Comments are in a sidebar instead of inlined, and support markdown and GitHub style "labels", & mentioning team members for notification purposes. Not open-source, appears to be a fledgling pre-revenue company.
Nothing. If it exists, I haven't seen it, and would love it if you'd file an issue, or make a Pull Request to update this page with information about plugins for other editors.