Hi! I’m the creator of this addon. When I first built it, I was just experimenting and didn’t expect it to reach over 20k downloads!

I’ve now created a new version, fully open source:
https://github.com/deyvidholz/sidenotes-v2

It includes a better UI, more features, and fixes for all known bugs.

The new version is currently under review by Opera moderators and should be available soon!