Found via https://github.com/mdn/webextensions-examples/tree/master/annotate-page Docs are at https://developer.mozilla.org/en-US/Add-ons/WebExtensions/user_interface/Sidebars Currently it only looks like it's supported by Firefox, and there's no "spec-level" information at present: https://browserext.github.io/browserext/ (from the [Browser Extension CG](https://www.w3.org/community/browserext/))
Found via https://github.com/mdn/webextensions-examples/tree/master/annotate-page
Docs are at https://developer.mozilla.org/en-US/Add-ons/WebExtensions/user_interface/Sidebars
Currently it only looks like it's supported by Firefox, and there's no "spec-level" information at present: https://browserext.github.io/browserext/ (from the Browser Extension CG)