To complement the collaborative features in #1930, it would be a good idea to also create a commenting system. That API can then be exposed to other developers (at Yoast we'd love to use it to mark SEO feedback in the text, for instance).

Here are some mockups of what this could look like. I've aimed for feature parity with Google Docs, and tried my best to stay within the Gutenberg design spectrum, but feedback or improvements on the UI and UX are very welcome.

Here's how it could work:

• An icon in the top bar shows how many comments a document has. Clicking that will move the regular inspector sidebar aside and slide in the comments sidebar.
• The default Overview tab shows the comments in context, like Google Docs does, and the Review tab turns them into a sortable 'to-do' kind of list.
• You can filter the comments by type (so from users, from plugins, resolved etc)
• Clicking the highlight in the text also highlights the comments, and vice versa.

@atimmer has also agreed to dedicate some time in the coming weeks to help figure out the technical specifications.

Adding comments to selected text or blocks via the comment button in the toolbar

Writing a comment

Comments marked up in the text

The overview tab, listing all comments in context

The Review tab

Some takes on what the filters could look like

Mobile version of the comments overview

Mobile version of a selected comment