Show caret position or at least the currently edited blocks by the collaborators

[youknowriad]

Caret position might require changes upstream in Gutenberg, I think it's fine to start with block highlighting first.

[epiqueras]

Selection is already tracked and exposed from the block editor for core-data undos, so no core changes should be needed.

[youknowriad]

@epiqueras I was thinking about how to render it, not how to track it. I guess a custom format might work

[epiqueras]

Yep, I was thinking that too.

[youknowriad]

This is already done. What's remaining is potentially assigning a custom color to each collaborator.