working on the Solution Sketches and after trying to explain the reason of both the Discussions panel and the comments activity filter, I think we HAVE to merge both inside the activity section, and let the user select how they want to "sort" or "group" comments.
Here's my proposal
Document Activity accessory panel header + filters
some notes about the sketch:
we will remove the Discussions button at the top of the accessory panel.
when the user filters the activity by "Comments" a new option appears on the right side of the tabs section, that let the user change the way comments are shown: either chronologically (current way) or by thread (the current discussions panel)
this section of the filters can be activated depending on the parent filtering on the right if needed. In this case, the only filter that needs extra customization is the comments filter.
This way, we can reduce the complexity on users when they need to understand commenting, and also this will be aligned with what Bea is working on for the future
New Idea
Based on Eric's comment, I realize that making the discussions a "sub-panel" knowing that is one of the most important ones sounds very silly tbh. so I had the idea of actually proposing the opposite: add the group by action to the current discussions panel and remove comments from the Document Activity. This will look like this:
This will make all the comment interactions equal as they are now, and also emphasize commenting on documents.
I'm more inclined now for this idea rather than the first one.