Ref T7447. Implements per-viewer comment hiding. Once a comment is obsolete or uninteresting, you can hide it completely.
This is sticky per-user.
My hope is that this will strike a better balance between concerns than some of the other approaches (conservative porting, summarization, hide-all).
Specifically, this adds a new action here:
Clicking it completely collapses the comment into a small icon on the previous line, and saves the comment state as hidden for you:
You can click the icon to reveal all hidden comments below the line.