When using Differential for Code Review, a reviewer can leave a comment on a particular line of code, starting a thread. The person who requested the code review can mark that thread as Done. The person who left the comment should also have that ability.

For instance:

Person A creates new PR/CL in Differential with arc diff.
Person B leaves a comment on line 48 that says "there's a typo here."
Person A fixes the typo and pushes a new build to the PR.

Either Person A or B should be able to close that comment thread. Right now, only Person A can.