You need to
before you can comment on or make changes to this bug.
People accidentally hit discard and then have no way to recover their comment.
1. Provide a temporary undo UI (gmail-style).
2. Leave a permanent placeholder on that line.
3. Have your discarded comment magically reappear if you try to add a comment back to that line.
2 and 3 are not mutually exclusive.
I vote for "undo"