## Summary When a file-level discussion exists and the file changes in a subsequent MR version, GitLab creates a system note ("changed this file in version X") as a reply in the thread via `SystemNotes::MergeRequestsService#diff_discussion_outdated`. If the original comment is then deleted, the system note becomes the root of the discussion thread. Replies to that system note are also preserved. This is standard GitLab behavior - `Notes::DestroyService` only destroys the individual note, with no cascade to other notes in the thread. The file discussions component should handle this edge case: a `position_type: "file"` discussion where the first note is a system note. Currently, these threads may not render correctly or at all in Rapid Diffs. ## Steps to reproduce 1. Create a file-level discussion on a file in an MR 2. Push a change to that file, creating a new MR version (GitLab adds a "changed this file in version X" system note to the thread) 3. Delete the original comment 4. The system note remains as the thread root and can be replied to ## Expected behavior File-level discussions rooted by a system note should render correctly in Rapid Diffs. ## Context Identified during review of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/226087. Stanislav agreed this is a valid use case to handle as a follow-up.