Draft Notes on Added Lines Don't Render When base_sha != start_sha

Problem

When the target branch (main) moves forward after an MR is created, base_sha (merge base) diverges from start_sha (current main HEAD). In this state:

  • Published notes (Discussions API) on added lines (new_line only) render correctly ✅
  • Draft notes (Draft Notes API) on added lines (new_line only) do not render on the diff ❌

This issue doesn't happen when the main has not moved forward (base_sha == start_sha)

How to reproduce

This MR sets up the test case viktomas/test-project!37

Setup:

  1. Branch tv/diff-line-test adds sum() and average() at end of src/utils.ts
  2. Main gets 6 new header lines, shifting all line numbers by +6
  3. diff_refs: base_sha=eaa82a4 ≠ start_sha=a377257

Test results on viktomas/test-project!37 (diffs)

Reproducer: curl Commands

Get diff_refs from the MR first:

curl -s -H "PRIVATE-TOKEN: $GITLAB_TOKEN" \
  "https://gitlab.com/api/v4/projects/viktomas%2Ftest-project/merge_requests/37" \
  | jq '.diff_refs'
# {
#   "base_sha": "eaa82a43a06204f6f9b6092950ceba92720773f0",
#   "start_sha": "a377257a71a83665cd2e5ae5a7e42b97526b6548",
#   "head_sha": "dc5ec65e783268ba590bc08245e1bd6ae3b989a6"
# }

✅ Published note — renders inline on the diff:

curl -s -X POST \
  -H "PRIVATE-TOKEN: $GITLAB_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "body": "This renders correctly on the added line",
    "position": {
      "position_type": "text",
      "base_sha": "eaa82a43a06204f6f9b6092950ceba92720773f0",
      "head_sha": "dc5ec65e783268ba590bc08245e1bd6ae3b989a6",
      "start_sha": "a377257a71a83665cd2e5ae5a7e42b97526b6548",
      "new_path": "src/utils.ts",
      "old_path": "src/utils.ts",
      "new_line": 48
    }
  }' \
  "https://gitlab.com/api/v4/projects/viktomas%2Ftest-project/merge_requests/37/discussions"

This shows on the diff

image

❌ Draft note — does NOT render inline (same position):

curl -s -X POST \
  -H "PRIVATE-TOKEN: $GITLAB_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "note": "This is invisible on the diff despite valid line_code",
    "position": {
      "position_type": "text",
      "base_sha": "eaa82a43a06204f6f9b6092950ceba92720773f0",
      "head_sha": "dc5ec65e783268ba590bc08245e1bd6ae3b989a6",
      "start_sha": "a377257a71a83665cd2e5ae5a7e42b97526b6548",
      "new_path": "src/utils.ts",
      "old_path": "src/utils.ts",
      "new_line": 48
    }
  }' \
  "https://gitlab.com/api/v4/projects/viktomas%2Ftest-project/merge_requests/37/draft_notes"

shows in the list of draft notes, but not on the diff

image

The only difference is the endpoint (discussions vs draft_notes) and field name (body vs note). The position is identical. The prerequisite is base_sha != start_sha (target branch moved after MR creation).

Edited by Tomas Vik