Re-enable legacy diff notes on merge request diffs

This re-enables legacy diff notes on the merge request diffs
This feature was not workig correctly after the Vue refactor
LegacyDiffNotes have no `position`, instead they only have a `line_code`
As an extra, this also re-enables commenting on legacy diffs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48873

8 files changed, 44 insertions, 20 deletions

diff --git a/app/assets/javascripts/diffs/components/diff_line_note_form.vue b/app/assets/javascripts/diffs/components/diff_line_note_form.vue
index a0dc381ccc7..0fa14615532 100644
--- a/app/assets/javascripts/diffs/components/diff_line_note_form.vue
+++ b/app/assets/javascripts/diffs/components/diff_line_note_form.vue
@@ -22,7 +22,7 @@ export default {
       type: Object,
       required: true,
     },
-    position: {
+    linePosition: {
       type: String,
       required: false,
       default: '',
@@ -81,7 +81,7 @@ export default {
         noteTargetLine: this.noteTargetLine,
         diffViewType: this.diffViewType,
         diffFile: selectedDiffFile,
-        linePosition: this.position,
+        linePosition: this.linePosition,
       });
 
       this.saveNote(postData)
diff --git a/app/assets/javascripts/diffs/components/parallel_diff_comment_row.vue b/app/assets/javascripts/diffs/components/parallel_diff_comment_row.vue
index 26417c350cb..3339c56cbb6 100644
--- a/app/assets/javascripts/diffs/components/parallel_diff_comment_row.vue
+++ b/app/assets/javascripts/diffs/components/parallel_diff_comment_row.vue
@@ -92,7 +92,7 @@ export default {
         :diff-file-hash="diffFileHash"
         :line="line.left"
         :note-target-line="line.left"
-        position="left"
+        line-position="left"
       />
     </td>
     <td class="notes_line new"></td>
@@ -111,7 +111,7 @@ export default {
         :diff-file-hash="diffFileHash"
         :line="line.right"
         :note-target-line="line.right"
-        position="right"
+        line-position="right"
       />
     </td>
   </tr>
diff --git a/app/assets/javascripts/diffs/constants.js b/app/assets/javascripts/diffs/constants.js
index f68afa44837..2795dddfc48 100644
--- a/app/assets/javascripts/diffs/constants.js
+++ b/app/assets/javascripts/diffs/constants.js
@@ -7,6 +7,7 @@ export const CONTEXT_LINE_TYPE = 'context';
 export const EMPTY_CELL_TYPE = 'empty-cell';
 export const COMMENT_FORM_TYPE = 'commentForm';
 export const DIFF_NOTE_TYPE = 'DiffNote';
+export const LEGACY_DIFF_NOTE_TYPE = 'LegacyDiffNote';
 export const NOTE_TYPE = 'Note';
 export const NEW_LINE_TYPE = 'new';
 export const OLD_LINE_TYPE = 'old';
diff --git a/app/assets/javascripts/diffs/store/mutations.js b/app/assets/javascripts/diffs/store/mutations.js
index 6dc5bf16c65..b6818f905e5 100644
--- a/app/assets/javascripts/diffs/store/mutations.js
+++ b/app/assets/javascripts/diffs/store/mutations.js
@@ -90,7 +90,8 @@ export default {
     const firstDiscussion = discussions[0];
     const isDiffDiscussion = firstDiscussion.diff_discussion;
     const hasLineCode = firstDiscussion.line_code;
-    const isResolvable = firstDiscussion.resolvable;
+    const isResolvable =
+      firstDiscussion.resolvable || (!firstDiscussion.resolvable && !firstDiscussion.position);
     const diffPosition = diffPositionByLineCode[firstDiscussion.line_code];
 
     if (
diff --git a/app/assets/javascripts/diffs/store/utils.js b/app/assets/javascripts/diffs/store/utils.js
index b7e52a8f37f..d521e4584ad 100644
--- a/app/assets/javascripts/diffs/store/utils.js
+++ b/app/assets/javascripts/diffs/store/utils.js
@@ -4,6 +4,7 @@ import {
   LINE_POSITION_LEFT,
   LINE_POSITION_RIGHT,
   TEXT_DIFF_POSITION_TYPE,
+  LEGACY_DIFF_NOTE_TYPE,
   DIFF_NOTE_TYPE,
   NEW_LINE_TYPE,
   OLD_LINE_TYPE,
@@ -60,7 +61,7 @@ export function getNoteFormData(params) {
       noteable_type: noteableType,
       noteable_id: noteableData.id,
       commit_id: '',
-      type: DIFF_NOTE_TYPE,
+      type: diffFile.diffRefs.startSha ? DIFF_NOTE_TYPE : LEGACY_DIFF_NOTE_TYPE,
       line_code: noteTargetLine.lineCode,
     },
   };
@@ -230,7 +231,16 @@ export function getDiffPositionByLineCode(diffFiles) {
         const { lineCode, oldLine, newLine } = line;
 
         if (lineCode) {
-          acc[lineCode] = { baseSha, headSha, startSha, newPath, oldPath, oldLine, newLine };
+          acc[lineCode] = {
+            baseSha,
+            headSha,
+            startSha,
+            newPath,
+            oldPath,
+            oldLine,
+            newLine,
+            lineCode,
+          };
         }
       });
     }
@@ -242,8 +252,14 @@ export function getDiffPositionByLineCode(diffFiles) {
 // This method will check whether the discussion is still applicable
 // to the diff line in question regarding different versions of the MR
 export function isDiscussionApplicableToLine(discussion, diffPosition) {
-  const originalRefs = convertObjectPropsToCamelCase(discussion.original_position.formatter);
-  const refs = convertObjectPropsToCamelCase(discussion.position.formatter);
+  const { lineCode, ...diffPositionCopy } = diffPosition;
 
-  return _.isEqual(refs, diffPosition) || _.isEqual(originalRefs, diffPosition);
+  if (discussion.original_position && discussion.position) {
+    const originalRefs = convertObjectPropsToCamelCase(discussion.original_position.formatter);
+    const refs = convertObjectPropsToCamelCase(discussion.position.formatter);
+
+    return _.isEqual(refs, diffPositionCopy) || _.isEqual(originalRefs, diffPositionCopy);
+  }
+
+  return lineCode === discussion.line_code;
 }
diff --git a/app/assets/javascripts/notes/stores/utils.js b/app/assets/javascripts/notes/stores/utils.js
index 8ccbdb4c130..e2dd81273f5 100644
--- a/app/assets/javascripts/notes/stores/utils.js
+++ b/app/assets/javascripts/notes/stores/utils.js
@@ -27,7 +27,11 @@ export const getQuickActionText = note => {
 
 export const reduceDiscussionsToLineCodes = selectedDiscussions =>
   selectedDiscussions.reduce((acc, note) => {
-    if (note.diff_discussion && note.line_code && note.resolvable) {
+    if (
+      note.diff_discussion &&
+      note.line_code &&
+      (note.resolvable || (!note.resolvable && !note.position))
+    ) {
       // For context about line notes: there might be multiple notes with the same line code
       const items = acc[note.line_code] || [];
       items.push(note);
diff --git a/changelogs/unreleased/mr-legacy-diff-notes.yml b/changelogs/unreleased/mr-legacy-diff-notes.yml
new file mode 100644
index 00000000000..38b161c2a37
--- /dev/null
+++ b/changelogs/unreleased/mr-legacy-diff-notes.yml
@@ -0,0 +1,5 @@
+---
+title: Correctly show legacy diff notes in the merge request changes tab
+merge_request:
+author:
+type: fixed
diff --git a/spec/features/merge_request/user_posts_diff_notes_spec.rb b/spec/features/merge_request/user_posts_diff_notes_spec.rb
index b6ed3686de2..fa148715855 100644
--- a/spec/features/merge_request/user_posts_diff_notes_spec.rb
+++ b/spec/features/merge_request/user_posts_diff_notes_spec.rb
@@ -200,23 +200,20 @@ describe 'Merge request > User posts diff notes', :js do
     end
 
     context 'with a new line' do
-      # TODO: https://gitlab.com/gitlab-org/gitlab-ce/issues/48034
-      xit 'allows commenting' do
-        should_allow_commenting(find('[id="2f6fcd96b88b36ce98c38da085c795a27d92a3dd_10_9"]').find(:xpath, '..'))
+      it 'allows commenting' do
+        should_allow_commenting(find('[id="2f6fcd96b88b36ce98c38da085c795a27d92a3dd_10_9"]'))
       end
     end
 
     context 'with an old line' do
-      # TODO: https://gitlab.com/gitlab-org/gitlab-ce/issues/48034
-      xit 'allows commenting' do
-        should_allow_commenting(find('[id="6eb14e00385d2fb284765eb1cd8d420d33d63fc9_22_22"]').find(:xpath, '..'))
+      it 'allows commenting' do
+        should_allow_commenting(find('[id="6eb14e00385d2fb284765eb1cd8d420d33d63fc9_22_22"]'))
       end
     end
 
     context 'with an unchanged line' do
-      # TODO: https://gitlab.com/gitlab-org/gitlab-ce/issues/48034
-      xit 'allows commenting' do
-        should_allow_commenting(find('[id="2f6fcd96b88b36ce98c38da085c795a27d92a3dd_7_7"]').find(:xpath, '..'))
+      it 'allows commenting' do
+        should_allow_commenting(find('[id="2f6fcd96b88b36ce98c38da085c795a27d92a3dd_7_7"]'))
       end
     end