Fixes suggestions being overwriten incorrectly

This fixes the suggestions in note being updated when it hasn't changed.
Instead of always updating the note, we compare to check if it matches
the existing note, if it does, we do nothing.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/235741

app/assets/javascripts/notes/stores/mutations.js

+import { isEqual } from 'lodash';
 import * as utils from './utils';
 import * as types from './mutation_types';
 import * as constants from '../constants';
@@ -31,7 +32,8 @@ export default {
         }
       }
 
-      note.base_discussion = undefined; // No point keeping a reference to this
+      // note.base_discussion = undefined; // No point keeping a reference to this
+      delete note.base_discussion;
       discussion.notes = [note];
 
       state.discussions.push(discussion);
@@ -220,6 +222,11 @@ export default {
   [types.UPDATE_NOTE](state, note) {
     const noteObj = utils.findNoteObjectById(state.discussions, note.discussion_id);
 
+    // Disable eslint here so we can delete the property that we no longer need
+    // in the note object
+    // eslint-disable-next-line no-param-reassign
+    delete note.base_discussion;
+
     if (noteObj.individual_note) {
       if (note.type === constants.DISCUSSION_NOTE) {
         noteObj.individual_note = false;
@@ -228,7 +235,10 @@ export default {
       noteObj.notes.splice(0, 1, note);
     } else {
       const comment = utils.findNoteObjectById(noteObj.notes, note.id);
-      noteObj.notes.splice(noteObj.notes.indexOf(comment), 1, note);
+
+      if (!isEqual(comment, note)) {
+        noteObj.notes.splice(noteObj.notes.indexOf(comment), 1, note);
+      }
     }
   },
 

app/assets/javascripts/vue_shared/components/markdown/suggestions.vue

@@ -64,6 +64,11 @@ export default {
   mounted() {
     this.renderSuggestions();
   },
+  beforeDestroy() {
+    if (this.suggestionsWatch) {
+      this.suggestionsWatch();
+    }
+  },
   methods: {
     renderSuggestions() {
       // swaps out suggestion(s) markdown with rich diff components
@@ -108,6 +113,13 @@ export default {
         },
       });
 
+      // We're using `$watch` as `suggestionsCount` updates do not
+      // propagate to this component for some unknown reason while
+      // using a traditional prop watcher.
+      this.suggestionsWatch = this.$watch('suggestionsCount', () => {
+        suggestionDiff.suggestionsCount = this.suggestionsCount;
+      });
+
       suggestionDiff.$on('apply', ({ suggestionId, callback, message }) => {
         this.$emit('apply', { suggestionId, callback, flashContainer: this.$el, message });
       });

changelogs/unreleased/ph-235741-fixSuggestionsUpdatingIncorrectly.yml

+---
+title: Fixed notes polling incorrectly overwriting suggestions in the DOM
+merge_request: 51988
+author:
+type: fixed

spec/frontend/notes/stores/mutation_spec.js

@@ -400,6 +400,19 @@ describe('Notes Store mutations', () => {
       expect(state.discussions[0].notes[0].note).toEqual('Foo');
     });
 
+    it('does not update existing note if it matches', () => {
+      const state = {
+        discussions: [{ ...individualNote, individual_note: false }],
+      };
+      jest.spyOn(state.discussions[0].notes, 'splice');
+
+      const updated = individualNote.notes[0];
+
+      mutations.UPDATE_NOTE(state, updated);
+
+      expect(state.discussions[0].notes.splice).not.toHaveBeenCalled();
+    });
+
     it('transforms an individual note to discussion', () => {
       const state = {
         discussions: [individualNote],