Resolve "Discussion counter is wrong on issue page"

Resolves the situation where the Discussion tab counter
would sometimes reflect the wrong count. We were relying
on the list of `allDiscussions` but it's more complicated
than that. We have to filter out system notes and count
all individual notes in threads.

Plus, we already have a getter for that.

https://gitlab.com/gitlab-org/gitlab/merge_requests/16567

app/assets/javascripts/notes/components/notes_app.vue

@@ -69,6 +69,7 @@ export default {
       'commentsDisabled',
       'getNoteableData',
       'userCanReply',
+      'discussionTabCounter',
     ]),
     noteableType() {
       return this.noteableData.noteableType;
@@ -95,13 +96,13 @@ export default {
       }
     },
     allDiscussions() {
-      if (this.discussonsCount) {
-        this.discussonsCount.textContent = this.allDiscussions.length;
+      if (this.discussionsCount && !this.isLoading) {
+        this.discussionsCount.textContent = this.discussionTabCounter;
       }
     },
   },
   created() {
-    this.discussonsCount = document.querySelector('.js-discussions-count');
+    this.discussionsCount = document.querySelector('.js-discussions-count');
 
     this.setNotesData(this.notesData);
     this.setNoteableData(this.noteableData);

ee/changelogs/unreleased/13370-fix-discussion-tab-counter-on-issues.yml

+---
+title: Fix Discussion tab counter on Issues
+merge_request:
+author:
+type: fixed

spec/features/issues/issues_tabs_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Issue page tabs', :js do
+  let(:user) { create(:user) }
+  let(:project) { create(:project, :public) }
+  let(:issue) { create(:issue, author: user, assignees: [user], project: project) }
+
+  describe 'discussions tab counter' do
+    before do
+      stub_licensed_features(design_management: true)
+      stub_feature_flags(design_management_flag: true)
+      allow(Ability).to receive(:allowed?) { true }
+    end
+
+    subject do
+      sign_in(user)
+
+      visit project_issue_path(project, issue)
+
+      wait_for_requests
+
+      find('#discussion')
+    end
+
+    context 'new issue' do
+      it 'displays count of 0' do
+        is_expected.to have_content('Discussion 0')
+      end
+    end
+
+    context 'issue with 2 system notes and 1 discussion' do
+      let!(:discussion) { create(:discussion_note_on_issue, noteable: issue, project: project, note: "This is good") }
+
+      before do
+        create(:system_note, noteable: issue, project: project, author: user, note: 'description updated')
+        create(:system_note, noteable: issue, project: project, author: user, note: 'description updated')
+      end
+
+      it 'displays count of 1' do
+        is_expected.to have_content('Discussion 1')
+      end
+
+      context 'with 1 reply' do
+        before do
+          create(:note, noteable: issue, in_reply_to: discussion, discussion_id: discussion.discussion_id, note: 'I also think this is good')
+        end
+
+        it 'displays count of 2' do
+          is_expected.to have_content('Discussion 2')
+        end
+      end
+    end
+  end
+end