Fix assign user dropdown in new issue form

Add default array value in case optional chaining falls through

app/assets/javascripts/gfm_auto_complete.js

@@ -251,9 +251,10 @@ class GfmAutoComplete {
           });
 
           // Cache assignees list for easier filtering later
-          assignees = SidebarMediator.singleton?.store?.assignees?.map(
-            assignee => `${assignee.username} ${assignee.name}`,
-          );
+          assignees =
+            SidebarMediator.singleton?.store?.assignees?.map(
+              assignee => `${assignee.username} ${assignee.name}`,
+            ) || [];
 
           const match = GfmAutoComplete.defaultMatcher(flag, subtext, this.app.controllers);
           return match && match.length ? match[1] : null;

changelogs/unreleased/210005-fix-assignee-quickaction-on-new-issue.yml

+---
+title: Fix assignee dropdown on new issue page
+merge_request: 26971
+author:
+type: fixed

spec/features/issues/gfm_autocomplete_spec.rb

@@ -284,16 +284,17 @@ describe 'GFM autocomplete', :js do
 
   context 'assignees' do
     let(:issue_assignee) { create(:issue, project: project) }
+    let(:unassigned_user) { create(:user) }
 
     before do
       issue_assignee.update(assignees: [user])
 
-      visit project_issue_path(project, issue_assignee)
-
-      wait_for_requests
+      project.add_maintainer(unassigned_user)
     end
 
     it 'lists users who are currently not assigned to the issue when using /assign' do
+      visit project_issue_path(project, issue_assignee)
+
       note = find('#note-body')
       page.within '.timeline-content-form' do
         note.native.send_keys('/as')
@@ -305,6 +306,19 @@ describe 'GFM autocomplete', :js do
       wait_for_requests
 
       expect(find('#at-view-users .atwho-view-ul')).not_to have_content(user.username)
+      expect(find('#at-view-users .atwho-view-ul')).to have_content(unassigned_user.username)
+    end
+
+    it 'shows dropdown on new issue form' do
+      visit new_project_issue_path(project)
+
+      textarea = find('#issue_description')
+      textarea.native.send_keys('/ass')
+      find('.atwho-view li', text: '/assign')
+      textarea.native.send_keys(:tab)
+
+      expect(find('#at-view-users .atwho-view-ul')).to have_content(unassigned_user.username)
+      expect(find('#at-view-users .atwho-view-ul')).to have_content(user.username)
     end
   end