Include author in assignee dropdown search

When searching for the author in the assignee dropdown it now correctly returns the user

Closes #22905

app/controllers/autocomplete_controller.rb

@@ -55,7 +55,13 @@ class AutocompleteController < ApplicationController
   def find_users
     @users =
       if @project
-        @project.team.users
+        user_ids = @project.team.users.map(&:id)
+
+        if params[:author_id].present?
+          user_ids << params[:author_id]
+        end
+
+        User.where(id: user_ids)
       elsif params[:group_id].present?
         group = Group.find(params[:group_id])
         return render_404 unless can?(current_user, :read_group, group)

changelogs/unreleased/assignee-dropdown-autocomplete.yml

@@ -0,0 +1,4 @@
+---
+title: Assignee dropdown now searches author of issue or merge request
+merge_request: 
+author: 

spec/features/issues/issue_sidebar_spec.rb

@@ -1,7 +1,9 @@
 require 'rails_helper'
 
 feature 'Issue Sidebar', feature: true do
-  let(:project) { create(:project) }
+  include WaitForAjax
+
+  let(:project) { create(:project, :public) }
   let(:issue) { create(:issue, project: project) }
   let!(:user) { create(:user)}
 
@@ -10,6 +12,37 @@ feature 'Issue Sidebar', feature: true do
     login_as(user)
   end
 
+  context 'assignee', js: true do
+    let(:user2) { create(:user) }
+    let(:issue2) { create(:issue, project: project, author: user2) }
+
+    before do
+      project.team << [user, :developer]
+      visit_issue(project, issue2)
+
+      find('.block.assignee .edit-link').click
+
+      wait_for_ajax
+    end
+
+    it 'shows author in assignee dropdown' do
+      page.within '.dropdown-menu-user' do
+        expect(page).to have_content(user2.name)
+      end
+    end
+
+    it 'shows author when filtering assignee dropdown' do
+      page.within '.dropdown-menu-user' do
+        find('.dropdown-input-field').native.send_keys user2.name
+        sleep 1 # Required to wait for end of input delay
+
+        wait_for_ajax
+
+        expect(page).to have_content(user2.name)
+      end
+    end
+  end
+
   context 'as a allowed user' do
     before do
       project.team << [user, :developer]