Show group milestones in autocomplete

app/services/projects/autocomplete_service.rb

@@ -5,7 +5,15 @@ module Projects
     end
 
     def milestones
-      @project.milestones.active.reorder(due_date: :asc, title: :asc).select([:iid, :title])
+      finder_params = {
+        project_ids: [@project.id],
+        state: :active,
+        order: { due_date: :asc, title: :asc }
+      }
+
+      finder_params[:group_ids] = [@project.group.id] if @project.group
+
+      MilestonesFinder.new(finder_params).execute.select([:iid, :title])
     end
 
     def merge_requests

changelogs/unreleased/group-milestone-references-system-notes.yml

+---
+title: Support Markdown references, autocomplete, and quick actions for group milestones
+merge_request:
+author:

doc/user/project/milestones/index.md

@@ -56,4 +56,5 @@ total merge requests and issues.
 
 ## Quick actions
 
-[Quick actions](../quick_actions.md) are available for assigning and removing project milestones only. [In the future](https://gitlab.com/gitlab-org/gitlab-ce/issues/34778), this will also apply to group milestones.
+[Quick actions](../quick_actions.md) are available for assigning and removing
+project and group milestones.

spec/services/projects/autocomplete_service_spec.rb

@@ -88,4 +88,31 @@ describe Projects::AutocompleteService do
       end
     end
   end
+
+  describe '#milestones' do
+    let(:user) { create(:user) }
+    let(:group) { create(:group) }
+    let(:project) { create(:project, group: group) }
+    let!(:group_milestone) { create(:milestone, group: group) }
+    let!(:project_milestone) { create(:milestone, project: project) }
+
+    let(:milestone_titles) { described_class.new(project, user).milestones.map(&:title) }
+
+    it 'includes project and group milestones' do
+      expect(milestone_titles).to eq([group_milestone.title, project_milestone.title])
+    end
+
+    it 'does not include closed milestones' do
+      group_milestone.close
+
+      expect(milestone_titles).to eq([project_milestone.title])
+    end
+
+    it 'does not include milestones from other projects in the group' do
+      other_project = create(:project, group: group)
+      project_milestone.update!(project: other_project)
+
+      expect(milestone_titles).to eq([group_milestone.title])
+    end
+  end
 end