Return ancestor group milestones for JSON requests

JSON requests are made for populating dropdowns and filters.
These should include ancestor milestones because ancestor
milestones can be assigned to the group's issues

app/controllers/groups/milestones_controller.rb

@@ -119,7 +119,9 @@ class Groups::MilestonesController < Groups::ApplicationController
   end
 
   def search_params
-    params.permit(:state, :search_title).merge(group_ids: group.id)
+    groups = request.format.json? ? group.self_and_ancestors.select(:id) : group.id
+
+    params.permit(:state, :search_title).merge(group_ids: groups)
   end
 end
 

changelogs/unreleased/27366-supergroup-milestone-not-showing-in-subgroup-board-issue-list.yml

+---
+title: Fix group issue list and group issue board filters not showing ancestor group
+  milestones
+merge_request: 23038
+author:
+type: fixed

spec/controllers/groups/milestones_controller_spec.rb

@@ -148,6 +148,19 @@ describe Groups::MilestonesController do
         expect(response).to have_gitlab_http_status(200)
         expect(response.content_type).to eq 'application/json'
       end
+
+      context 'for a subgroup' do
+        let(:subgroup) { create(:group, parent: group) }
+
+        it 'includes ancestor group milestones' do
+          get :index, params: { group_id: subgroup.to_param }, format: :json
+
+          milestones = json_response
+
+          expect(milestones.count).to eq(1)
+          expect(milestones.first['title']).to eq('group milestone')
+        end
+      end
     end
 
     context 'external authorization' do