Fix bug with project pagination

When projects were listed after groups, the projects that would also
have been listed on the last page containing groups would be repeated.

app/finders/group_descendants_finder.rb

@@ -59,8 +59,7 @@ class GroupDescendantsFinder
                               .page(params[:page]).per(per_page)
                               .select(GROUP_SELECTS)
 
-    paginated_projects = projects.with_route.page(subgroup_page - group_page_count)
-                           .per(per_page - subgroups_with_counts.size)
+    paginated_projects = paginate_projects_after_groups(subgroups_with_counts)
 
     if params[:filter]
       ancestors_for_project_search = ancestors_for_groups(Group.where(id: paginated_projects.select(:namespace_id)))
@@ -70,6 +69,31 @@ class GroupDescendantsFinder
     @children = subgroups_with_counts + paginated_projects
   end
 
+  def paginate_projects_after_groups(loaded_subgroups)
+    # We adjust the pagination for projects for the combination with groups:
+    # - We limit the first page (page 0) where we show  projects:
+    #   Page size = 20: 17 groups, 3 projects
+    # - We ofset the page to start at 0 after the group pages:
+    #   3 pages of projects:
+    #   - currently on page 3: Show page 0 (first page) limited to the number of
+    #     projects that still fit the page (no offset)
+    #   - currently on page 4: Show page 1 show all projects, offset by the number
+    #     of projects shown on project-page 0.
+    group_page_count = loaded_subgroups.total_pages
+    subgroup_page = loaded_subgroups.current_page
+    group_last_page_count = subgroups.page(group_page_count).count
+    project_page = subgroup_page - group_page_count
+    offset = if project_page.zero? || group_page_count.zero?
+               0
+             else
+               per_page - group_last_page_count
+             end
+
+    projects.with_route.page(project_page)
+      .per(per_page - loaded_subgroups.size)
+      .padding(offset)
+  end
+
   def direct_child_groups
     GroupsFinder.new(current_user,
                      parent: parent_group,

spec/controllers/groups_controller_spec.rb

@@ -152,13 +152,15 @@ describe GroupsController do
 
   describe 'GET #show' do
     context 'pagination' do
+      let(:per_page) { 3 }
+
       before do
-        allow(Kaminari.config).to receive(:default_per_page).and_return(2)
+        allow(Kaminari.config).to receive(:default_per_page).and_return(per_page)
       end
 
       context 'with only projects' do
         let!(:other_project) { create(:project, :public, namespace: group) }
-        let!(:first_page_projects) { create_list(:project, Kaminari.config.default_per_page, :public, namespace: group ) }
+        let!(:first_page_projects) { create_list(:project, per_page, :public, namespace: group ) }
 
         it 'has projects on the first page' do
           get :show, id: group.to_param, sort: 'id_desc'
@@ -174,9 +176,9 @@ describe GroupsController do
       end
 
       context 'with subgroups and projects', :nested_groups do
-        let!(:first_page_subgroups) { create_list(:group, Kaminari.config.default_per_page, parent: group) }
+        let!(:first_page_subgroups) { create_list(:group,  per_page, :public,  parent: group) }
         let!(:other_subgroup) { create(:group, :public, parent: group) }
-        let!(:project) { create(:project, :public, namespace: group) }
+        let!(:next_page_projects) { create_list(:project, per_page, :public, namespace: group) }
 
         it 'contains all subgroups' do
           get :children, id: group.to_param, sort: 'id_asc', format: :json
@@ -187,7 +189,7 @@ describe GroupsController do
         it 'contains the project and group on the second page' do
           get :children, id: group.to_param, sort: 'id_asc', page: 2, format: :json
 
-          expect(assigns(:children)).to contain_exactly(other_subgroup, project)
+          expect(assigns(:children)).to contain_exactly(other_subgroup, *next_page_projects.take(per_page - 1))
         end
       end
     end