Allow issue reference filter to recognize design tab links

This inspects the path group (if available) to see if it matches
the known `/designs` subpath.

ee/lib/ee/banzai/filter/issue_reference_filter.rb

+# frozen_string_literal: true
+
+module EE
+  module Banzai
+    module Filter
+      module IssueReferenceFilter
+        extend ActiveSupport::Concern
+
+        prepended do
+          extend ::Gitlab::Utils::Override
+
+          override :object_link_text_extras
+          def object_link_text_extras(issue, matches)
+            super + design_link_extras(issue, matches.names.include?('path') && matches[:path])
+          end
+
+          private
+
+          def design_link_extras(issue, path)
+            if path == '/designs' && read_designs?(issue)
+              ['designs']
+            else
+              []
+            end
+          end
+
+          def read_designs?(issue)
+            Ability.allowed?(current_user, :read_design, issue)
+          end
+        end
+      end
+    end
+  end
+end

ee/spec/lib/banzai/filter/issue_reference_filter_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Banzai::Filter::IssueReferenceFilter do
+  include FilterSpecHelper
+  include DesignManagementTestHelpers
+
+  describe '#object_link_text_extras' do
+    before do
+      enable_design_management(enabled)
+    end
+
+    let(:enabled) { true }
+    let_it_be(:issue) { create(:issue) }
+
+    let(:project) { issue.project }
+    let(:matches) { ::Issue.link_reference_pattern.match(input_text) }
+    let(:current_user) { project.owner }
+    let(:extras) { subject.object_link_text_extras(issue, matches) }
+
+    subject do
+      render_context = Banzai::RenderContext.new(project, current_user)
+      context = { project: project, current_user: current_user, render_context: render_context }
+      described_class.new(input_text, context)
+    end
+
+    context 'the link does not go to the designs tab' do
+      let(:input_text) { Gitlab::Routing.url_helpers.project_issue_url(issue.project, issue) }
+
+      it 'does not include designs' do
+        expect(extras).not_to include('designs')
+      end
+    end
+
+    context 'the link goes to the designs tab' do
+      let(:input_text) { url_for_designs(issue) }
+
+      it 'includes designs' do
+        expect(extras).to include('designs')
+      end
+
+      context 'design management is disabled' do
+        let(:enabled) { false }
+
+        it 'does not include designs in the extras' do
+          expect(extras).not_to include('designs')
+        end
+      end
+    end
+  end
+end

ee/spec/support/helpers/design_management_test_helpers.rb

@@ -29,6 +29,10 @@ module DesignManagementTestHelpers
     Gitlab::Routing.url_helpers.designs_project_issue_url(design.project, design.issue, path_options)
   end
 
+  def url_for_designs(issue)
+    Gitlab::Routing.url_helpers.designs_project_issue_url(issue.project, issue)
+  end
+
   private
 
   def act_on_designs(designs, &block)