FIX: Only use lastViewedTopic when going 'back' to a topic list (#22594)

Using the lastViewedTopicId indiscriminately can cause strange scrolling behavior when navigating to a **different** topic list after viewing a topic. We only want to refocus the topic when going 'back' to the same topic list which originally triggered the navigation.

spec/system/page_objects/components/topic_list.rb

@@ -34,6 +34,10 @@ module PageObjects
         find("#{TOPIC_LIST_BODY_SELECTOR} a", text: title).click
       end
 
+      def visit_topic(topic)
+        find("#{topic_list_item_class(topic)} a.raw-topic-link").click
+      end
+
       private
 
       def topic_list_item_class(topic)

spec/system/topic_list_focus_spec.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+describe "Topic list focus", type: :system do
+  let!(:topics) { Fabricate.times(10, :post).map(&:topic) }
+
+  before { Fabricate(:admin) }
+
+  let(:discovery) { PageObjects::Pages::Discovery.new }
+  let(:topic) { PageObjects::Pages::Topic.new }
+
+  def focussed_topic_id
+    page.evaluate_script(
+      "document.activeElement.closest('.topic-list-item')?.dataset.topicId",
+    )&.to_i
+  end
+
+  it "refocusses last clicked topic when going back to topic list" do
+    visit("/")
+    expect(page).to have_css("body.navigation-topics")
+    expect(discovery.topic_list).to have_topics
+
+    # Click a topic
+    discovery.topic_list.visit_topic(topics[5])
+    expect(topic).to have_topic_title(topics[5].title)
+
+    # Going back to the topic-list should re-focus
+    page.go_back
+    expect(page).to have_css("body.navigation-topics")
+    expect(focussed_topic_id).to eq(topics[5].id)
+
+    # Click topic again
+    discovery.topic_list.visit_topic(topics[5])
+    expect(topic).to have_topic_title(topics[5].title)
+
+    # Visiting a topic list another way should not focus
+    find(".sidebar-section-link[data-link-name='everything']").click
+    expect(page).to have_css("body.navigation-topics")
+    expect(focussed_topic_id).to eq(nil)
+  end
+end