FIX: new topic indicator on the mobile categories page (#12271)

Regression with new dismiss button. We need the same solution to indicate if the topic was seen for category_list as done for topic_list:
https://github.com/discourse/discourse/blob/master/app/models/topic_list.rb#L123

Meta: https://meta.discourse.org/t/dismissed-new-topics-still-show-blue-dots-in-categories-view/181596

app/models/category_list.rb

@@ -66,6 +66,7 @@ def find_relevant_topics
     @all_topics.each do |t|
       # hint for the serializer
       t.include_last_poster = true if @options[:include_topics]
+      t.dismissed = dismissed_topic?(t)
       @topics_by_id[t.id] = t
     end
 
@@ -75,6 +76,15 @@ def find_relevant_topics
     end
   end
 
+  def dismissed_topic?(topic)
+    if @guardian.current_user
+      @dismissed_topic_users_lookup ||= DismissedTopicUser.lookup_for(@guardian.current_user, @all_topics)
+      @dismissed_topic_users_lookup.include?(topic.id)
+    else
+      false
+    end
+  end
+
   def find_categories
     @categories = Category.includes(
       :uploaded_background,

spec/models/category_list_spec.rb

@@ -95,18 +95,22 @@
       let!(:topic2) { Fabricate(:topic, category: topic_category, bumped_at: 5.minutes.ago) }
       let!(:topic3) { Fabricate(:topic, category: topic_category, bumped_at: 2.minutes.ago) }
       let!(:pinned) { Fabricate(:topic, category: topic_category, pinned_at: 10.minutes.ago, bumped_at: 10.minutes.ago) }
+      let!(:dismissed_topic_user) { Fabricate(:dismissed_topic_user, topic: topic2, user: user) }
 
       def displayable_topics
         category_list = CategoryList.new(Guardian.new(user), include_topics: true)
-        category_list.categories.find { |c| c.id == topic_category.id }.displayable_topics.map(&:id)
+        category_list.categories.find { |c| c.id == topic_category.id }.displayable_topics
       end
 
       it "returns pinned topic first" do
-        expect(displayable_topics).to eq([pinned.id, topic3.id])
+        expect(displayable_topics.map(&:id)).to eq([pinned.id, topic3.id])
 
         TopicUser.change(user.id, pinned.id, cleared_pinned_at: pinned.pinned_at + 10)
 
-        expect(displayable_topics).to eq([topic3.id, topic2.id])
+        expect(displayable_topics[0].dismissed).to eq(false)
+        expect(displayable_topics[1].dismissed).to eq(true)
+
+        expect(displayable_topics.map(&:id)).to eq([topic3.id, topic2.id])
       end
     end