FIX: Do not show cached post after toggling "Show original post" and scrolling past 21st post

app/helpers/topics_helper.rb

@@ -22,6 +22,8 @@ def categories_breadcrumb(topic)
   end
 
   def localize_topic_view_content(topic_view)
+    return if cookies.key?(ContentLocalization::SHOW_ORIGINAL_COOKIE)
+
     crawl_locale = params[Discourse::LOCALE_PARAM].presence || SiteSetting.default_locale
 
     LocalizationAttributesReplacer.replace_topic_attributes(topic_view.topic, crawl_locale)

frontend/discourse/app/components/topic-localized-content-toggle.gjs

@@ -30,7 +30,19 @@ export default class TopicLocalizedContentToggle extends Component {
       });
     }
 
-    this.router.refresh();
+    const postStream = this.args.topic?.postStream;
+    if (postStream) {
+      const currentURL = this.router.currentURL;
+      // this is required to clear the post stream cache
+      // otherwise the old posts before the toggle will be shown
+      postStream.removeAllPosts();
+      await this.router.refresh();
+      // refreshing clears the post number,
+      // and postStream.refresh nearPost does not load to the correct post
+      if (this.router.currentURL !== currentURL) {
+        this.router.replaceWith(currentURL);
+      }
+    }
   }
 
   get title() {

frontend/discourse/app/models/post-stream.js

@@ -649,6 +649,11 @@ export default class PostStream extends RestModel {
     postIds.forEach((id) => delete identityMap[id]);
   }
 
+  removeAllPosts() {
+    const posts = Object.values(this._identityMap).filter((p) => p);
+    this.removePosts(posts);
+  }
+
   // Returns a post from the identity map if it's been inserted.
   findLoadedPost(id) {
     return this._identityMap[id];

spec/requests/topics_controller_spec.rb

@@ -3637,6 +3637,84 @@ def extract_post_stream
         end
       end
     end
+
+    describe "#posts with content localization" do
+      fab!(:localized_post) do
+        post = Fabricate(:post, user:, locale: "en", cooked: "<p>Original EN</p>")
+        Fabricate(:post_localization, post:, locale: "ja", cooked: "<p>Translated JA</p>")
+        post
+      end
+      fab!(:localized_topic) { localized_post.topic }
+      fab!(:localized_post2) do
+        post =
+          Fabricate(
+            :post,
+            user:,
+            topic: localized_topic,
+            locale: "ja",
+            cooked: "<p>Original 2 JA</p>",
+          )
+        Fabricate(:post_localization, post:, locale: "en", cooked: "<p>Translated 2 EN</p>")
+        post
+      end
+
+      before do
+        SiteSetting.content_localization_enabled = true
+        I18n.locale = "en"
+      end
+
+      context "when show_original cookie is not set" do
+        it "returns translated posts" do
+          get "/t/#{localized_topic.id}/posts.json"
+
+          expect(response.status).to eq(200)
+
+          body = response.parsed_body
+          posts = body["post_stream"]["posts"]
+
+          expect(posts.first["cooked"]).to eq("<p>Original EN</p>")
+          expect(posts.second["cooked"]).to eq("<p>Translated 2 EN</p>")
+        end
+
+        it "returns translated posts when loading specific post_ids" do
+          get "/t/#{localized_topic.id}/posts.json", params: { post_ids: [localized_post2.id] }
+
+          expect(response.status).to eq(200)
+
+          body = response.parsed_body
+          posts = body["post_stream"]["posts"]
+
+          expect(posts.first["cooked"]).to eq("<p>Translated 2 EN</p>")
+        end
+      end
+
+      context "when show_original cookie is set" do
+        before { cookies[ContentLocalization::SHOW_ORIGINAL_COOKIE] = "true" }
+
+        it "returns original posts" do
+          get "/t/#{localized_topic.id}/posts.json"
+
+          expect(response.status).to eq(200)
+
+          body = response.parsed_body
+          posts = body["post_stream"]["posts"]
+
+          expect(posts.first["cooked"]).to eq("<p>Original EN</p>")
+          expect(posts.second["cooked"]).to eq("<p>Original 2 JA</p>")
+        end
+
+        it "returns original posts when loading specific post_ids" do
+          get "/t/#{localized_topic.id}/posts.json", params: { post_ids: [localized_post2.id] }
+
+          expect(response.status).to eq(200)
+
+          body = response.parsed_body
+          posts = body["post_stream"]["posts"]
+
+          expect(posts.first["cooked"]).to eq("<p>Original 2 JA</p>")
+        end
+      end
+    end
   end
 
   describe "#feed" do

spec/system/content_localization_spec.rb

@@ -192,6 +192,63 @@
         expect(post_history_modal.previous_locale).to have_content("English (US)")
       end
     end
+
+    context "when loading 20+ posts in stream", trace: true do
+      before do
+        highest = topic.highest_post_number
+        22.times do |i|
+          post_number = i + highest + 1
+          post =
+            Fabricate(
+              :post,
+              topic: topic,
+              locale: "ja",
+              raw: "Japanese content for post #{post_number}",
+              cooked: "<p>日本語コンテンツ #{post_number}</p>",
+            )
+
+          Fabricate(
+            :post_localization,
+            post:,
+            locale: "en",
+            cooked: "<p>English translation #{post_number}</p>",
+          )
+        end
+      end
+
+      let(:post_21_obj) { PageObjects::Components::Post.new(21) }
+
+      it "respects the show_original toggle for posts loaded dynamically when scrolling (20+ posts)" do
+        sign_in(site_local_user)
+        visit("/")
+
+        topic_page.visit_topic(topic)
+
+        expect(post_3_obj.post).to have_content("A general is one who ..")
+        expect(topic_page).to have_post_content(post_number: 3, content: "A general is one who ..")
+
+        5.times do
+          break if page.has_css?("#post_21 .cooked", visible: :all, wait: 0)
+          page.execute_script("window.scrollTo(0, document.body.scrollHeight)")
+        end
+
+        expect(page).to have_css("#post_21")
+        expect(topic_page).to have_post_content(post_number: 21, content: "English translation 21")
+
+        page.find(TOGGLE_LOCALIZE_BUTTON_SELECTOR).click
+        expect(post_3_obj.post).to have_content("将とは、智・信・仁・勇・厳なり。")
+
+        5.times do
+          break if page.has_css?("#post_21 .cooked", visible: :all, wait: 0)
+          page.execute_script("window.scrollTo(0, document.body.scrollHeight)")
+        end
+
+        expect(post_21_obj.post).to have_content("日本語コンテンツ 21")
+
+        page.refresh
+        expect(post_3_obj.post).to have_content("将とは、智・信・仁・勇・厳なり。")
+      end
+    end
   end
 
   context "for site settings" do