Render pin-to-bottom post live without a page reload

app/controllers/discourse_mod_categories/messages_controller.rb

@@ -87,6 +87,7 @@ def update_topic
                reply_prompt: topic.custom_fields[TOPIC_REPLY_PROMPT_FIELD].to_s,
                reply_prompt_max_tl: topic.custom_fields[TOPIC_REPLY_PROMPT_TL_FIELD],
                pinned_post_id: topic.custom_fields[TOPIC_PINNED_POST_FIELD],
+               pinned_post: DiscourseModCategories.serialized_pinned_post(topic),
                require_reply_approval: !!topic.custom_fields[TOPIC_REQUIRE_REPLY_APPROVAL_FIELD],
                private_note: topic.custom_fields[TOPIC_PRIVATE_NOTE_FIELD].to_s,
                private_note_position:

assets/javascripts/discourse/connectors/topic-area-bottom/topic-footer-message.gjs

@@ -40,6 +40,7 @@ export default class TopicFooterMessage extends Component {
 
   @tracked footerMessage = topicFooterMessage(this.topic);
   @tracked pinnedPostId = this.topic?.mod_topic_pinned_post_id || null;
+  @tracked pinnedPostPayload = this.topic?.mod_topic_pinned_post || null;
   @tracked cookedFooterMessage = null;
 
   constructor() {
@@ -84,6 +85,7 @@ export default class TopicFooterMessage extends Component {
   readTopicState(topic) {
     this.footerMessage = topicFooterMessage(topic);
     this.pinnedPostId = topic?.mod_topic_pinned_post_id || null;
+    this.pinnedPostPayload = topic?.mod_topic_pinned_post || null;
     this.cookFooterMessage();
   }
 
@@ -106,10 +108,18 @@ export default class TopicFooterMessage extends Component {
     return this.cookedFooterMessage;
   }
 
+  // Prefer the topic-attached payload (serialized server-side and returned
+  // by the pin endpoint) so the bottom copy renders immediately, even when
+  // the pinned post lives outside the currently-loaded post-stream window.
+  // The `postStream.posts` lookup is the historical fallback — kept so a
+  // stale topic-view that predates the new field still renders.
   get pinnedPost() {
     if (!this.pinnedPostId) {
       return null;
     }
+    if (this.pinnedPostPayload?.id === this.pinnedPostId) {
+      return this.pinnedPostPayload;
+    }
     return (
       this.topic?.postStream?.posts?.find((p) => p.id === this.pinnedPostId) ||
       null

assets/javascripts/discourse/initializers/mod-pin-post.js

@@ -43,6 +43,7 @@ export default {
                 }
               );
               topic?.set("mod_topic_pinned_post_id", result.pinned_post_id);
+              topic?.set("mod_topic_pinned_post", result.pinned_post || null);
               if (topic) {
                 appEvents.trigger("discourse-mod:messages-updated", topic);
                 // Re-render the stream so the in-stream pin badge appears

assets/stylesheets/topic-footer-message.scss

@@ -69,9 +69,11 @@
 // The pinned post renders as a regular post at the bottom of the topic
 // (right after the post stream), not as a footer banner.
 .topic-footer-pinned-post {
-  margin: 0;
-  padding: 1em 0;
-  border-top: 1px solid var(--primary-low);
+  margin: 2em 0 0;
+  padding: 1em 1.25em;
+  border: 1px solid var(--primary-low);
+  border-radius: var(--d-border-radius, 0.25em);
+  background: var(--primary-very-low);
 
   .pinned-post {
     display: flex;

spec/requests/mod_messages_spec.rb

@@ -99,6 +99,35 @@
       expect(topic.reload.custom_fields["mod_topic_pinned_post_id"]).to eq(first_post.id)
     end
 
+    it "returns the pinned post's render payload so the frontend can render without a reload" do
+      sign_in(moderator)
+
+      put "/discourse-mod-categories/topic/#{topic.id}.json",
+          params: {
+            pinned_post_id: first_post.id,
+          }
+
+      expect(response.status).to eq(200)
+      payload = response.parsed_body["pinned_post"]
+      expect(payload).to be_present
+      expect(payload["id"]).to eq(first_post.id)
+      expect(payload["post_number"]).to eq(first_post.post_number)
+      expect(payload["cooked"]).to eq(first_post.cooked)
+      expect(payload["username"]).to eq(first_post.user.username)
+      expect(payload["avatar_template"]).to eq(first_post.user.avatar_template)
+    end
+
+    it "returns a null pinned_post when unpinning" do
+      topic.custom_fields["mod_topic_pinned_post_id"] = first_post.id
+      topic.save_custom_fields(true)
+      sign_in(moderator)
+
+      put "/discourse-mod-categories/topic/#{topic.id}.json", params: { pinned_post_id: "" }
+
+      expect(response.status).to eq(200)
+      expect(response.parsed_body["pinned_post"]).to be_nil
+    end
+
     it "lets a moderator unpin by sending a blank pinned_post_id" do
       topic.custom_fields["mod_topic_pinned_post_id"] = first_post.id
       topic.save_custom_fields(true)

spec/requests/mod_serialization_spec.rb

@@ -29,6 +29,29 @@
     expect(json["mod_topic_pinned_post_id"]).to eq(post.id)
   end
 
+  it "exposes the pinned post's render payload alongside the id" do
+    topic.custom_fields["mod_topic_pinned_post_id"] = post.id
+    topic.save_custom_fields(true)
+
+    get "/t/#{topic.id}.json"
+
+    expect(response.status).to eq(200)
+    payload = response.parsed_body["mod_topic_pinned_post"]
+    expect(payload).to be_present
+    expect(payload["id"]).to eq(post.id)
+    expect(payload["post_number"]).to eq(post.post_number)
+    expect(payload["cooked"]).to eq(post.cooked)
+    expect(payload["username"]).to eq(post.user.username)
+    expect(payload["avatar_template"]).to eq(post.user.avatar_template)
+  end
+
+  it "returns a null mod_topic_pinned_post when no post is pinned" do
+    get "/t/#{topic.id}.json"
+
+    expect(response.status).to eq(200)
+    expect(response.parsed_body["mod_topic_pinned_post"]).to be_nil
+  end
+
   it "leaves the topic fields nil when nothing has been set" do
     get "/t/#{topic.id}.json"
 
@@ -37,6 +60,7 @@
     expect(json["mod_topic_footer_message"]).to be_nil
     expect(json["mod_topic_reply_prompt"]).to be_nil
     expect(json["mod_topic_pinned_post_id"]).to be_nil
+    expect(json["mod_topic_pinned_post"]).to be_nil
   end
 
   it "exposes the category new-topic prompt in the categories list" do

spec/system/moderator_messages_spec.rb

@@ -428,6 +428,15 @@ def visit_at(target)
 
       find(".mod-pin-post-to-bottom").click
       expect(page).to have_css(".topic-footer-pinned-post", wait: 10)
+      # The username and avatar in the bottom copy come from the server
+      # response payload (mod_topic_pinned_post), not from the loaded
+      # post-stream. Asserting they render confirms the live-render fix
+      # for the "footer empty until reload" bug.
+      expect(page).to have_css(
+        ".topic-footer-pinned-post .pinned-post-username",
+        text: post.user.username,
+      )
+      expect(page).to have_css(".topic-footer-pinned-post .pinned-post-avatar")
       shot("26_post_pinned_to_bottom")
 
       expect(topic.reload.custom_fields["mod_topic_pinned_post_id"]).to be_present

sub_plugins/mod_categories.rb

@@ -55,6 +55,29 @@ module ::DiscourseModCategories
   TOPIC_PROMPT_CHECKLIST_FIELD = "mod_topic_prompt_checklist"
   USER_TOPIC_CHECKLIST_FIELD = "mod_topic_checklist_accepted"
 
+  # Render data for the topic's pinned-to-bottom post, or nil when the topic
+  # has no pinned post (or the pinned post has been deleted out from under
+  # the custom field). Shared by the `:mod_topic_pinned_post` serializer and
+  # the `update_topic` controller response so a freshly-pinned post renders
+  # the bottom copy live, without a page reload, even when the post isn't in
+  # the currently-loaded post-stream window.
+  def self.serialized_pinned_post(topic)
+    return nil unless topic
+    id = topic.custom_fields[TOPIC_PINNED_POST_FIELD]
+    return nil if id.blank?
+    post = topic.posts.find_by(id: id.to_i)
+    return nil unless post
+    user = post.user
+    {
+      id: post.id,
+      post_number: post.post_number,
+      cooked: post.cooked,
+      username: user&.username,
+      name: user&.name,
+      avatar_template: user&.avatar_template,
+    }
+  end
+
   # The current checklist config, or nil when none is set. Shape:
   #   { "version" => Integer, "items" => [{ "label" =>, "url" => }],
   #     "updated_at" => ISO8601 String }
@@ -368,6 +391,13 @@ class Engine < ::Rails::Engine
   add_to_serializer(:topic_view, :mod_topic_pinned_post_id) do
     object.topic.custom_fields[DiscourseModCategories::TOPIC_PINNED_POST_FIELD]
   end
+  # The pinned post's render data, attached to the topic so the bottom-copy
+  # connector renders without needing the post to be in the currently-loaded
+  # `postStream.posts` window — pinning a post far above the current scroll
+  # position would otherwise leave the footer blank until reload.
+  add_to_serializer(:topic_view, :mod_topic_pinned_post) do
+    DiscourseModCategories.serialized_pinned_post(object.topic)
+  end
   add_to_serializer(:topic_view, :mod_topic_require_reply_approval) do
     !!object.topic.custom_fields[DiscourseModCategories::TOPIC_REQUIRE_REPLY_APPROVAL_FIELD]
   end