feat: make it easier to add content after the first post (#4050)

framework/core/js/src/forum/components/PostStream.js

@@ -78,11 +78,24 @@ export default class PostStream extends Component {
         content = <PostLoading />;
       }
 
-      return (
+      const postStreamElement = (
         <div className="PostStream-item" {...attrs}>
           {content}
         </div>
       );
+
+      // If we're on the first post, call the afterFirstPostItems method and add any additional elements.
+      if (i === 0 && this.afterFirstPostItems().toArray().length > 0) {
+        // Using m.fragment to return multiple elements without an enclosing container
+        return m.fragment({ ...attrs }, [
+          postStreamElement,
+          <div className="PostStream-item PostStream-afterFirstPost" key="afterFirstPost">
+            {this.afterFirstPostItems().toArray()}
+          </div>,
+        ]);
+      }
+
+      return postStreamElement;
     });
 
     if (!viewingEnd && posts[this.stream.visibleEnd - this.stream.visibleStart - 1]) {
@@ -117,6 +130,15 @@ export default class PostStream extends Component {
     );
   }
 
+  /**
+   * @returns {ItemList<import('mithril').Children>}
+   */
+  afterFirstPostItems() {
+    const items = new ItemList();
+
+    return items;
+  }
+
   /**
    * @returns {ItemList<import('mithril').Children>}
    */