DEV: Reduce the number of IntersectionObservers (#22933)

…by adding the `chat/track-message` modifier only to the last thread rather than all of them

(plus: don't use Ember's `lastObject`)

plugins/chat/assets/javascripts/discourse/components/chat/thread-list.hbs

@@ -15,13 +15,10 @@
       {{#each this.sortedThreads as |thread|}}
         <Chat::ThreadList::Item
           @thread={{thread}}
-          {{chat/track-message
-            (if
-              (eq thread this.sortedThreads.lastObject)
-              this.loadThreads
-              (fn (noop))
-            )
-          }}
+          {{(if
+            (eq thread this.lastThread)
+            (modifier "chat/track-message" this.loadThreads)
+          )}}
         />
       {{else}}
         {{#if this.threadsCollection.fetchedOnce}}

plugins/chat/assets/javascripts/discourse/components/chat/thread-list.js

@@ -57,6 +57,10 @@ export default class ChatThreadList extends Component {
       });
   }
 
+  get lastThread() {
+    return this.sortedThreads[this.sortedThreads.length - 1];
+  }
+
   get shouldRender() {
     return !!this.args.channel;
   }