FIX: ensures scrollable-list is passing scroller element (#36736)

Following idle pane refactoring we incorrectly were expecting the
scroller from the scrollable-list when we now expect the component
(chat-channel/chat-thread) to pass it down.

We have tests for this behavior generally but testing the exact
scrolling moment is generally brittle, I won't add another test for now.

plugins/chat/assets/javascripts/discourse/components/chat-channel.gjs

@@ -475,6 +475,7 @@ export default class ChatChannel extends Component {
 
       DatesSeparatorsPositioner.apply(this.scroller);
       this.paneState.updatePendingContentFromScrollState({
+        scroller: this.scroller,
         fetchedOnce: this.messagesLoader.fetchedOnce,
         canLoadMoreFuture: this.messagesLoader.canLoadMoreFuture,
         state,
@@ -506,6 +507,7 @@ export default class ChatChannel extends Component {
       this.chatChannelScrollPositions.delete(this.args.channel.id);
     } else {
       this.paneState.updatePendingContentFromScrollState({
+        scroller: this.scroller,
         fetchedOnce: this.messagesLoader.fetchedOnce,
         canLoadMoreFuture: this.messagesLoader.canLoadMoreFuture,
         state,

plugins/chat/assets/javascripts/discourse/components/chat-thread.gjs

@@ -133,6 +133,7 @@ export default class ChatThread extends Component {
 
       DatesSeparatorsPositioner.apply(this.scroller);
       this.paneState.updatePendingContentFromScrollState({
+        scroller: this.scroller,
         fetchedOnce: this.messagesLoader.fetchedOnce,
         canLoadMoreFuture: this.messagesLoader.canLoadMoreFuture,
         state,
@@ -163,6 +164,7 @@ export default class ChatThread extends Component {
       this.fetchMoreMessages({ direction: FUTURE });
     } else {
       this.paneState.updatePendingContentFromScrollState({
+        scroller: this.scroller,
         fetchedOnce: this.messagesLoader.fetchedOnce,
         canLoadMoreFuture: this.messagesLoader.canLoadMoreFuture,
         state,

plugins/chat/assets/javascripts/discourse/lib/chat-pane-state.js

@@ -173,11 +173,16 @@ export default class ChatPaneState {
    * @param {number} [options.distanceThresholdPixels]
    */
   updatePendingContentFromScrollState(options) {
-    const { fetchedOnce, canLoadMoreFuture, state, distanceThresholdPixels } =
-      options;
+    const {
+      scroller,
+      fetchedOnce,
+      canLoadMoreFuture,
+      state,
+      distanceThresholdPixels,
+    } = options;
 
     this.#updateHasPendingContentBelow({
-      scroller: state?.scroller,
+      scroller,
       fetchedOnce,
       canLoadMoreFuture,
       distanceToBottomPixels: state?.distanceToBottom?.pixels ?? 0,