Fix issue with thread panel not updating when it loads on first render (#8382)

src/components/structures/ThreadPanel.tsx

@@ -198,9 +198,10 @@ const ThreadPanel: React.FC<IProps> = ({
     useEffect(() => {
         const room = mxClient.getRoom(roomId);
         room.createThreadsTimelineSets().then(() => {
-            setRoom(room);
+            return room.fetchRoomThreads();
+        }).then(() => {
             setFilterOption(ThreadFilterType.All);
-            room.fetchRoomThreads();
+            setRoom(room);
         });
     }, [mxClient, roomId]);
 
@@ -286,8 +287,8 @@ const ThreadPanel: React.FC<IProps> = ({
                     sensor={card.current}
                     onMeasurement={setNarrow}
                 />
-                { timelineSet && (
-                    <TimelinePanel
+                { timelineSet
+                    ? <TimelinePanel
                         key={timelineSet.getFilter()?.filterId ?? (roomId + ":" + filterOption)}
                         ref={timelinePanel}
                         showReadReceipts={false} // No RR support in thread's MVP
@@ -311,7 +312,8 @@ const ThreadPanel: React.FC<IProps> = ({
                         permalinkCreator={permalinkCreator}
                         disableGrouping={true}
                     />
-                ) }
+                    : <div className="mx_AutoHideScrollbar" />
+                }
             </BaseCard>
         </RoomContext.Provider>
     );

src/components/structures/TimelinePanel.tsx

@@ -1188,11 +1188,8 @@ class TimelinePanel extends React.Component<IProps, IState> {
         const onLoaded = () => {
             if (this.unmounted) return;
 
-            // clear the timeline min-height when
-            // (re)loading the timeline
-            if (this.messagePanel.current) {
-                this.messagePanel.current.onTimelineReset();
-            }
+            // clear the timeline min-height when (re)loading the timeline
+            this.messagePanel.current?.onTimelineReset();
             this.reloadEvents();
 
             // If we switched away from the room while there were pending