Fix blank timeline when thread root is UTD

Author: germain-gg

src/components/structures/TimelinePanel.tsx

@@ -1637,7 +1637,9 @@ class TimelinePanel extends React.Component<IProps, IState> {
         let i = events.length - 1;
         let userMembership = "leave";
         for (; i >= 0; i--) {
-            const timeline = room.getTimelineForEvent(events[i].getId());
+            const timeline = isThreadTimeline
+                ? room.getTimelineForEvent(events[i].getId())
+                : room.getUnfilteredTimelineSet().getTimelineForEvent(events[i].getId()!);
             if (!timeline) {
                 // Somehow, it seems to be possible for live events to not have
                 // a timeline, even though that should not happen. :(