populate up filtered timelineSets vaguely correctly

lib/models/event-timeline-set.js

@@ -237,8 +237,8 @@ EventTimelineSet.prototype.addEventsToTimeline = function(events, toStartOfTimel
 
     if (!toStartOfTimeline && timeline == this._liveTimeline) {
         throw new Error(
-            "Room.addEventsToTimeline cannot be used for adding events to " +
-            "the live timeline - use EventTimelineSet.addLiveEvents instead"
+            "EventTimelineSet.addEventsToTimeline cannot be used for adding events to " +
+            "the live timeline - use Room.addLiveEvents instead"
         );
     }
 

lib/models/room.js

@@ -469,6 +469,33 @@ Room.prototype.getOrCreateFilteredTimelineSet = function(filter) {
     reEmit(this, timelineSet, [ "Room.timeline" ]);
     this._filteredTimelineSets[filter.filterId] = timelineSet;
     this._timelineSets.push(timelineSet);
+
+    // populate up the new timelineSet with filtered events from our live
+    // unfiltered timeline.
+    //
+    // XXX: This is risky as our timeline
+    // may have grown huge and so take a long time to filter.
+    // see https://github.com/vector-im/vector-web/issues/2109
+
+    var unfilteredLiveTimeline = this._timelineSets[0].getLiveTimeline();
+
+    unfilteredLiveTimeline.getEvents().forEach(function(event) {
+        timelineSet.addLiveEvent(event);
+    });
+
+    timelineSet.getLiveTimeline().setPaginationToken(
+        unfilteredLiveTimeline.getPaginationToken(EventTimeline.BACKWARDS),
+        EventTimeline.BACKWARDS
+    );
+
+    // alternatively, we could try to do something like this to try and re-paginate
+    // in the filtered events from nothing, but Mark says it's an abuse of the API
+    // to do so:
+    //
+    // timelineSet.resetLiveTimeline(
+    //      unfilteredLiveTimeline.getPaginationToken(EventTimeline.FORWARDS)
+    // );
+
     return timelineSet;
 };