Merge pull request #1354 from matrix-org/dbkr/fix_super_slow_room_change

Fix room change sometimes being very slow

src/components/structures/TimelinePanel.js

@@ -344,9 +344,16 @@ var TimelinePanel = React.createClass({
                 newState[canPaginateOtherWayKey] = true;
             }
 
-            this.setState(newState);
-
-            return r;
+            // Don't resolve until the setState has completed: we need to let
+            // the component update before we consider the pagination completed,
+            // otherwise we'll end up paginating in all the history the js-sdk
+            // has in memory because we never gave the component a chance to scroll
+            // itself into the right place
+            return new Promise((resolve) => {
+                this.setState(newState, () => {
+                    resolve(r);
+                });
+            });
         });
     },