Add canResetTimeline callback and thread it through to TimelinePanel

src/components/structures/LoggedInView.js

@@ -81,6 +81,13 @@ export default React.createClass({
         return this._scrollStateMap[roomId];
     },
 
+    canResetTimelineInRoom: function(roomId) {
+        if (!this.refs.roomView) {
+            return true;
+        }
+        return this.refs.roomView.canResetTimeline();
+    },
+
     _onKeyDown: function(ev) {
             /*
             // Remove this for now as ctrl+alt = alt-gr so this breaks keyboards which rely on alt-gr for numbers

src/components/structures/MatrixChat.js

@@ -806,9 +806,31 @@ module.exports = React.createClass({
      * (useful for setting listeners)
      */
     _onWillStartClient() {
+        var self = this;
         var cli = MatrixClientPeg.get();
 
-        var self = this;
+        // Allow the JS SDK to reap timeline events. This reduces the amount of
+        // memory consumed as the JS SDK stores multiple distinct copies of room
+        // state (each of which can be 10s of MBs) for each DISJOINT timeline. This is
+        // particularly noticeable when there are lots of 'limited' /sync responses
+        // such as when laptops unsleep.
+        // https://github.com/vector-im/riot-web/issues/3307#issuecomment-282895568
+        cli.setCanResetTimelineCallback(function(roomId) {
+            console.log("Request to reset timeline in room ", roomId, " viewing:", self.state.currentRoomId);
+            if (roomId !== self.state.currentRoomId) {
+                // It is safe to remove events from rooms we are not viewing.
+                return true;
+            }
+            // We are viewing the room which we want to reset. It is only safe to do
+            // this if we are not scrolled up in the view. To find out, delegate to
+            // the timeline panel. If the timeline panel doesn't exist, then we assume
+            // it is safe to reset the timeline.
+            if (!self.refs.loggedInView) {
+                return true;
+            }
+            return self.refs.loggedInView.canResetTimelineInRoom(roomId);
+        });
+
         cli.on('sync', function(state, prevState) {
             self.updateStatusIndicator(state, prevState);
             if (state === "SYNCING" && prevState === "SYNCING") {

src/components/structures/RoomView.js

@@ -490,6 +490,13 @@ module.exports = React.createClass({
         }
     },
 
+    canResetTimeline: function() {
+        if (!this.refs.messagePanel) {
+            return true;
+        }
+        return this.refs.messagePanel.canResetTimeline();
+    },
+
     // called when state.room is first initialised (either at initial load,
     // after a successful peek, or after we join the room).
     _onRoomLoaded: function(room) {

src/components/structures/TimelinePanel.js

@@ -431,6 +431,10 @@ var TimelinePanel = React.createClass({
         }
     },
 
+    canResetTimeline: function() {
+        return this.refs.messagePanel && this.refs.messagePanel.isAtBottom();
+    },
+
     onRoomRedaction: function(ev, room) {
         if (this.unmounted) return;