Avoid rerendering during Room unmount

might speed up room changing by a few milliseconds

src/components/views/rooms/ReadReceiptMarker.js

@@ -53,6 +53,11 @@ module.exports = React.createClass({
         // this room
         readReceiptInfo: React.PropTypes.object,
 
+        // A function which is used to check if the parent panel is being
+        // unmounted, to avoid unnecessary work. Should return true if we
+        // are being unmounted.
+        checkUnmounting: React.PropTypes.func,
+
         // callback for clicks on this RR
         onClick: React.PropTypes.func,
     },
@@ -81,6 +86,13 @@ module.exports = React.createClass({
             return;
         }
 
+        // checking the DOM properties can force a re-layout, which can be
+        // quite expensive; so if the parent messagepanel is being unmounted,
+        // then don't bother with this.
+        if (this.props.checkUnmounting && this.props.checkUnmounting()) {
+            return;
+        }
+
         var avatarNode = ReactDOM.findDOMNode(this);
         rrInfo.top = avatarNode.offsetTop;
         rrInfo.left = avatarNode.offsetLeft;