Increase performance when receiving messages (#5375)

webapp/components/post_view/post_view_cache.jsx

@@ -33,7 +33,7 @@ export default class PostViewCache extends React.Component {
 
     componentWillUnmount() {
         if (UserStore.getCurrentUser()) {
-            AsyncClient.viewChannel('');
+            AsyncClient.viewChannel('', this.state.currentChannelId || '');
         }
         ChannelStore.removeChangeListener(this.onChannelChange);
     }