diff --git a/static/js/notifications.js b/static/js/notifications.js
index c84af7671d..03f6f69e04 100644
--- a/static/js/notifications.js
+++ b/static/js/notifications.js
@@ -496,6 +496,7 @@ exports.received_messages = function (messages) {
             return;
         }
         if (!unread.message_unread(message)) {
+            // The message is already read; Zulip is currently in focus.
             return;
         }