diff --git a/src/UserActivity.js b/src/UserActivity.js
index 145b23e36e..b49bcf1a91 100644
--- a/src/UserActivity.js
+++ b/src/UserActivity.js
@@ -1,5 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2019 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -33,7 +34,7 @@ class UserActivity {
         this._attachedTimers = [];
         this._activityTimeout = new Timer(CURRENTLY_ACTIVE_THRESHOLD_MS);
         this._onUserActivity = this._onUserActivity.bind(this);
-        this._onDocumentBlurred = this._onDocumentBlurred.bind(this);
+        this._onWindowBlurred = this._onWindowBlurred.bind(this);
         this._onPageVisibilityChanged = this._onPageVisibilityChanged.bind(this);
         this.lastScreenX = 0;
         this.lastScreenY = 0;
@@ -74,8 +75,8 @@ class UserActivity {
         document.onmousemove = this._onUserActivity;
         document.onkeydown = this._onUserActivity;
         document.addEventListener("visibilitychange", this._onPageVisibilityChanged);
-        document.addEventListener("blur", this._onDocumentBlurred);
-        document.addEventListener("focus", this._onUserActivity);
+        window.addEventListener("blur", this._onWindowBlurred);
+        window.addEventListener("focus", this._onUserActivity);
         // can't use document.scroll here because that's only the document
         // itself being scrolled. Need to use addEventListener's useCapture.
         // also this needs to be the wheel event, not scroll, as scroll is
@@ -116,7 +117,7 @@ class UserActivity {
         }
     }
 
-    _onDocumentBlurred() {
+    _onWindowBlurred() {
         this._activityTimeout.abort();
     }