refactor: update appJS.setTimeoutFocusDependent()

assets/JsHelper/JsHelper.js

@@ -993,8 +993,7 @@ appJS.systemMsg = {
 /**
  * Set a timeout but only execute it when browser tab has focused
  *
- * NOTE: as is this cannot coexist together with other `window.onblur` and `window.onfocus` handlers.
- * Tried using `window.addEventListener('blur')` but it didn't work, which is confirmed by {@link https://stackoverflow.com/questions/11955774/javascript-window-addeventlistenerblur-wont-work Javascript, window.addEventListener('blur') won't work}
+ * Can use `document.hasFocus()` at any time to check current status.
  *
  * @param {callable} callback : Function to call on timeout
  * @param {integer} duration : Milliseconds after which the earliest possible call to the function should be.
@@ -1004,22 +1003,20 @@ appJS.setTimeoutFocusDependent = function(callback, duration) {
 	var hasFocus = true;
 	var skippedDueToNotFocused = false;
 
-	if (window.onblur || window.onfocus) {
-		alert('Cant set focus-dependent timeout because another onblur or onfocus handler has already been set.');
-	}
-
-	window.onblur = function() {
+	var blurHandler = function() {
 		hasFocus = false;
 	};
-	window.onfocus = function() {
+	var focusHandler = function() {
 		if (skippedDueToNotFocused) {
 			callback();
-			window.onblur = null;
-			window.onfocus = null;
+			window.removeEventListener('blur', blurHandler);
+			window.removeEventListener('focus', focusHandler);
 			return;
 		}
 		hasFocus = true;
 	};
+	window.addEventListener('blur', blurHandler);
+	window.addEventListener('focus', focusHandler);
 
 	setTimeout(function() {
 		if (hasFocus) {