OO-3759: double check the opener is on the same location

Check if the opener is null or not in the same location as the OpenOLAT
location (or catch the exception if we are not allowed to read the
location of the opener)

src/main/java/org/olat/core/gui/control/winmgr/_content/serverpart.html

 ## Set height to 0 to fix IE rendering issue (div would consume some visible space)
 <div style="height:0px">
-<script type="text/javascript">
+<script>
 /* <![CDATA[ */ 
 var stopped = true;
 function initPolling () {
@@ -30,11 +30,29 @@ var timestampLastPoll = new Date().getTime();
 // set timestamp cookie to inform other windows that they are outdated
 var sbtimestamp = new Date().getTime();
 var sbcookie = 'OLAT-UI-TIMESTAMP';
-if (window.opener == null) document.cookie = sbcookie+'='+sbtimestamp+'; path=/; SameSite=strict';
+if (thickCheckStandBy()) {
+	document.cookie = sbcookie+'='+sbtimestamp+'; path=/; SameSite=strict';
+}
 
 ## starts an interval which checks every second whether to send an poll request based on
 ## the pollperiod or not 10 min after the last click the poll process stops
 
+function thickCheckStandBy() {
+	var check = false;
+	try {
+		if (window.opener == null) {
+			check = true;
+		## if the opener is an external location, it will produce an exception
+		} else if(window.opener.location.hostname != document.location.hostname) {
+			check = true;
+		}
+	} catch(e) {
+		if(window.console) console.log(e);
+		check = true;
+	}
+	return check;
+}
+
 function tick () {
 	o_info.poller = jQuery.periodic({period: $pollperiod, decay:1.005, max_period: Number.MAX_VALUE}, function() {
 		if ( !o_info.linkbusy && (this.period > 1000) && (pollError < 2) ) {
@@ -65,7 +83,7 @@ function tick () {
 		// if window is not a popup window, become a standby window 
 		// in case the cookie timestamp is newer than the own one set while loading
 		// because another window holds the current UI which works
-		if (window.opener == null) {
+		if (thickCheckStandBy()) {
 			var p = sbcookie + "=";
 			var ca = document.cookie.split(';');
 			for(var i=0;i < ca.length;i++) {