Tests: Make the beforeunload event tests work regardless of extensions

Some browser extensions, like React DevTools, send messages to the content area.
Since our beforeunload event test listens for all messages, it used to catch
those as well, failing the test.

Add a `source` field to the payload JSON and check for it before treating the
message as coming from our own test to make sure the test passes even with such
browser extensions installed.

Closes gh-5478

2 files changed, 20 insertions, 10 deletions

diff --git a/test/data/event/onbeforeunload.html b/test/data/event/onbeforeunload.html
index 11ad1964a..30053967a 100644
--- a/test/data/event/onbeforeunload.html
+++ b/test/data/event/onbeforeunload.html
@@ -4,17 +4,18 @@
 	<script>
 		function report( event ) {
 			var payload = {
+				source: "jQuery onbeforeunload iframe test",
 				event: event.type
 			};
-			return parent.postMessage( JSON.stringify(payload), "*" );
+			return parent.postMessage( JSON.stringify( payload ), "*" );
 		}
 
 		jQuery( window ).on( "beforeunload", function( event ) {
 			report( event );
-		}).on( "load", function( event ) {
-			setTimeout(function() {
+		} ).on( "load", function( event ) {
+			setTimeout( function() {
 				window.location.reload();
-			}, 50);
-		});
+			}, 50 );
+		} );
 	</script>
 </html>
diff --git a/test/unit/event.js b/test/unit/event.js
index 38a805b37..ac309d22d 100644
--- a/test/unit/event.js
+++ b/test/unit/event.js
@@ -1448,13 +1448,22 @@ QUnit[ /(ipad|iphone|ipod)/i.test( navigator.userAgent ) ? "skip" : "test" ](
 	var done = assert.async();
 
 	window.onmessage = function( event ) {
-		var payload = JSON.parse( event.data );
+		try {
+			var payload = JSON.parse( event.data );
 
-		assert.ok( payload.event, "beforeunload", "beforeunload event" );
+			// Ignore unrelated messages
+			if ( payload.source === "jQuery onbeforeunload iframe test" ) {
+				assert.ok( payload.event, "beforeunload", "beforeunload event" );
 
-		iframe.remove();
-		window.onmessage = null;
-		done();
+				iframe.remove();
+				window.onmessage = null;
+				done();
+			}
+		} catch ( e ) {
+
+			// Messages may come from other sources, like browser extensions;
+			// some may not be valid JSONs and thus cannot be `JSON.parse`d.
+		}
 	};
 
 	iframe.appendTo( "#qunit-fixture" );