Fix #14379: AJAX requests on unload

(cherry picked from commit f9d41ac6)

Conflicts:

	src/ajax/xhr.js

test/data/ajax/onunload.html

+<!doctype html>
+<html>
+<head>
+	<meta http-equiv="Content-type" content="text/html; charset=utf-8"/>
+	<title>onunload ajax requests (#14379)</title>
+	<script src="../../jquery.js"></script>
+</head>
+<body>
+	<form id="navigate" action="about:blank"></form>
+	<script>
+		jQuery( window ).on( "unload", function() {
+			var ajaxStatus;
+			jQuery.ajax({
+				url: "../name.html",
+				async: false,
+				complete: function( xhr, status ) {
+					ajaxStatus = status;
+				}
+			});
+			parent.iframeCallback( ajaxStatus );
+		});
+
+		jQuery(function() {
+			setTimeout(function() {
+				document.getElementById( "navigate" ).submit();
+			}, 0 );
+		});
+	</script>
+</body>
+</html>

test/unit/ajax.js

@@ -1565,6 +1565,11 @@ module( "ajax", {
 		}
 	} );
 
+	testIframeWithCallback( "#14379 - jQuery.ajax() on unload", "ajax/onunload.html", function( status ) {
+		expect( 1 );
+		strictEqual( status, "success", "Request completed" );
+	});
+
 //----------- jQuery.ajaxPrefilter()
 
 	ajaxTest( "jQuery.ajaxPrefilter() - abort", 1, {