Index: src/wp-admin/js/post.js
===================================================================
--- src/wp-admin/js/post.js	(revision 38162)
+++ src/wp-admin/js/post.js	(working copy)
@@ -407,17 +407,25 @@
 			return;
 		}
 
-		$.ajax({
-			type: 'POST',
-			url: ajaxurl,
-			async: false,
-			data: {
-				action: 'wp-remove-post-lock',
-				_wpnonce: $('#_wpnonce').val(),
-				post_ID: $('#post_ID').val(),
-				active_post_lock: $('#active_post_lock').val()
-			}
+		var formData = new FormData();
+		var data = {
+			action: 'wp-remove-post-lock',
+			_wpnonce: $('#_wpnonce').val(),
+			post_ID: $('#post_ID').val(),
+			active_post_lock: $('#active_post_lock').val()
+		};
+
+		$.each(data, function(key, value) {
+			formData.append(key, value);
 		});
+
+		if (!navigator.sendBeacon || !navigator.sendBeacon(ajaxurl, formData)) {
+			$.post({
+				async: false,
+				data: data,
+				url: ajaxurl
+			});
+		}
 	});
 
 	// multi-taxonomies