Index: src/wp-admin/js/nav-menu.js
===================================================================
--- src/wp-admin/js/nav-menu.js	(revision 34775)
+++ src/wp-admin/js/nav-menu.js	(working copy)
@@ -619,6 +619,11 @@
 			// Use the right edge if RTL.
 			menuEdge += api.isRTL ? api.menuList.width() : 0;
 
+			// Prevent infinite scrolling while dragging an item
+			api.menuList.on('scroll', function() {
+				$(this).scrollLeft(0);
+			});
+
 			api.menuList.sortable({
 				handle: '.menu-item-handle',
 				placeholder: 'sortable-placeholder',