Fixes background items with no groups (#2241)

lib/timeline/component/Group.js

@@ -502,8 +502,7 @@ Group.prototype.order = function() {
 Group.prototype._updateVisibleItems = function(orderedItems, oldVisibleItems, range) {
   var visibleItems = [];
   var visibleItemsLookup = {}; // we keep this to quickly look up if an item already exists in the list without using indexOf on visibleItems
-   
-  if (!this.isVisible) {
+  if (!this.isVisible && this.groupId != "__background__") {
     for (var i = 0; i < oldVisibleItems.length; i++) {
       var item = oldVisibleItems[i];
       if (item.displayed) item.hide();
@@ -557,7 +556,6 @@ Group.prototype._updateVisibleItems = function(orderedItems, oldVisibleItems, ra
     });
   }
 
-
   // finally, we reposition all the visible items.
   for (var i = 0; i < visibleItems.length; i++) {
     var item = visibleItems[i];