Ok background areas seem to work fine now

lib/timeline/component/Group.js

@@ -1,7 +1,6 @@
 var util = require('../../util');
 var stack = require('../Stack');
 var RangeItem = require('./item/RangeItem');
-var DateUtil = require('../DateUtil');
 
 /**
  * @constructor Group

lib/timeline/component/ItemSet.js

@@ -578,15 +578,15 @@ ItemSet.prototype._updateUngrouped = function() {
       ungrouped.hide();
       delete this.groups[UNGROUPED];
 
-//      var background = this.groups[BACKGROUND];
-//      for (itemId in this.items) {
-//        if (this.items.hasOwnProperty(itemId)) {
-//          item = this.items[itemId];
-//          if ((item instanceof BackgroundItem)) {
-//            background.add(item);
-//          }
-//        }
-//      }
+      for (itemId in this.items) {
+        if (this.items.hasOwnProperty(itemId)) {
+          item = this.items[itemId];
+          item.parent && item.parent.remove(item);
+          var groupId = this._getGroupId(item.data);
+          var group = this.groups[groupId];
+          group && group.add(item) || item.hide();
+        }
+      }
     }
   }
   else {
@@ -777,7 +777,7 @@ ItemSet.prototype._getType = function (itemData) {
 ItemSet.prototype._getGroupId = function (itemData) {
   var type = this._getType(itemData);
   if (type == 'background') {
-    return itemData.group != undefined ? itemData.group : BACKGROUND;
+   return this.groupsData && itemData.group != undefined ? itemData.group : BACKGROUND;
   }
   else {
     return this.groupsData ? itemData.group : UNGROUPED;

lib/timeline/component/item/BackgroundItem.js

@@ -1,5 +1,6 @@
 var Hammer = require('../../../module/hammer');
 var Item = require('./Item');
+var BackgroundGroup = require('../BackgroundGroup');
 var RangeItem = require('./RangeItem');
 
 /**
@@ -185,18 +186,18 @@ BackgroundItem.prototype.repositionY = function(margin) {
   // and in the case of no subgroups:
   else {
     // we want backgrounds with groups to only show in groups.
-    if (this.data.group !== undefined) {
-      height = this.parent.height;
-      // same alignment for items when orientation is top or bottom
-      this.dom.box.style.top = this.parent.top + 'px';
-      this.dom.box.style.bottom = '';
-    }
-    else {
+    if (this.parent instanceof BackgroundGroup) {
       // if the item is not in a group:
       height = Math.max(this.parent.height, this.parent.itemSet.body.domProps.centerContainer.height);
       this.dom.box.style.top = onTop ? '0' : '';
       this.dom.box.style.bottom = onTop ? '' : '0';
     }
+    else {
+      height = this.parent.height;
+      // same alignment for items when orientation is top or bottom
+      this.dom.box.style.top = this.parent.top + 'px';
+      this.dom.box.style.bottom = '';
+    }
   }
   this.dom.box.style.height = height + 'px';
 };