fix(timeline): #2725 background items positioning when `orientation: top` (#2831)

* Fix background items when orientation top

lib/timeline/component/item/BackgroundItem.js

@@ -143,6 +143,7 @@ BackgroundItem.prototype.repositionX = RangeItem.prototype.repositionX;
  */
 BackgroundItem.prototype.repositionY = function(margin) {
   var height;
+  var orientation = this.options.orientation.item;
 
   // special positioning for subgroups
   if (this.data.subgroup !== undefined) {
@@ -154,7 +155,6 @@ BackgroundItem.prototype.repositionY = function(margin) {
 
     this.dom.box.style.height = this.parent.subgroups[itemSubgroup].height + 'px';
 
-    var orientation = this.options.orientation.item;
     if (orientation == 'top') { 
       this.dom.box.style.top = this.parent.top + this.parent.subgroups[itemSubgroup].top + 'px';
     } else {
@@ -170,8 +170,8 @@ BackgroundItem.prototype.repositionY = function(margin) {
       height = Math.max(this.parent.height,
           this.parent.itemSet.body.domProps.center.height,
           this.parent.itemSet.body.domProps.centerContainer.height);
+      this.dom.box.style.bottom = orientation == 'bottom' ? '0' : '';
       this.dom.box.style.top = orientation == 'top' ? '0' : '';
-      this.dom.box.style.bottom = orientation == 'top' ? '' : '0';
     }
     else {
       height = this.parent.height;