Fixed group height being applied to group background panel as well

src/timeline/component/Group.js

@@ -176,7 +176,8 @@ Group.prototype.redraw = function(range, margin, restack) {
   resized = util.updateProperty(this.props.label, 'height', this.dom.inner.clientHeight) || resized;
 
   // apply new height
-  foreground.style.height  = height + 'px';
+  this.dom.background.style.height  = height + 'px';
+  this.dom.foreground.style.height  = height + 'px';
   this.dom.label.style.height = height + 'px';
 
   // update vertical position of items after they are re-stacked and the height of the group is calculated