Fixes #2261 Bugs with mousewheel events (#2262)

lib/timeline/Core.js

@@ -207,14 +207,14 @@ Core.prototype._create = function (container) {
     event.preventDefault();
   }
 
-  if (this.dom.center.addEventListener) {
+  if (this.dom.centerContainer.addEventListener) {
     // IE9, Chrome, Safari, Opera
-    this.dom.center.addEventListener("mousewheel", onMouseWheel.bind(this), false);
+    this.dom.centerContainer.addEventListener("mousewheel", onMouseWheel.bind(this), false);
     // Firefox
-    this.dom.center.addEventListener("DOMMouseScroll", onMouseWheel.bind(this), false);
+    this.dom.centerContainer.addEventListener("DOMMouseScroll", onMouseWheel.bind(this), false);
   } else {
     // IE 6/7/8
-    this.dom.center.attachEvent("onmousewheel", onMouseWheel.bind(this));
+    this.dom.centerContainer.attachEvent("onmousewheel", onMouseWheel.bind(this));
   }
 
   function onMouseScrollSide(event) {