Edit in JSFiddle

Ext.window.Window.override({
    animateTarget: Ext.getDoc(),
    maximize: function(){
      this.callParent([true]);
    },
    restore:function(){
      this.callParent([true]);
    }
});

Ext.define('MyWindow', {
    extend: 'Ext.window.Window',

    height: 250,
    width: 400,
    title: 'My Window',
    maximizable: true
});

Ext.onReady(function(){
   var w = new MyWindow();
   w.show();
});