It is presently impossible to subclass without providing a full replacement of the static 'create' method, because it hard codes

The solution is to replace the class with 'this':

Ext.extend(f, this);
This permits a subclass to provide an augmented implementation of the create method such as:

Code: = function (config, o) {
    var f =, o);
    Ext.apply(f.prototype, config, {
		foo : 1,
		bar: false
    return f;