It appears there may be an issue or design flaw within Kendo UI that needs attention.
As a temporary fix, you can try replacing the kendoWindow widget and modifying the "windowObject" function to also accommodate your customized window subclasses:
function windowObject(element, name) {
var contentElement = element.children(KWINDOWCONTENT);
return contentElement.data("kendoWindow") || contentElement.data("kendoCustomWindow") || contentElement.data("kendo" + name);
}
Here is an example of the fixed code: http://jsbin.com/OfIHOm/1/edit
Update:
In the latest update, Q2 2013 SP1 (version 2013.2.918), the code from the private function windowObject has been relocated to the method _object.
This allows for subclassing of kendoWindow just like any other widget, but it is still recommended to modify kendoWindow's _object method as follows:
/**
* Modify kendoWindow's _object method to include our new widget
*/
ui.Window.fn._object = function (element) {
var content = element.children(KWINDOWCONTENT);
return content.data("kendoWindow") || content.data("kendoCustomWindow") || content.data("kendo" + this.options.name);
};
For the updated version, refer to: http://jsfiddle.net/lhoeppner/qj2HL/