Hi Alan,
We have customized the ChromelessWindow style in our samples and we have removed the triggers in customized style which results in cause for your issue.
We have modified one of our sample of Chromeless window and achieved your requirement. In the sample, we have added triggers for the property 'ResizeMode' when the value is 'Noresize' is the change made in the sample to achieve this behavior.
Kindly download the modified sample.
Please let us know if case of any queries.
Regards,
Dinesh Kumar P
Chromeless window610583507_6bdb0208.zip