We raise the PopupButton.PopupClosed event right in the property changed handler for the IsPopupOpen property. I believe the Popup.IsOpen binding in the PopupButton's Template hasn't been dispatched by WPF yet (it would be DataBind dispatcher priority) at the time the event is raised.
If you do a Dispatcher.BeginInvoke call to show your MessageBox with DataBind priority from that event handler, it should work around the issue for now.
We updated our code to raise the PopupClosed event in a Dispatcher.BeginInvoke on our end with DataBind priority. That should fix it for the next maintenance release.
[Modified 11 days ago]
Invoking the dialog did fix the issue in my application. Thanks.
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