The BackKeyPress event is exposed by the Page class which is not part of our framework and we cannot control the behavior of this event, i.e. we cannot prevent it from firing.
I believe that you are subscribing for the BackKeyPress event before we do and therefore you get the args.Cancel value as false. Probably it be useful for you to know that we handle this event as soon as the GroupPicker popup is opened and therefore you might try to subscribe for it after we do so that you can get the args.Cancel == true value.
I hope this helps.
All the best,
the Telerik team
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