This question has been asked several times and the answer is that this is not what RadNotification does. It facilitates a timed appearance of a popup and an easy way to perform a request that will renew the session, but it does not monitor the session, nor can it know of other tabs/windows:
What you can consider is to always keep the session alive in your tabs. You could do that without the notification. Or, you could call the ContinueSession() function when needed (e.g. when the notification has been shown, even without the user pressing the button) so that the session never ends automatically.
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