The IsTransitioning is set to true during the content swap something like:
IsTransitioning = true;
Content = newContent;
IsTransitioning = false;
So it does not work in your scenario.
However the scenario you describe sounds reasonable enough so we will consider a feature that would expose IsAnimating property that will behave the way you have described.
For the moment you will have to create a wrapper control around the RadTransitionControl that would expose such property or implement it different logic in the view model. The Animation duration is known prior to content change so when the IsTransitioning get set to true you can disable the buttons, start a timer and re-enable the buttons when the timer runs out. This would be quite tricky though.
the Telerik team
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