RadTransitionControl makes an extensive use of shaders for its transitions. XBAP's security policy does not alow the use of shaders. I believe the reason is they use hardware support and video card. You can not use anything like transparent popups and shaders (including DropShadowEffects) in XBAP so we fallback to more generic transition that use CLR animations. This means in XBAP we can only animate common properties like RenderTransform, Opacity etc. With the lack of Silverlight's perspective transformation we are pretty much stuck to the Fade effect.
If you want to use such transitions consider click-once application instead of XBAP.
I am really sorry for the inconvenience.
the Telerik team
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