I understood from the connection newsletter that there are two folders in your Windows profile for which you need to clear their contents in order for the Visual Studio designer to work with updating the version of Visual Studio 2013 3.
Two folders:
For Blend: AppData \ Local \ Microsoft \ VisualStudio \ 12.0 \ Blend \ ShadowCache
For Visual Studio 2013: AppData \ Local \ Microsoft \ VisualStudio \ 12.0 \ Designer \ ShadowCache
Do not delete these folders, just delete the contents of these folders, and you will return to work with the XAML designer.
Luke loescher
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