Thank you for writing.
Currently our controls support only basic design-time support by providing simple metadata to the designer run-time. This is why you will not see more advanced editors for the controls than already provided by the run-time.
The case of RadWindow is different than all the other controls. RadWindow is intended to occupy zero space in the container that it is inserted in. At run-time RadWindow dynamically inserts a Canvas panel which displays the content of the window. Therefore what you are seeing during design-time, is not the Canvas panel, but RadWindow itself and you are inserting the controls in RadWindow, not the Canvas. It turned out that there is no way to delegate the creation of objects to the canvas panel in Expression Blend.
Currently you can design how your controls will look like in the window by placing them in an appropriate panel. After you do this you can simply cut the generated XAML and paste it as the value of RadWindow's Content property.
Please write again if you have more questions.
the Telerik team
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