As SnOrfus says, apparently this can create some conflict when you start doing hard coding, although this is unusual for what happens to you, since I can work on creating tab controls, tabs, and manipulating through the design view.
My recommendation would be 1. Verify that a later service pack for Visual Studio is installed. (Check for updates for VS2008) 2. - Start your design with a fresh one, working only on the design. 3. Try using "Wrappanel" or "stackPanel" instead of the canvas or grid inside the tabitem (which will help you with the layout).
If you are using a WrapPanel, you may need to view the scrolling and wrapping behavior of its contents.
Hope this helps.
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