Hello Paul Brun,
We are aware of this issue and have logged it in our public issue tracking system. You can monitor its process via following link: http://www.telerik.com/support/pits.aspx#/public/silverlight/3737 .
Can you please tell us which was the last version you used where context appears correctly?
Alternatively you could set the background property of the tree view explicitly (for example - transparent).
Please let us know if you need more info or the provided workaround is not working for you.
the Telerik team
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