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Actually this is a known problem with Silverlight controls. The MouseOver value is not synchronized to the mouse position when the control become visible.
To visualize the problem you can try the attached example. In WPF everything is OK but in Silverlight its pretty good visible. All the controls (RadButton, standard SL Button and TextBox) in the test application behave in this manner. Please note we are setting the focus in code behind just to facilitate your test.
Of course if you find a way to fight the issue we will highly appreciate to get a sample.
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