The property FontWeight of the RadPanelBarItem affects its child elements and in Silverlight you cannot forbid this behavior with Interaction triggers. A possible workaround could be the use of the MouseMove event of the RadPanelBar. You can iterate through the children of the hovered RadPanelBarItem and reset their Fontweight.
sender, MouseEventArgs e)
RadPanelBarItem hoveredItem = (e.OriginalSource
RadPanelBarItem childItem = item
childItem.FontWeight = FontWeights.Normal;
I hope this helps you. Feel free to ask if you need more info.
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