I thought I'd share some javascript code that will automatically deselect a click on a link that has target="_blank" since it is a popup window. Put in OnClientItemClicked="RadPanelBarClicked" your RadPanelBar declaration.
function RadPanelBarClicked(sender, eventArgs) {
// Dont' select links with target.
var item = eventArgs.get_item();
var target = item.get_target();
if (target)
{
item.set_selected(
false);
}
}