//
// OnUpdateOverItem: This method is a listener for the UpdateOverItemEvent. When it is received,
// it means that the sender is the closest UIElementItem to the mouse (closest in the sense of the
// tree, not geographically).
static void OnUpdateOverItem(object sender, RoutedEventArgs args)
{
// Mark this object as the tree view item over which the mouse
// is currently positioned.
_currentItem = sender as UIElement;
// Tell that item to re-calculate the IsMouseDirectlyOverItem property
_currentItem.InvalidateProperty(IsMouseDirectlyOverItemProperty);
// Prevent this event from notifying other tree view items higher in the tree.
args.Handled = true;
}