I will post the answer from your support ticket also here in the forum.
"To achieve your requirement you can subscribe for the PreviewCollapsed event of the RadPanelBar control and handle it when all items are collapsing. Here is an example in code for this approach:
sender, Telerik.Windows.RadRoutedEventArgs e)
var panelBar = (RadPanelBar)sender;
var areAllItemsCollapsing = panelBar.Items.OfType<RadPanelBarItem>().All(x => !x.IsExpanded);
e.Handled = areAllItemsCollapsing;
I am afraid that, I can't suggest you a XAML-only approach.
Telerik by Progress
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