As far as understand you, when a new item arrives, say X, it is placed in a group depending on its AreaName property. Then when it moves to area b, you are creating a new item Xb and you are adding it to the Timeline ItemsSource. And when X moves again you create Xc and again add it to ItemsSource.
In order to be able select the item X across all groups (areas in your scenario), you need to know that Xa, Xb and Xc all represent the same item X, but in different groups.
You can for example have a property named Origin which holds a reference to the original object X. Given that, you should be able to use the SelectionChanged event like this:
I hope this helps.
sender, SelectionChangeEventArgs e)
(e.AddedItems.Count > 0)
var origin = (e.AddedItems.Last()
var allItemsWithTheSameOrigin = (timeline.ItemsSource
.Where(p => p.Origin == origin);
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