Thank you for writing.
Expanding an item is not supported, however, you could use a tooltip to show the whole text of the desired item. For the purpose you can use the VisualItemFormatting event:
sender, VisualItemFormattingEventArgs args)
args.VisualItem.ToolTipText = args.VisualItem.Text;
I hope that you will find this useful. Let us know should you have any other questions.
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