Thank you for the attached picture.
I looked into the described requirement, however I am afraid that currently it is not possible to achieve what you are going for in the RadTreeView or the RadTreeListView. Basically the logic for drawing the tree lines is internal to both of the controls. For the RadTreeListView, the lines are added entirely in code. In the case of the RadTreeView, they exist in RadTreeViewItem's ControlTemplate as Rectangles and their width and height is set in code. However, I am afraid that it is not possible to modify the template of the RadTreeViewItem to achieve the described customization without breaking the current representation of the lines.
Keeping all of that in mind, the only thing that I can suggest is to log in a feature request in our feedback portal
for this functionality.
I hope this helps. Let me know if I can be of any further assistance.
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