NavigationView.MenuItems RelativeSource doesn't work,How to binding?

[scarletNnn]

First one can work,but second doesn't work

[JohnTasler]

Since you found a syntax that works, I suggest you stick with that. Is there any reason to keep this bug open?

I think this is "By Design". While you think that NavigationView is the direct parent of the NavigationViewItem, that may only be true in the Logical Tree. I'm not sure why that would not be true in the Visual Tree, it could possibly be that the NavigationViewItem is contained in a Popup, rather then directly within the NavigationView. The AncestorLevel is looking through the NavigationViewItem's Visual Tree, which is dependent on the ControlTemplate of the control.

For FindAncestor mode, the optional property AncestorLevel can help disambiguate the ancestor lookup in cases where there is possibly more than one ancestor of that type existing in the element tree.

I don't think you need to disambiguate that parent control, so just don't specify the AncestorLevel property in the binding.