Thank you for the provided sample project.
I was able to reproduce the observed on your end behavior and I investigated it. Here are my findings:
Basically, the reason for it is the complexity of the nested RadListBox in a RadListBoxItem. In such a case, when dragging an item over the target RadListBox, sometimes the mouse is over a RadListBoxItem from the parent listbox and sometimes the mouse is over a RadListBoxItem from the child listbox.
For the custom drop visual indicator to be placed correctly in such a custom scenario you can create a class inheriting LinearDropVisualProvider
and override its GetLocation
method. Then you can check which container is received in that method and change the drop position accordingly. I am attaching the sample project modified to demonstrate what I have in mind. Note, that I have implemented the custom LinearDropVisualProvider for the top RadListBox with RadExpander, however you can use a similar approach for the other listbox.
I hope you find this helpful.
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