Hello Benoit Chevalier,
Unfortunately if you want the events to raise, you will have to add "telerik:RadDragAndDropManager.AllowDrop="True"", either in each control definition, or in an implicit style. Other than that, you cannot drop on a control with a AllowDrop = "True". As for the visual indications, you can use the DragCue functionality to obtain the desired behavior.
Hope you find this information helpful!
the Telerik team
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