PositionChanging and PositionChanged events are used to detect the dragging and dropping of any nodes within the diagram canvas. In these events, you will get the source node and the current position of the node from the corresponding event arguments.
For more information regarding the PositionChanged and PositionChanging events, please refer to the following UG link.
https://blazor.syncfusion.com/documentation/diagram/nodes/events#position-change-event
If the requirement is to detect the target node after the drop, then you can use the DragDrop event, where you will get the target node from the event arguments. The DragDrop event can be invoked by enabling the AllowDrop constraints in the target node, which shows the highlighters when you drag the node and mouse hover on another node.
For more information regarding the DragDrop event, please refer to the following UG link:
https://blazor.syncfusion.com/documentation/diagram/events#dragdrop-event
Note: The OnDrop (IBlazorDropEventArgs args) event belongs to our Classic SfDiagram control. We suggest you use SfDiagramComponent since it will provide better performance than SfDiagram and SfDiagramComponent will be actively developed in the future. You can use SfDiagramComponent to build a rich diagram editor-style application. For your reference, we have shared our online help documentation and a demo link.
UG: https://blazor.syncfusion.com/documentation/diagram/getting-started
Demo: https://blazor.syncfusion.com/demos/diagramcomponent/flowchart?theme=fluent