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When the user attempts to click and drag an item in the SfListBox, the dragged item may not be the one clicked. It seems as though there is a delay before the ListBox's logic decides that a drag is taking place. Perhaps the mouse pointer has to move a certain distance, or something like that. If the mouse pointer is no longer over the item where the mousedown event took place, then the item dragged will be an adjacent item (the one over which the mouse pointer was positioned when the ListBox logic decides that a drag is taking place).
Correct behaviour would be for the object over which the mousedown event took place to be dragged instead.
This is illustrated by the attached gif. This is the WASM ListBoxSample project from the GitHub, with AllowDragAndDrop set on the SfListBox. The user clicks "Bugatti Chiron" to select it. Next, the user moves the mouse pointer down so that it is close to the border between "Bugatti Chiron" and "Bugatti Veyron Super Sport". Then the user presses down the mouse button and quickly moves the pointer down over the border. The result is that "Bugatti Veyron Super Sport" is dragged and not "Bugatti Chiron". "Bugatti Chiron" remains the selected item even though another item is being dragged, which tells us that the mousedown event did actually happen over "Bugatti Chiron".
NB. The sample project is on version 19.4.0.48 of the SyncFusion NuGet packages. If you update the sample project to version 22.1.34 and modify Program.cs to allow it to build (by removing the argument in the call to AddSyncfusionBlazor()), the sample project does not work without additional changes ("Uncaught (in promise) Error: System.ArgumentException: The call to 'DragEndAsync' expects '9' parameters, but received '7'.") However, I have verified in my own application that I can still make the incorrect dragging behaviour occur in version 22.1.34.