Переопределить getDragTracker в editPart:
@Override
public DragTracker getDragTracker(Request request)
{
// Create a drag tracker that will enforce single selection
return new DragEditPartsTracker(this)
{
protected void performSelection()
{
if (hasSelectionOccurred())
return;
setFlag(FLAG_SELECTION_PERFORMED, true);
getCurrentViewer().select(getSourceEditPart());
}
};
}