procedure TForm1.AdvPolyList1DragDrop(Sender, Source: TObject; X, Y: Integer);
var
  it, itdrop: TCustomItem;
begin
  it := (Source as TAdvPolyList).DropItem;
  itdrop := AdvPolyList1.List.ItemAtXY(X, Y);
  if Assigned(itdrop) then
    AdvPolyList1.InsertItem(itdrop.Index, TCustomItemClass(it.ClassType)).Assign(it)
  else
    AdvPolyList1.AddItem(TCustomItemClass(it.ClassType)).Assign(it);
end;

procedure TForm1.AdvPolyList1DragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  Accept := Source is TAdvPolyList;
end;