Code:

procedure TForm1.TreeView1DragOver(Sender, Source: TObject; X, Y: Integer;

  State: TDragState; var Accept: Boolean);

begin

  if (y < 15) then {On the upper edge - should scroll up }

    SendMessage(TreeView1.Handle, WM_VSCROLL, SB_LINEUP, 0)

  else if (TreeView1.Height - y < 15) then { On the lower edge - should scroll down }

    SendMessage(TreeView1.Handle, WM_VSCROLL, SB_LINEDOWN, 0);

end;