当高度不够的时候

 
 
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);
 
 
 
    Accept
:= Source =
Sender;
    if not Accept then
exit;
end;