在DragDrop的时候，滚动TreeView？
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;