void __fastcall TfMain::TreeViewDragOver(TObject *Sender, TObject *Source,
      int X, int Y, TDragState State, bool &Accept)
{
 if(Y<15)
 {
     SendMessage(TreeView->Handle,WM_VSCROLL,SB_LINEUP,0);
 }
 else if(TreeView->Height-Y < 15)
 {
     SendMessage(TreeView->Handle,WM_VSCROLL,SB_LINEDOWN,0);
 }
}