function Form1.ItemAtPos(TabControlHandle : HWND; X, Y : Integer) : Integer;
var
  HitTestInfo : TTCHitTestInfo;
  HitIndex : Integer;
begin
  HitTestInfo.pt.x := X;
  HitTestInfo.pt.y := Y;
  HitTestInfo.flags := 0;
  HitIndex := SendMessage(TabControlHandle, TCM_HITTEST, 0, Longint(@HitTestInfo));
  Result := HitIndex;
end;