{
 MS did not see fit to send a notification to the listview when the user
 Double - clicks on the header. But the header control class does have the
 CS_DBLCLKS Style, so it does get WM_LBUTTONDBLCLK messages, it just does
 not do anything with them. To get at these messages requires API - Style
 subclassing of the header control.
}
 uses commctrl;

 function HeaderProc(wnd: HWND; Msg: Cardinal; wParam: wParam; lParam: lParam): Longint;
  stdcall;
 var
  hti: THDHitTestInfo;
 begin
  Result := CallWindowProc(Pointer(GetWindowLong(wnd, GWL_USERDATA)),
  wnd, Msg, wParam, lParam);
  if Msg = WM_LBUTTONDBLCLK then
  begin
  FillChar(hti, SizeOf(hti), 0);
  hti.Point := SmallPointToPoint(TSmallPoint(lParam));
  if SendMessage(wnd, HDM_HITTEST, 0, Longint(@hti)) >= 0 then
  if hti.Flags = HHT_ONHEADER then
  // would usually send a custom notification to GetParent(wnd) here
  Form1.Memo1.Lines.Add(Format('doubleclick on header item %d', [hti.Item]));
  end;
 end;
 procedure TForm1.Button1Click(Sender: TObject);
 var
  wnd: HWND;
  buffer: array [0..80] of Char;
  oldProc: Integer;
 begin
  wnd := GetWindow(Listview1.Handle, GW_CHILD);
  if wnd <> 0 then
  begin
  Windows.GetClassname(wnd, buffer, SizeOf(buffer));
  memo1.Text := buffer;
  if (GetClassLong(wnd, GCL_STYLE) and CS_DBLCLKS) <> 0 then
  begin
  Memo1.Lines.Add('Has doubleclicks style');
  oldproc := GetWIndowLong(wnd, GWL_WNDPROC);
  if GetWindowLong(wnd, GWL_USERDATA) <> 0 then
  raise Exception.Create('Cannot sublcass header, USERDATA already in use');
  SetWIndowLong(wnd, GWL_USERDATA, oldproc);
  SetWindowLong(wnd, GWL_WNDPROC, Integer(@HeaderProc));
  end;
  end
  else
  Memo1.Text := 'No child';
 end;