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procedure TForm1.Webbrowser1DocumentComplete(ASender: TObject;
  const pDisp: IDispatch; var URL: OleVariant);
const
  LVS_THUMBNAIL  = $702D;
var
  wnd: HWND;
begin
  wnd := FindWindowEx(Self.Handle, 0, 'Shell Embedding', nil);
  if wnd <> 0 then
  begin
    wnd := FindWindowEx(wnd, 0, 'SHELLDLL_DefView', nil);
    if wnd <> 0 then
      SendMessage(wnd, WM_COMMAND, LVS_THUMBNAIL, 0 );
  end;
end;