Inno Setup: Combo box closing immediately on drop-down (which starts an application)

Question

I am getting a list of Application Pools and then populating the combo box with the names of app pools. The problem is that when when the OnDropDown event is called the combo box opens for a fraction of a second and then closes straight away. It does not remain "dropped down". I do see all the app pools in the combo box. Here is my code:

function GetApplicationPoolList() : TArrayOfString;
var
  i, nExitCode: Integer;
  sFileLines: TArrayOfString;
  sTempFileName, sListAppPoolCmd: String;  
begin

    sListAppPoolCmd := ExpandConstant('{sys}') + '\inetsrv\appcmd list apppool /text:name'; 
    sTempFileName := ExpandConstant('{tmp}') + '\appPoolList.txt';
    if not ExecAppCmd(Format('%s > %s',[sListAppPoolCmd, sTempFileName]), nExitCode) then begin
      MsgBox('Could not get app pools', mbError, MB_OK);
    end else begin      
      LoadStringsFromFile(sTempFileName, sFileLines);
    end
    Result := sFileLines;
end;

// ==============================================

procedure OnAppPoolComboBoxDropDown(Sender: TObject);
var
  sAppPoolList: TArrayOfString;
  i: Integer;
begin

  // Clear existing
  appPoolComboBox.Items.Clear;
  // Populate the combo box with the application pools
  sAppPoolList := GetApplicationPoolList;  

  For i := 0 to GetArrayLength (sAppPoolList) - 1 do
    begin
      // ComboBox with Application Pool Names   
      appPoolComboBox.Items.Add(sAppPoolList[i]);      
    end;
  appPoolComboBox.ItemIndex := 0;
end;


function ExecAppCmd(params :String; nExitCode: Integer) :Boolean;
var
  execSuccessfully :Boolean;
  resultCode :Integer;
begin
  execSuccessfully := Exec('cmd.exe', '/c ' + '' + ' ' + params, '', SW_HIDE, ewWaitUntilTerminated, resultCode); 
  nExitCode := resultCode;   
  Result := execSuccessfully and (resultCode = 0);
end;

I am not sure what is happening here. Any advice is appreciated.

EDIT: ExecAppCmd seems to be the issue, commenting it out makes the combo box behave normally... Though not sure why

Answer 1

The drop-down closes probably because the combo box loses focus momentarily as the application is starting.

I'd say it's a bad practice to call an application on drop-down, as it takes time and it ruins a user experience. Populate the combo box earlier, like from CurPageChanged.