when display the popup should block dropdownlist. I close the popup when I enable the dropdownlist

Hi experts, I have 2 web pages: a father and a popup.
The parent page is
1. edit icon (in a grid view)
2. combo (parent page)
When I press the edit icon calls the popup window. The problem is that this popup appears sometimes combo parent page (overlaps)
I am using asp.net, vb.net, vstudio 2005 and ajax.

what comes to mind is:
1. when you display the popup should block dropdownlist
2. I close the popup when I enable the dropdownlist
If you have another idea would be great

code behind
CType(e.Row.FindControl("ibtnEditarCandidato"), ImageButton).Attributes("OnClick") = "javascript:return parent.pAbrir_MdpopupCandidatosFinalistas('M','" + strNroProvision + "','" + strIdFinalista + "');"

javascript
function pAbrir_MdpopupCandidatosFinalistas(pstrAccion,p_INTIDNUMEROSOLICITUD,p_intidfinalistas){
        var ctrllblAccionModal= document.getElementById('ctl00_lblAccionModal');
          var ctrlpnlModal= document.getElementById('ctl00_pnlModal');
          var btnCerrar = document.getElementById('ctl00_btnCerrarModal');
          var strHtml ;
          
          //btnCerrar.style.display='';                 
          
        
            strHtml='<iframe runat="server" id="ifrModal" width="100%" height="230px" frameborder="0"scrolling="auto" marginheight="0" marginwidth="0" ';
         
           ctrllblAccionModal.innerText=">> Candidato Finalista ";
           strHtml=strHtml + 'src="../ModalPopup/frmEditCandidatosFinalistas.aspx?QS_INTIDNUMEROSOLICITUD='+ p_INTIDNUMEROSOLICITUD +'&QS_intidfinalistas='+ p_intidfinalistas +'"></iframe><br />';
           
            document.getElementById('divFrameMasterModal').innerHTML= strHtml ;
            ctrlpnlModal.style.width = "520px";
            ctrlpnlModal.style.height= "290px";
            fNavegadorCombo(true);
            //fCargarLoadModalWBC(ctrlpnlModal);
            $find('ctl00_mdlModal')._xCoordinate = 280
            $find('ctl00_mdlModal')._yCoordinate = 100
            $find('ctl00_mdlModal').show();
       
     }

The datagrid this in an update panel, i attached the file

dropdownlistOVERLAPS.JPG
block-dropdownlistHTMgenerated.txt