JSFiddle - React, Tailwind, and code Playground

HTML

<div class="popup_Container">
        <div class="popup_Titlebar" id="PopupHeader">
            <div class="TitlebarLeft">
                Confirmação de Escolha</div>
            <div class="TitlebarRight" onclick="$get('ButtonDeleteCancel').click();">
            </div>
        </div>
        <div class="popup_Body">
            <p>
                <asp:Label ID="lblmensagem" runat="server"></asp:Label>
            </p>
        </div>
        <div class="popup_Buttons">
            
            <input id="ButtonDeleleOkay" type="button" value="Confirmar" />
            <input id="ButtonDeleteCancel" type="button" value="Cancelar" />
        </div>
    </div>

CSS

.popupContainer
{
    background-color:#fffeb2;
    border:2px solid #000000;
    padding: 0px 0px 0px 0px;
}

JavaScript

function resize_popup_container()
{
    var windowWidth = document.documentElement.clientWidth;
    var newWidth = windowWidth - 120;
    var newLeft = (windowWidth - newWidth) / 2;
    $('#popup_container').css('width', newWidth + 'px');
    $('#popup_container').css('left', newLeft + 'px');
}

$(function(){
    $(window).resize(function() {
        resize_popup_container();
    });
    
    resize_popup_container();
});