In the @page Directive set the property MaintainScrollPositionOnPostback and check.

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="CS.aspx.cs" Inherits="CS" MaintainScrollPositionOnPostback = "true" %>

Also refer the below article.

ASP.Net MaintainScrollPositionOnPostback not working in Firefox and Chrome

As per your code what i saw is you are focusing to other control after postback that’s why your scroll bar move to the top.

So you need to add the below script in order to maintain the scrollbar.

jQuery

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    $(document).ready(function () {
        HTMLInputElement.prototype.scrollIntoView = function (a) { this.scrollIntoViewIfNeeded(); }
        HTMLSelectElement.prototype.scrollIntoView = function (a) { this.scrollIntoViewIfNeeded(); }
        HTMLAreaElement.prototype.scrollIntoView = function (a) { this.scrollIntoViewIfNeeded(); }
    });
</script>