How to fixed the header of the GridView that is inside Repeater?

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C#

ASP.NET

CSS

Hello everybody,

I have a GridView which is nested inside a Repeater. Now I want to fix the header of this GridView. I used many article and posts and each time I got the same error in the looking and design which is: the header goes out of the GridView. It means that the GridView has a vertical and horizontal scrolls. When I try to fix the header, the header goes out of the GridView area and I don't know why.

For example, I use the following simple CSS code:

<style>
          .header
          {
            position:absolute;
          }
        </style>

and the ASP.NET code:

<div style="width:700px; height:300px; overflow:auto; overflow-y:auto;">
                <asp:GridView ID="GridView1" runat="server" 
                                AllowSorting="True" 
                                CellPadding="3" 
                                DataSourceID="SqlDataSource1" 
                                ClientIDMode="Static"
                                CssClass="mGrid"
                                AlternatingRowStyle-CssClass="alt" 
                                RowStyle-HorizontalAlign="Center" 
                                OnRowDataBound="GridView1_RowDataBound" OnPreRender="GridView1_PreRender">
                    <AlternatingRowStyle BackColor="White" ForeColor="#284775" />
                    <HeaderStyle Font-Bold = "true" ForeColor="Black" CssClass="header" Height="20px"/> 
                    <Columns>
                    </Columns>
                    <EditRowStyle BackColor="#999999" />
                    <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                    <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
                    <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
                    <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
                    <SortedAscendingCellStyle BackColor="#E9E7E2" />
                    <SortedAscendingHeaderStyle BackColor="#506C8C" />
                    <SortedDescendingCellStyle BackColor="#FFFDF8" />
                    <SortedDescendingHeaderStyle BackColor="#6F8DAE" />
                </asp:GridView>
                </div>

And I don't know why I am having this problem. Any help please?