Page getting postback in update panel

Hi. I am using script manager and update panel in my aspx page. In the page, I have few dropdown controls , button and gridview. when I click button or there is a postback in dropdown, there is a flickering. How to avoid screen flickering while using the scriptmanager. Below is the code used :

  <form id="form1" runat="server">
        <asp:ScriptManager ID="scriptmanager1" runat="server">  
</asp:ScriptManager>  
        <div class="container-fluid">
          <div>      
          <asp:UpdatePanel ID="updatepnl" runat="server">  
<ContentTemplate> 
 <div >
  <label class="form-label" for="ddlAppln">Select Application</label>
 <div class="dropdown">
  <asp:DropDownList CssClass="btn btn-secondary dropdown-toggle" runat="server" ID="ddlAppln" AutoPostBack="true" >
                            <asp:ListItem Enabled="true" Text="Select Application" Value="-1"></asp:ListItem>
                            <asp:ListItem Text="T1" Value="T1"></asp:ListItem>
                            <asp:ListItem Text="T2" Value="T2"></asp:ListItem>
                            <asp:ListItem Text="T3" Value="T3"></asp:ListItem>                          
                        </asp:DropDownList>
                    </div> </div>
// other controls like dropdown , button, gridview
        </div>
            </ContentTemplate>  
</asp:UpdatePanel>  
        </div>      
    </form>

Is there any code which needs to be added in page load ?