Problem with focus on textbox

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C#

ASP.NET

Javascript

I have a login page, i have set default button property of form with login button. so when I press enter key it must login.

when i am not using focus on ony one of the textboxes(username,password) it working fine.
but when i have used focus method on any one of the textbox for the first time when i press enter key its not accepting, for second time its accepting

above case is when i have saved my password in chrome and when i open login page it automatically loads textbox values i.e., username,password.

following is the code:

ASPX:

<form id="form1" runat="server" defaultbutton="btnSubmit">
        <div>
            <table>
                    <tr>
                        <td>UserName:
                        </td>
                        <td>
                            <asp:TextBox ID="txtUserName" runat="server"></asp:TextBox>
                            <asp:RequiredFieldValidator ID="rfvUserName" runat="server" ErrorMessage="Enter UserName" ControlToValidate="txtUserName"></asp:RequiredFieldValidator>
                        </td>
                    </tr>
                    <tr>
                        <td>Password:
                        </td>
                        <td>
                            <asp:TextBox ID="txtPassword" runat="server" TextMode="Password"></asp:TextBox>
                            <asp:RequiredFieldValidator ID="rfvPassword" runat="server" ErrorMessage="Enter Password" ControlToValidate="txtPassword"></asp:RequiredFieldValidator>
                        </td>
                    </tr>
                    <tr>
                        <td colspan="2">
                            <asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click" />
                        </td>
                    </tr>
                </table>
        </div>
    </form>

CS Code:

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            form1.Attributes.Add("onkeydown", "javascript: return WebForm_FireDefaultButton (event, '" + btnSubmit.ClientID + "')");
            txtUserName.Focus();
        }
    }
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        Response.Redirect("home.aspx");
    }

please do help me here.




thanks in advance.