Need click twice to active a textbox

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ASP.NET

In am aspx page, there are a couple of textbox (e.g. txtBoxA and txtBoxB). Ideally, after entering values in txtBoxA, clicking a TAB and RETURN key will make txtBoxB activated. However, when the page is initially loaded, it needs to click twice to activate txtBoxB. How can this problem be debugged? Thanks.
The related codes are below:

<table>
    <tr>
        <td class="style1">
            <asp:TextBox ID="txtUser" runat="server" Font-Size="Small"  AutoPostBack="true"
                TabIndex="2" onkeydown="onFocus(event);"
                Text =""
                ></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td class="style1">
            <asp:TextBox ID="txtPassword" ClientID="txtPassword" runat="server" Font-Size="Small" AutoPostBack="true"
                TextMode="Password" TabIndex="3" >
            </asp:TextBox>
        </td>

    function onFocus(event) {
        var e1 = document.getElementById('<%=txtUser.ClientID%>').value;
        if (e1.length > 5) {
            if (event.keyCode == 13) {      // RETURN
                event.keyCode = 9;
                return event.keyCode;
            }
        }
    }
    </tr>
</table>