Asp.net Login control Textbox loses focus when keyboard appears (Chrome iOs)

Question

I am experiencing what seems to be a minor bug on Chrome (ver. 30.0.1599.16) on my iPad (iOS 7.0.3). I have an aspx page with a login control which renders the following HTML code:

Script in Head

<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['form1'];
if (!theForm) {
    theForm = document.form1;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
</script>

Controls

<td align="right">
  <label for="UserName">User Name:</label>
</td>
<td>
  <input name="lgnBlock$UserName" type="text" id="UserName" />
  <span id="UserNameRequired" title="User Name is required." style="visibility:hidden;">*</span>
</td>

Bottom Script

<script type="text/javascript">
//<![CDATA[
var Page_Validators =  new Array(document.getElementById("UserNameRequired"), document.getElementById("PasswordRequired"));
//]]>
</script>

<script type="text/javascript">
//<![CDATA[
var UserNameRequired = document.all ? document.all["UserNameRequired"] : document.getElementById("UserNameRequired");
UserNameRequired.controltovalidate = "UserName";
UserNameRequired.errormessage = "User Name is required.";
UserNameRequired.validationGroup = "lgnBlock";
UserNameRequired.evaluationfunction = "RequiredFieldValidatorEvaluateIsValid";
UserNameRequired.initialvalue = "";
var PasswordRequired = document.all ? document.all["PasswordRequired"] : document.getElementById("PasswordRequired");
PasswordRequired.controltovalidate = "Password";
PasswordRequired.errormessage = "Password is required.";
PasswordRequired.validationGroup = "lgnBlock";
PasswordRequired.evaluationfunction = "RequiredFieldValidatorEvaluateIsValid";
PasswordRequired.initialvalue = "";
//]]>
</script>


<script type="text/javascript">
//<![CDATA[

var Page_ValidationActive = false;
if (typeof(ValidatorOnLoad) == "function") {
    ValidatorOnLoad();
}

function ValidatorOnSubmit() {
    if (Page_ValidationActive) {
        return ValidatorCommonOnSubmit();
    }
    else {
        return true;
    }
}
        //]]>
</script>

When I tap on the UserName textbox to input the name the onscreen keyboard appears but the control loses focus and I have to tap again in order to fill it in. This DOES NOT happen with Safari on my iPad. I found this similar question but the answer reported as solution does not fit my case.

Any suggestions?

p.s. The javascript with validation are part of the Login Control