When I add an option to a select the select gets narrower - asp.net

Question

Inherited an app with a page that has a link that calls the javascript function addValueClick(), when I do this a dialog box pops up, I type in some text, and then the text gets put in the select box. Every time a new option is added to the select it gets about 5 pixels narrower. I can't figure out why this is happening, but it only happens in IE7
Here is the javascript:
function addValueClick()
{
var newValue = prompt("Please enter a new value.","");
if (newValue != null && newValue != "")
{
var lst = document.getElementById("lstValues");
var opt = document.createElement("option");
opt.setAttribute("selected", "true");
opt.appendChild(document.createTextNode(newValue));
lst.appendChild(opt);
updateBtns();
copyValues();
}
}
function copyValues()
{
var frm = document.forms[0];
var lst = frm.elements["lstValues"];
var hid = frm.elements["hidValues"];
var xml = "<root>";
for (var i = 0; i < lst.options.length; i++)
{
xml += "<value seq_num=\"" + (i + 1) + "\">" +
lst.options[i].text + "</value>";
}
xml += "</root>";
hid.value = xml;
}
function updateBtns()
{
var lst = document.getElementById("lstValues");
var iSelected = lst.selectedIndex;
var lnkEdit = document.getElementById("lnkEditValue");
var lnkDelete = document.getElementById("lnkDeleteValue");
var lnkUp = document.getElementById("lnkValueUp");
var lnkDown = document.getElementById("lnkValueDown");
if (iSelected == -1)
{
lnkEdit.style.visibility = "hidden";
lnkDelete.style.visibility = "hidden";
lnkUp.style.visibility = "hidden";
lnkDown.style.visibility = "hidden";
}
else
{
lnkEdit.style.visibility = "visible";
lnkDelete.style.visibility = "visible";
if (iSelected == 0)
lnkUp.style.visibility = "hidden";
else
lnkUp.style.visibility = "visible";
if (iSelected == lst.options.length - 1)
lnkDown.style.visibility = "hidden";
else
lnkDown.style.visibility = "visible";
}
}
EDIT:
The HTML, it's actually ASP.NET. All the listValueChanged() method does is call updateButtons() above.
<tr>
<TD class=body vAlign=top noWrap align=right><b>Values:</TD>
<TD class=body vAlign=top noWrap align=left><asp:ListBox id="lstValues" runat="server" onchange="lstValuesChange();" Rows="9" onselectedindexchanged="lstValues_SelectedIndexChanged"></asp:ListBox></TD>
<TD class=body vAlign=top noWrap align=left>
<TABLE id="Table2" cellSpacing="2" cellPadding="2" border="0">
<TR>
<TD noWrap>
<asp:HyperLink id="lnkAddValue" runat="server" NavigateUrl="javascript:addValueClick();">Add</asp:HyperLink></TD>
</TR>
<TR>
<TD noWrap>
<asp:HyperLink id="lnkEditValue" runat="server" NavigateUrl="javascript:editValueClick();">Edit</asp:HyperLink></TD>
</TR>
<TR>
<TD noWrap>
<asp:HyperLink id="lnkDeleteValue" runat="server" NavigateUrl="javascript:deleteValueClick();">Delete</asp:HyperLink></TD>
</TR>
<TR>
<TD noWrap> </TD>
</TR>
<TR>
<TD noWrap>
<asp:HyperLink id="lnkValueUp" runat="server" NavigateUrl="javascript:valueUpClick();">Up</asp:HyperLink></TD>
</TR>
<TR>
<TD noWrap>
<asp:HyperLink id="lnkValueDown" runat="server" NavigateUrl="javascript:valueDownClick();">Down</asp:HyperLink></TD>
</TR>
</TABLE>
</TD>
</TR>

Answer 1

It may have more to do with the html than the script. What is the select contained in? Based on the JS, I don't think there are any problems here.

Answer 2

I know that adding options via SelectElement.innerHTML = '...'; fails in IE (bug 274).
But I don't know about adding options via .createElement() failing... although I'm not overly surprised.
You can use the JavaScript new Option(); syntax to create them, I'm fairly certain that works without failure.

Answer 3

The issue was where the Option was being added to the Select I changed it to the following and it works perfectly:
function addValueClick()
{
var newValue = prompt("Please enter a new value.","");
if (newValue != null && newValue != "")
{
var lst = document.getElementById("lstValues");
var opt = document.createElement("option");
opt.text = newValue;
opt.value = newValue;
try {
lst.add(opt, null); // real browsers
}
catch (ex) {
lst.add(opt); // IE
}
updateBtns();
copyValues();
}
}