Focussing problem with dynamic select element

[Jonathan Parkes]

I am having a problem with a select element that is modified at run-time
in JavaScript. An additional option is added if the user enters any text
in an adjacent textbox. The option is removed if the textbox is cleared.
I have this functionality working fine, but there is a side-effect. If
the user enters some text in the textbox, then clicks the combo box, it
drops down and is immediately the list is retracted again. It requires a
further two clicks to get the list to drop down again.

Here is the relevant snippet of code:

<select id="cboReservationDate">
<option value="0">Current</option>
<option value="1">Monday</option>
<option value="2">Tuesday</option>
{etc. for other working days}
</select>
<input id="txtTicketNumber" onkeypress="txtTicketNumber_onkeypress();"
onchange="txtTicketNumber_onchange();" value="">

<script language="javascript" type="text/javascript">
<!--
//only allow characters 0-9
function txtTicketNumber_onkeypress()
{
if ((window.event.keyCode < 48) || (window.event.keyCode > 57))
window.event.keyCode = 0;
}

//if there are any data in the ticket number textbox, add an "all"
option to the reservation date combo box
function txtTicketNumber_onchange()
{
var indexOfAll;

if (txtTicketNumber.value == '')
{
//if "all" is the currently selected value, reset to "current"
if (cboReservationDate.value == '999')
cboReservationDate.value = '0';

//remove "all" option from date combo if it exists
if ((indexOfAll = indexOfAllOption()) > -1)
cboReservationDate.options.remove(indexOfAll);
}
else
{
//add "all" option to date combo if it is not already there
if ((indexOfAll = indexOfAllOption()) == -1)
{
var oAllOption = document.createElement('OPTION');
oAllOption.text = 'All';
oAllOption.value = '999';

cboReservationDate.options.add(oAllOption);
}
}
}

function indexOfAllOption()
{
for (i=0; i < cboReservationDate.options.length; i++)
if (cboReservationDate.options.value == '999')
return i;

return -1;
}

Thanks in advance to anyone who can help!

Regards,

Jon.