That's exactly how it's supposed to work, but it doesn't.  I'm using Infragistics 2014.1 build 2150 in IE9.  I have cascading dropdowns *inside* an UpdatePanel as following.  When the first dropdown is selected, then the second populates correctly.  Then the first time and only the first time that I change the value of the second dropdown the onbeforeunload is called.  It's not called when I change the value the second time.  It's seems like an IE9 issue.  I haven't tried other IE versions.  This doesn't happen in Chrome, but of course my target browser is IE9.  Any suggestions?

<script type="text/javascript">
var dirty = false;
var skipConfirm = false;
var autoDestroyEligDate = true;
window.onbeforeunload = confirmExit;

function confirmExit() {
if (dirty && !skipConfirm) {
return "You have attempted to leave this page without saving your changes.";
}
}

function SetDirtyFlag() {
dirty = true;
}

....

</script>

...
<asp:UpdatePanel runat="server" ID="UpdatePanel1" ChildrenAsTriggers="False" UpdateMode="Conditional">
<Triggers>
<asp:PostBackTrigger runat="server" ControlID="btnSave"/>
<asp:PostBackTrigger runat="server" ControlID="btnReset"/>
<asp:AsyncPostBackTrigger runat="server" ControlID="cmbDeptOwner" EventName="ValueChanged"/>
</Triggers>
<ContentTemplate>

....

<ig:WebDropDown ID="cmbDeptOwner" runat="server"
BackColor="White" BorderColor="Silver" BorderStyle="Solid" BorderWidth="1px"
ForeColor="Black"
Width="220px" DisplayMode="DropDownList"
OnSelectionChanged="cmbDeptOwner_SelectionChanged">
<AutoPostBackFlags SelectionChanged="On" />
<ClientEvents SelectionChanging="ReturnTrueIfNotEditable"
DropDownOpening="ReturnTrueIfNotEditable" SelectionChanged="SetDirtyFlag" />
</ig:WebDropDown>

.........

<ig:WebDropDown ID="cmbDescription" runat="server"
BorderColor="Silver" BorderStyle="Solid" BorderWidth="1px"
Width="220px"
ForeColor="Black" DisplayMode="DropDownList" CurrentValue="">
<ClientEvents SelectionChanged="SetDirtyFlag" />
</ig:WebDropDown>

.....

</ContentTemplate>
</asp:UpdatePanel>
</asp:Content>