DropDownList not firing onbeforeunload when AutoPostBack="True"

I have implemented an "unsaved changes" warning using techniques described on these pages:

Client/JS Framework for "Unsaved Data" Protection?

http://kenbrowning.blogspot.com/2009/01/using-jquery-to-standardize.html

This works well except for a DropDownList on the page. It does an AutoPostBack, and I want onbeforeunload to fire because unsaved changes will be lost, but it isn't working. Should it be raising the onbeforeunload event? Can I somehow make it raise the event?

Edit:

The DropDownList is inside an UpdatePanel, so that means it isn't unloading the page and that would be why onbeforeunload isn't being triggered. Is there any way I can trigger the event programmatically? Or do I have to roll my own imitation Confirm dialog?

Edit2

I now have a solution that adds the dialog to asynchronous postbacks from an UpdatePanel. I have edited the original script, adding the call to setConfirmAsyncPostBack() as described in my solution.

Here is my JavaScript:

/****Scripts to warn user of unsaved changes****/

//https://stackoverflow.com/questions/140460
//http://jonstjohn.com/node/23

//Activates the confirm message onbeforeunload.
function setConfirmUnload(on) {

    setConfirmAsyncPostBack();

    if (on) {
        removeCheckFromNoWarnClasses();
        fixIEonBeforeUnload();
        window.onbeforeunload = unloadMessage
        return;
    }

    window.onbeforeunload = null
}

function unloadMessage() {

    return 'You have unsaved chang开发者_开发问答es.';
}

//Moves javascript from href to onclick to prevent IE raising onbeforeunload unecessarily
//http://kenbrowning.blogspot.com/2009/01/using-jquery-to-standardize.html
function fixIEonBeforeUnload() {
 
    if (!$.browser.msie)
        return;
    $('a').filter(function() {
        return (/^javascript\:/i).test($(this).attr('href'));
    }).each(function() {
        var hrefscript = $(this).attr('href');
        hrefscript = hrefscript.substr(11);
        $(this).data('hrefscript', hrefscript);
    }).click(function() {
        var hrefscript = $(this).data('hrefscript');
        eval(hrefscript);
        return false;
    }).attr('href', '#');
}

//Removes warnings from Save buttons, links, etc, that have been can be given "no-warn" or "no-warn-validate" css class
//"no-warn-validate" inputs/links will only remove warning after successful validation
//use the no-warn-validate class on buttons/links that cause validation. 
//use the no-warn class on controls that have CausesValidation=false (e.g. a "Save as Draft" button).
function removeCheckFromNoWarnClasses() {
  
    $('.no-warn-validate').click(function() {
        if (Page_ClientValidate == null || Page_ClientValidate()) {
            setConfirmUnload(false);
        }
    });

    $('.no-warn').click(function() {
        setConfirmUnload(false);
    });
}

//Adds client side events to all input controls to switch on confirmation onbeforeunload
function enableUnsavedChangesWarning() {
 
    $(':input').one('change', function() {
        window.onbeforeunload = function() {
            return 'You have unsaved changes.';
        }
    });

    removeCheckFromNoWarnClasses();
}

And in my ASP.NET page, when the user makes a change:

    if (changed)
    {
        ...
        //Confirm unload if there are unsaved changes. 
        //NB we also have to call fixIEonBeforeUnload() to fix links, done in in page load to include links that are rendered during callbacks
        ScriptManager.RegisterStartupScript(Page, GetType(), "unsavedchanges", "setConfirmUnload(true);", true);
    }
    else
        ...

Also see How to prevent AutoPostBack when DropDownlist is selected using jQuery

//http://msdn.microsoft.com/en-us/magazine/cc163413.aspx
//https://stackoverflow.com/questions/2424327/prevent-asp-net-dopostback-from-jquery-submit-within-updatepanel
//Adds an event handler to confirm unsaved changes when an asynchronous postback is initialised by an UpdatePanel
function setConfirmAsyncPostBack() {

    if (typeof (Sys.WebForms) === "undefined" || typeof (Sys.WebForms.PageRequestManager) === "undefined")
        return;

    var prm = Sys.WebForms.PageRequestManager.getInstance();
    prm.add_initializeRequest(confirmAsyncPostBack);
}

//An event handler for asynchronous postbacks that confirms unsaved changes, cancelling the postback if they are not confirmed
//Adds the confirmation to elements that have a css class of "warn"
function confirmAsyncPostBack(sender, args) {
    if (window.onbeforeunload != null && args.get_postBackElement().className == "warn" && !unloadConfirmed())
        args.set_cancel(true);
}

//Displays a confirmation dialog that imitates the dialog displayed by onbeforeunload
function unloadConfirmed() {

    var confirmed = confirm("Are you sure you want to navigate away from this page?\n\n" + unloadMessage() + "\n\nPress OK to continue or Cancel to stay on the current page.");
    if (confirmed)
        window.onbeforeunload = null;
    return confirmed;
}