Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);

// Showing and hiding the alwaysvisiblecontrolextender's panel was taken from http://forums.asp.net/p/1300667/2542826.aspx .
function BeginRequestHandler(sender, args)
{
    $find("ctl00_AlwaysVisibleControlExtender1").get_element().style.display = 'block';
}

function EndRequestHandler(sender, args)
{
    $find("ctl00_AlwaysVisibleControlExtender1").get_element().style.display = 'none';
}