Automation.AddStructureChangedEventHandler doesn't work?

I'm trying to use System.Windows.Automation library to do some UI testing, and was able to make some progress, but I am unable to subscribe to the event of creating a popup window in my app. I've tried to use Automation.AddStructureChangedEventHandler on the root (desktop object) as well as on the window, but that didn't work. I also tried using different scopes, which also didn't help.

AutomationElement desktop = AutomationElement.RootElement;
AutomationElement app = desktop
    .FindFirst(TreeScope.Children,
        new PropertyCondition(AutomationElement.NameProperty,
            "Name of the App", PropertyConditionFlags.IgnoreCase));

ActivateWindow(app);

AutomationElement appWindow = app
    .FindFirst(TreeScope.Children,
        new PropertyCondition(AutomationElement.ControlTypeProperty,
            ControlType.Window));

// Find a button that opens a popup window and click it
AutomationElement button = appWindow
    .FindFirst(TreeScope.Children, Condition.TrueCondition)
    .FindAll(TreeScope.Children, Condition.TrueCondition)[8];

MoveMouseToAndClick(button);

Automation.AddStructureChangedEventHandler(desktop, TreeScope.Descendants, setupWindowOpen);

The setupWindowOpen handler fires sometimes, but it looks like that happens for other apps, not mine (I'm seeing Internet Explorer ids on the sender element object). Thanks in advance.