Question

I move focus to the Popup on its opening:

wcl:FocusHelper.IsFocused="{Binding RelativeSource={RelativeSource Self}, Path=IsOpen}"

FocusHelper class code:

public static class FocusHelper
{
     public static readonly DependencyProperty IsFocusedProperty =
            DependencyProperty.RegisterAttached("IsFocused", typeof(bool?), typeof(FocusHelper), new FrameworkPropertyMetadata(IsFocusedChanged));

        public static bool? GetIsFocused(DependencyObject element)
        {
            if (element == null)
            {
                throw new ArgumentNullException("element");
            }

            return (bool?)element.GetValue(IsFocusedProperty);
        }

        public static void SetIsFocused(DependencyObject element, bool? value)
        {
            if (element == null)
                throw new ArgumentNullException("element");

            element.SetValue(IsFocusedProperty, value);
        }

        private static void IsFocusedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var fe = (FrameworkElement)d;

            if (e.OldValue == null)
            {
                fe.GotFocus += ElementGotFocus;
                fe.LostFocus += ElementLostFocus;
                fe.IsVisibleChanged += ElementIsVisibleChanged;
            }

            if (e.NewValue == null)
            {
                fe.GotFocus -= ElementGotFocus;
                fe.LostFocus -= ElementLostFocus;
                fe.IsVisibleChanged -= ElementIsVisibleChanged;
                return;
            }

            if ((bool)e.NewValue)
            {
                fe.SetFocusWithin();
            }
        }

        private static void ElementIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            var fe = (FrameworkElement)sender;

            if (fe.IsVisible 
                && (bool)(((FrameworkElement) sender).GetValue(IsFocusedProperty))) // Bring focus to just become visible element.
                fe.Focus();
        }

        private static void ElementGotFocus(object sender, RoutedEventArgs e)
        {
            ((FrameworkElement)sender).SetCurrentValue(IsFocusedProperty, true);
        }

        private static void ElementLostFocus(object sender, RoutedEventArgs e)
        {
            ((FrameworkElement)sender).SetCurrentValue(IsFocusedProperty, false);
        }

        /// <summary>
        /// Tries to set focus to the element or any other element inside this one.
        /// Tab index is respected
        /// </summary>
        /// <param name="element"></param>
        /// <returns></returns>
        public static bool SetFocusWithin(this DependencyObject element)
        {
            if (element == null)
                throw new ArgumentNullException("element");

            var inputElement = element as IInputElement;
            if (inputElement == null || !inputElement.Focus())
            {
                var children = element.GetVisualChildrenSortedByTabIndex().Where(child => !(child is Control) || (bool)child.GetValue(Control.IsTabStopProperty) );
                return children.Any(SetFocusWithin);
            }

            return true;
        }
    }

ElementTreeHelper class part :

public static IEnumerable<DependencyObject> GetVisualChildren(this DependencyObject parent)
        {
            if (parent == null)
                throw new ArgumentNullException("parent");

            var count = VisualTreeHelper.GetChildrenCount(parent);
            for (var i = 0; i < count; i++)
                yield return VisualTreeHelper.GetChild(parent, i);
        }

        public static IEnumerable<DependencyObject> GetVisualChildrenSortedByTabIndex(this DependencyObject parent)
        {
            if (parent == null)
                throw new ArgumentNullException("parent");

            return parent.GetVisualChildren().OrderBy(KeyboardNavigation.GetTabIndex);
        }

The problem is that
var count = VisualTreeHelper.GetChildrenCount(parent) == 0 when parent is Popup.

UPDATE

The answer is here

Answer 1

The Popup doesn’t host the Child within itself. Instead it creates a PopupRoot (internal class) that is the root visual of a new HWND created to host the popup’s content within a separate top level window (or child window in xbap). The Child of the Popup is hosted within an AdornerDecorator within that PopupRoot.