/// <summary>
        /// Returns an observable that returns a SW.DragDropKeyStates value
        /// whenever it changes, even if one of the events involved
        /// is handled or occurs in a sibling.
        /// </summary>
        /// <param name="that">The element to listen to.</param>
        /// <param name="initialState">The initial state SW.DragDropKeyStates.
        /// </param>
        /// <returns>An observable that returns a SW.DragDropKeyStates value
        /// whenever it changes, even if one of the events involved
        /// is handled.</returns>
        internal static IObservable <SW.DragDropKeyStates> GetKeyStateChangedOnSelfAndSiblingsAlways(this UIElement that, SW.DragDropKeyStates initialState)
        {
            IEnumerable <UIElement> popupRoots =
                VisualTreeExtensions.GetVisualDescendants(that)
                .OfType <Popup>()
                .Select(popup => popup.Child)
                .Where(popupRoot => popupRoot != null);

            return(GetKeyStateChanged(
                       popupRoots
                       .Select(popupRoot => popupRoot.GetMouseLeftButtonDownAlways())
                       .Aggregate(that.GetMouseLeftButtonDownAlways(), (left, right) => left.Merge(right)),
                       popupRoots
                       .Select(popupRoot => popupRoot.GetMouseLeftButtonUpAlways())
                       .Aggregate(that.GetMouseLeftButtonUpAlways(), (left, right) => left.Merge(right)),
                       popupRoots
                       .Select(popupRoot => popupRoot.GetKeyUpAlways())
                       .Aggregate(that.GetKeyUpAlways(), (left, right) => left.Merge(right)),
                       popupRoots
                       .Select(popupRoot => popupRoot.GetKeyDownAlways())
                       .Aggregate(that.GetKeyDownAlways(), (left, right) => left.Merge(right)),
                       initialState));
        }
 /// <summary>
 /// Returns an observable that returns a SW.DragDropKeyStates value
 /// whenever it changes, even if one of the events involved
 /// is handled.
 /// </summary>
 /// <param name="that">The element to listen to.</param>
 /// <param name="initialState">The initial state SW.DragDropKeyStates.
 /// </param>
 /// <returns>An observable that returns a SW.DragDropKeyStates value
 /// whenever it changes, even if one of the events involved
 /// is handled.</returns>
 internal static IObservable <SW.DragDropKeyStates> GetKeyStateChangedAlways(
     this UIElement that,
     SW.DragDropKeyStates initialState)
 {
     return
         (GetKeyStateChanged(that.GetMouseLeftButtonDownAlways(), that.GetMouseLeftButtonUpAlways(), that.GetKeyDownAlways(), that.GetKeyUpAlways(), initialState));
 }