/// <summary>
/// Returns an observable that wraps the MouseMove event on an instance
/// and all of it's siblings.
/// </summary>
/// <param name="that">The instance to retrieve the event for.</param>
/// <returns>An observable that wraps the MouseMove event on an instance
/// and all of it's siblings.</returns>
internal static IObservable <IEvent <MouseEventArgs> > GetMouseMoveOnSelfAndSiblings(this UIElement that)
{
IEnumerable <UIElement> popupRoots =
that
.GetVisualDescendants()
.OfType <Popup>()
.Select(popup => popup.Child)
.Where(popupRoot => popupRoot != null);
return
(popupRoots
.Select(popupRoot => popupRoot.GetMouseMoveWithOriginalSource())
.Aggregate(that.GetMouseMoveWithOriginalSource(), (left, right) => left.Merge(right)));
}