NavigationView: Fix bug where items with empty MenuItemsSource would still spawn flyout (#3860)

* Update test page, fix bug

* PR feedback

* Switch to ItemsSourceView

* Fix test failures hopefully

Author: marcelwgn

dev/NavigationView/NavigationView.cpp

@@ -29,6 +29,7 @@
 #include <ItemsRepeater.common.h>
 #include "NavigationViewItemExpandingEventArgs.h"
 #include "NavigationViewItemCollapsedEventArgs.h"
+#include "InspectingDataSource.h"
 
 // General items
 static constexpr auto c_togglePaneButtonName = L"TogglePaneButton"sv;
@@ -5313,7 +5314,13 @@ void NavigationView::Collapse(const winrt::NavigationViewItem& item)
 
 bool NavigationView::DoesNavigationViewItemHaveChildren(const winrt::NavigationViewItem& nvi)
 {
-    return nvi.MenuItems().Size() > 0 || nvi.MenuItemsSource() != nullptr || nvi.HasUnrealizedChildren();
+    if (nvi.MenuItemsSource()) {
+        if (const auto sourceView = winrt::make<InspectingDataSource>(nvi.MenuItemsSource()))
+        {
+            return sourceView.Count() > 0;
+        }
+    }
+    return nvi.MenuItems().Size() > 0 || nvi.HasUnrealizedChildren();
 }
 
 void NavigationView::ToggleIsExpandedNavigationViewItem(const winrt::NavigationViewItem& nvi)

dev/NavigationView/TestUI/Hierarchical/HierarchicalNavigationViewDataBinding.xaml.cs

@@ -69,9 +69,11 @@ public HierarchicalNavigationViewDataBinding()
             var categories8 = new ObservableCollection<Category>();
             categories8.Add(new Category("Menu Item 16 (Selectable)", "MI16", "Icon", categories9, true));
 
+            var categories10 = new ObservableCollection<Category>();
+
             categories.Add(new Category("Menu Item 1", "MI1", "Icon", categories2, false));
             categories.Add(new Category("Menu Item 6 (Selectable)", "MI6", "Icon", categories4, true));
-            categories.Add(new Category("Menu Item 10", "MI10", "Icon", null, true));
+            categories.Add(new Category("Menu Item 10", "MI10", "Icon", categories10, true));
             categories.Add(new Category("Menu Item 11", "MI11", "Icon", categories6, false));
             categories.Add(new Category("Menu Item 15", "MI15", "Icon", categories8, false));