add VerifyExpandCollapseChevronVisibility API test

dev/NavigationView/NavigationView_ApiTests/NavigationViewTests.cs

@@ -759,5 +759,70 @@ public void VerifyNavigationViewItemInFooterDoesNotCrash()
                 Verify.IsTrue(true);
             });
         }
+
+        [TestMethod]
+        public void VerifyExpandCollapseChevronVisibility()
+        {
+            NavigationView navView = null;
+            NavigationViewItem parentItem = null;
+            ObservableCollection<string> children = null;
+
+            RunOnUIThread.Execute(() =>
+            {
+                navView = new NavigationView();
+                Content = navView;
+
+                children = new ObservableCollection<string>();
+                parentItem = new NavigationViewItem() { Content = "ParentItem", MenuItemsSource = children };
+
+                navView.MenuItems.Add(parentItem);
+
+                navView.Width = 1008; // forces the control into Expanded mode so that the menu renders
+                Content.UpdateLayout();
+
+                UIElement chevronUIElement = (UIElement)FindVisualChildByName(parentItem, "ExpandCollapseChevron");
+                Verify.IsTrue(chevronUIElement.Visibility == Visibility.Collapsed, "chevron should have been collapsed as NavViewItem has no children");
+
+                // Add a child to our NavigationView parentItem. This should make the chevron visible.
+                children.Add("Undo");
+                Content.UpdateLayout();
+
+                Verify.IsTrue(chevronUIElement.Visibility == Visibility.Visible, "chevron should have been visible as NavViewItem now has children");
+
+                // Remove all children from our NavigationView parentItem. This should collapse the chevron
+                children.Clear();
+                Content.UpdateLayout();
+
+                Verify.IsTrue(chevronUIElement.Visibility == Visibility.Collapsed, "chevron should have been collapsed as NavViewItem no longer has children");
+            });
+        }
+
+        private static DependencyObject FindVisualChildByName(FrameworkElement parent, string name)
+        {
+            if (parent.Name == name)
+            {
+                return parent;
+            }
+
+            int childrenCount = VisualTreeHelper.GetChildrenCount(parent);
+
+            for (int i = 0; i < childrenCount; i++)
+            {
+                FrameworkElement childAsFE = VisualTreeHelper.GetChild(parent, i) as FrameworkElement;
+
+                if (childAsFE != null)
+                {
+                    DependencyObject result = FindVisualChildByName(childAsFE, name);
+
+                    if (result != null)
+                    {
+                        return result;
+                    }
+                }
+            }
+
+            return null;
+        }
+
     }
 }