fix(droid): add workaround for NavView leaving blank on IsBackButtonV…

…isible update

src/Uno.UI.RuntimeTests/Tests/Microsoft_UI_Xaml_Controls/Given_NavigationView.cs

@@ -1,4 +1,19 @@
-using System.Threading.Tasks;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Media;
+using Windows.UI;
+using Uno.Extensions;
+using Uno.Extensions.Specialized;
+using Uno.UI.Extensions;
+using Uno.UI.RuntimeTests.Helpers;
+
+using static Private.Infrastructure.TestServices;
+using MUXC = Microsoft/* UWP don't rename */.UI.Xaml.Controls;
+
 #if WINAPPSDK
 using Uno.UI.Extensions;
 #elif __IOS__
@@ -7,12 +22,6 @@
 using AppKit;
 #else
 #endif
-using Microsoft.UI.Xaml;
-using Microsoft.UI.Xaml.Controls;
-using static Private.Infrastructure.TestServices;
-using Windows.UI;
-using Microsoft.UI.Xaml.Media;
-using Uno.UI.RuntimeTests.Helpers;
 
 namespace Uno.UI.RuntimeTests.Tests.Microsoft_UI_Xaml_Controls
 {
@@ -73,5 +82,98 @@ public async Task When_SelectedItem_Set_Before_Load_And_Theme_Changed()
 #endif
 			}
 		}
+
+		[TestMethod]
+		public async Task When_IsBackButtonVisible_Toggled()
+		{
+			// unoplatform/uno#19516
+			// droid-specific: There is a bug where setting IsBackButtonVisible would "lock" the size of all NVIs
+			// preventing resizing on items expansion/collapse.
+
+			var sut = new MUXC.NavigationView()
+			{
+				Height = 500,
+				IsBackButtonVisible = MUXC.NavigationViewBackButtonVisible.Collapsed,
+				IsPaneToggleButtonVisible = false,
+				PaneDisplayMode = NavigationViewPaneDisplayMode.Left,
+				CompactModeThresholdWidth = 10,
+				ExpandedModeThresholdWidth = 50,
+			};
+			sut.ItemInvoked += (s, e) =>
+			{
+				// manual trigger for deepest/inner-most items
+				if (e.InvokedItemContainer is NavigationViewItem nvi &&
+					nvi.MenuItems.Count == 0)
+				{
+					sut.IsBackButtonVisible = NavigationViewBackButtonVisible.Visible;
+				}
+			};
+
+			var nvis = new Dictionary<string, NavigationViewItem>();
+			//AddItems(sut.MenuItems, "", count: 4, depth: 1, maxDepth: 2);
+			AddItems(sut.MenuItems, "", count: 3, depth: 1, maxDepth: 3);
+			void AddItems(IList<object> target, string prefix, int count, int depth, int maxDepth)
+			{
+				for (int i = 0; i < count; i++)
+				{
+					var header = prefix + (char)('A' + i);
+					var item = new NavigationViewItem() { Content = header };
+
+					if (depth < maxDepth) AddItems(item.MenuItems, header, count, depth + 1, maxDepth);
+
+					target.Add(item);
+					nvis.Add(header, item);
+				}
+			}
+
+			// for debugging
+			var panel = new StackPanel();
+			void AddTestButton(string label, Action action)
+			{
+				var button = new Button() { Content = label };
+				button.Click += (s, e) => action();
+				panel.Children.Add(button);
+			}
+			AddTestButton("InvalidateMeasure", () => sut
+				.EnumerateDescendants().OfType<ItemsRepeater>()
+				.ForEach(x => x.InvalidateMeasure()));
+			AddTestButton("IsBackButtonVisible toggle", () => sut.IsBackButtonVisible = sut.IsBackButtonVisible == NavigationViewBackButtonVisible.Collapsed
+				? NavigationViewBackButtonVisible.Visible
+				: NavigationViewBackButtonVisible.Collapsed);
+			panel.Children.Add(sut);
+
+			await UITestHelper.Load(panel, x => x.IsLoaded);
+
+			var initialHeight = nvis["B"].ActualHeight;
+
+			nvis["B"].IsExpanded = true;
+			await UITestHelper.WaitForIdle();
+			var partiallyExpandedHeight = nvis["B"].ActualHeight;
+
+			nvis["BB"].IsExpanded = true;
+			await UITestHelper.WaitForIdle();
+			var fullyExpandedHeight = nvis["B"].ActualHeight;
+
+			// trigger the bug
+			await Task.Delay(2000); // necessary
+			sut.IsBackButtonVisible = NavigationViewBackButtonVisible.Visible;
+			await UITestHelper.WaitForIdle();
+
+			nvis["BB"].IsExpanded = false;
+			await UITestHelper.WaitForIdle();
+			var partiallyCollapsedHeight = nvis["B"].ActualHeight;
+
+			nvis["B"].IsExpanded = false;
+			await UITestHelper.WaitForIdle();
+			var fullyCollapsedHeight = nvis["B"].ActualHeight;
+
+			// sanity check
+			Assert.IsTrue(initialHeight < partiallyExpandedHeight, $"Expanding 'B' should increase item 'B' height: {initialHeight} -> {partiallyExpandedHeight}");
+			Assert.IsTrue(partiallyExpandedHeight < fullyExpandedHeight, $"Expanding 'BB' should increase item 'B' height: {partiallyExpandedHeight} -> {fullyExpandedHeight}");
+
+			// verifying fix
+			Assert.IsTrue(fullyExpandedHeight > partiallyCollapsedHeight, $"Collapsing 'BB' should reduce item 'B' height: {fullyExpandedHeight} -> {partiallyCollapsedHeight}");
+			Assert.IsTrue(partiallyCollapsedHeight > fullyCollapsedHeight, $"Collapsing 'B' should reduce item 'B' height: {partiallyCollapsedHeight} -> {fullyCollapsedHeight}");
+		}
 	}
 }

src/Uno.UI/Microsoft/UI/Xaml/Controls/NavigationView/NavigationView.cs

@@ -9,12 +9,14 @@
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
 using System.Globalization;
+using System.Linq;
 using System.Numerics;
 using Microsoft/* UWP don't rename */.UI.Xaml.Automation.Peers;
 using Microsoft/* UWP don't rename */.UI.Xaml.Controls.AnimatedVisuals;
 using Uno.Disposables;
 using Uno.Foundation.Logging;
 using Uno.UI.DataBinding;
+using Uno.UI.Extensions;
 using Uno.UI.Helpers.WinUI;
 using Windows.ApplicationModel.Core;
 using Windows.Foundation;
@@ -4353,6 +4355,16 @@ private void OnPropertyChanged(DependencyPropertyChangedEventArgs args)
 			{
 				backButton.UpdateLayout();
 			}
+
+			// workaround for unoplatform/uno#19516 where toggling IsBackButtonVisible would lock the size of all NVIs
+			if (m_appliedTemplate && IsLoaded)
+			{
+				foreach (var ir in this.EnumerateDescendants().OfType<ItemsRepeater>())
+				{
+					ir.InvalidateMeasure();
+				}
+			}
+
 			UpdatePaneLayout();
 		}