fix(navigationview): reconcile menu items in place to preserve expansion

src/Reactor/Core/Reconciler.Mount.cs

@@ -1450,10 +1450,7 @@ private WinUI.NavigationView MountNavigationView(NavigationViewElement nav, Acti
         }
         if (nav.Content is not null) nv.Content = Mount(nav.Content, requestRerender);
         if (nav.SelectedTag is not null)
-        {
-            foreach (var mi in nv.MenuItems.OfType<WinUI.NavigationViewItem>())
-                if (mi.Tag as string == nav.SelectedTag) { nv.SelectedItem = mi; break; }
-        }
+            nv.SelectedItem = FindNavItemByTag(nv.MenuItems, nav.SelectedTag);
         SetElementTag(nv, nav);
         if (nav.OnSelectedTagChanged is not null)
             nv.SelectionChanged += (s, args) =>

src/Reactor/Core/Reconciler.Update.cs

@@ -1750,17 +1750,17 @@ void FinalizeOldPage(UIElement? oldCtrl, Element? oldElem, object? oldRoute)
         if (!double.IsNaN(n.CompactModeThresholdWidth) && nv.CompactModeThresholdWidth != n.CompactModeThresholdWidth) nv.CompactModeThresholdWidth = n.CompactModeThresholdWidth;
         if (!double.IsNaN(n.ExpandedModeThresholdWidth) && nv.ExpandedModeThresholdWidth != n.ExpandedModeThresholdWidth) nv.ExpandedModeThresholdWidth = n.ExpandedModeThresholdWidth;
 
+        // Reconcile menu items in place rather than clear-and-rebuild. The old
+        // rebuild recreated every NavigationViewItem on each render, and since
+        // per-item expansion (IsExpanded) lives only on the live container — it
+        // is not modeled in NavigationViewItemData — every re-render snapped all
+        // expanded hierarchical items shut. Because consumers typically pass a
+        // fresh MenuItems array each render (e.g. built via LINQ), the
+        // ReferenceEquals guard never held and the rebuild fired constantly,
+        // producing the "expand then collapse" flash and "child click collapses
+        // parent". Reusing containers by Tag preserves IsExpanded across renders.
         if (!ReferenceEquals(o.MenuItems, n.MenuItems))
-        {
-            nv.MenuItems.Clear();
-            foreach (var item in n.MenuItems)
-            {
-                if (item.IsHeader)
-                    nv.MenuItems.Add(new WinUI.NavigationViewItemHeader { Content = item.Content });
-                else
-                    nv.MenuItems.Add(CreateNavItem(item));
-            }
-        }
+            ReconcileNavMenuItems(nv.MenuItems, o.MenuItems, n.MenuItems);
 
         // AutoSuggestBox / PaneFooter / PaneCustomContent reconcile in place
         // when possible so the controls keep focus / scroll state across re-renders.
@@ -1835,6 +1835,123 @@ void FinalizeOldPage(UIElement? oldCtrl, Element? oldElem, object? oldRoute)
         return null;
     }
 
+    /// <summary>
+    /// Brings a live NavigationView menu-item collection into agreement with
+    /// <paramref name="newData"/> while reusing existing <see cref="WinUI.NavigationViewItem"/>
+    /// containers (matched by Tag) so their runtime <c>IsExpanded</c>/selection
+    /// state survives the re-render. Recurses into hierarchical children.
+    /// </summary>
+    private void ReconcileNavMenuItems(
+        global::System.Collections.Generic.IList<object> live,
+        NavigationViewItemData[]? oldData,
+        NavigationViewItemData[] newData)
+    {
+        // Fast path: the structure (order of headers + item Tags) is unchanged —
+        // the overwhelmingly common case, since menus are usually static. Update
+        // each container in place without detaching anything, which keeps every
+        // container's expansion, selection and animation state pristine.
+        if (NavStructureMatches(live, newData))
+        {
+            for (int i = 0; i < newData.Length; i++)
+            {
+                var data = newData[i];
+                if (data.IsHeader)
+                {
+                    if (live[i] is WinUI.NavigationViewItemHeader h && !Equals(h.Content, data.Content))
+                        h.Content = data.Content;
+                }
+                else if (live[i] is WinUI.NavigationViewItem nvi)
+                {
+                    var oldItem = oldData is not null && i < oldData.Length ? oldData[i] : null;
+                    UpdateNavItemInPlace(nvi, oldItem, data);
+                }
+            }
+            return;
+        }
+
+        // Structure changed (items added / removed / reordered): snapshot the
+        // existing containers by Tag so matches can be reused, then rebuild the
+        // collection in the new order. Reused containers retain their IsExpanded.
+        var reusable = new global::System.Collections.Generic.Dictionary<string, WinUI.NavigationViewItem>();
+        foreach (var item in live)
+            if (item is WinUI.NavigationViewItem nvi && nvi.Tag is string tag)
+                reusable[tag] = nvi;
+
+        var oldByTag = new global::System.Collections.Generic.Dictionary<string, NavigationViewItemData>();
+        if (oldData is not null)
+            foreach (var d in oldData)
+                if (!d.IsHeader)
+                    oldByTag[d.Tag ?? d.Content] = d;
+
+        live.Clear();
+        foreach (var data in newData)
+        {
+            if (data.IsHeader)
+            {
+                live.Add(new WinUI.NavigationViewItemHeader { Content = data.Content });
+                continue;
+            }
+
+            var key = data.Tag ?? data.Content;
+            if (reusable.TryGetValue(key, out var nvi))
+                UpdateNavItemInPlace(nvi, oldByTag.GetValueOrDefault(key), data);
+            else
+                nvi = CreateNavItem(data);
+            live.Add(nvi);
+        }
+    }
+
+    /// <summary>True when the live collection already matches the new data in
+    /// count and in the sequence of headers / item Tags.</summary>
+    private static bool NavStructureMatches(
+        global::System.Collections.Generic.IList<object> live,
+        NavigationViewItemData[] newData)
+    {
+        if (live.Count != newData.Length) return false;
+        for (int i = 0; i < newData.Length; i++)
+        {
+            var data = newData[i];
+            if (data.IsHeader)
+            {
+                if (live[i] is not WinUI.NavigationViewItemHeader) return false;
+            }
+            else
+            {
+                if (live[i] is not WinUI.NavigationViewItem nvi) return false;
+                if ((nvi.Tag as string) != (data.Tag ?? data.Content)) return false;
+            }
+        }
+        return true;
+    }
+
+    /// <summary>Updates a reused NavigationViewItem's Content/Icon (only when
+    /// changed) and reconciles its children, leaving IsExpanded untouched.</summary>
+    private void UpdateNavItemInPlace(WinUI.NavigationViewItem nvi, NavigationViewItemData? oldData, NavigationViewItemData data)
+    {
+        if (!Equals(nvi.Content, data.Content)) nvi.Content = data.Content;
+
+        var newTag = data.Tag ?? data.Content;
+        if (!Equals(nvi.Tag, newTag)) nvi.Tag = newTag;
+
+        // Re-resolve the icon only when the icon data actually changed (IconData
+        // records compare by value), so the FontIcon/SymbolIcon isn't reallocated
+        // on every render.
+        bool iconChanged = oldData is null
+            || !Equals(oldData.IconElement, data.IconElement)
+            || oldData.Icon != data.Icon;
+        if (iconChanged)
+        {
+            var icon = ResolveIcon(data.IconElement, data.Icon);
+            if (icon is not null) nvi.Icon = icon;
+            else if (nvi.Icon is not null) nvi.Icon = null;
+        }
+
+        if (data.Children is { Length: > 0 } children)
+            ReconcileNavMenuItems(nvi.MenuItems, oldData?.Children, children);
+        else if (nvi.MenuItems.Count > 0)
+            nvi.MenuItems.Clear();
+    }
+
     private UIElement? UpdateTitleBar(TitleBarElement o, TitleBarElement n, WinUI.TitleBar titleBar, Action requestRerender)
     {
         titleBar.Title = n.Title;

src/Reactor/Core/V1Protocol/Descriptor/Descriptors/NavigationViewDescriptor.cs

@@ -123,8 +123,9 @@ internal static class NavigationViewDescriptor
 
     private static void ApplyMenuAndSelection(WinUI.NavigationView control, NavigationViewElement? oldElement, NavigationViewElement element)
     {
-        if (oldElement is null || !ReferenceEquals(oldElement.MenuItems, element.MenuItems))
+        if (oldElement is null)
         {
+            // Mount: build fresh.
             control.MenuItems.Clear();
             foreach (var item in element.MenuItems)
             {
@@ -133,6 +134,13 @@ private static void ApplyMenuAndSelection(WinUI.NavigationView control, Navigati
                     : CreateNavItem(item));
             }
         }
+        else if (!ReferenceEquals(oldElement.MenuItems, element.MenuItems))
+        {
+            // Update: reconcile in place so reused containers keep IsExpanded.
+            // Mirrors Reconciler.ReconcileNavMenuItems (legacy arm) — see the note
+            // there on why a clear-and-rebuild collapses hierarchical items.
+            ReconcileMenuItems(control.MenuItems, oldElement.MenuItems, element.MenuItems);
+        }
 
         if (oldElement is null
             || oldElement.SelectedTag != element.SelectedTag
@@ -142,6 +150,107 @@ private static void ApplyMenuAndSelection(WinUI.NavigationView control, Navigati
         }
     }
 
+    private static void ReconcileMenuItems(
+        global::System.Collections.Generic.IList<object> live,
+        NavigationViewItemData[]? oldData,
+        NavigationViewItemData[] newData)
+    {
+        if (StructureMatches(live, newData))
+        {
+            for (int i = 0; i < newData.Length; i++)
+            {
+                var data = newData[i];
+                if (data.IsHeader)
+                {
+                    if (live[i] is WinUI.NavigationViewItemHeader h && !Equals(h.Content, data.Content))
+                        h.Content = data.Content;
+                }
+                else if (live[i] is WinUI.NavigationViewItem nvi)
+                {
+                    var oldItem = oldData is not null && i < oldData.Length ? oldData[i] : null;
+                    UpdateNavItemInPlace(nvi, oldItem, data);
+                }
+            }
+            return;
+        }
+
+        var reusable = new global::System.Collections.Generic.Dictionary<string, WinUI.NavigationViewItem>();
+        foreach (var item in live)
+            if (item is WinUI.NavigationViewItem nvi && nvi.Tag is string tag)
+                reusable[tag] = nvi;
+
+        var oldByTag = new global::System.Collections.Generic.Dictionary<string, NavigationViewItemData>();
+        if (oldData is not null)
+            foreach (var d in oldData)
+                if (!d.IsHeader)
+                    oldByTag[d.Tag ?? d.Content] = d;
+
+        live.Clear();
+        foreach (var data in newData)
+        {
+            if (data.IsHeader)
+            {
+                live.Add(new WinUI.NavigationViewItemHeader { Content = data.Content });
+                continue;
+            }
+
+            var key = data.Tag ?? data.Content;
+            if (reusable.TryGetValue(key, out var nvi))
+                UpdateNavItemInPlace(nvi, oldByTag.GetValueOrDefault(key), data);
+            else
+                nvi = CreateNavItem(data);
+            live.Add(nvi);
+        }
+    }
+
+    private static bool StructureMatches(
+        global::System.Collections.Generic.IList<object> live,
+        NavigationViewItemData[] newData)
+    {
+        if (live.Count != newData.Length) return false;
+        for (int i = 0; i < newData.Length; i++)
+        {
+            var data = newData[i];
+            if (data.IsHeader)
+            {
+                if (live[i] is not WinUI.NavigationViewItemHeader) return false;
+            }
+            else
+            {
+                if (live[i] is not WinUI.NavigationViewItem nvi) return false;
+                if ((nvi.Tag as string) != (data.Tag ?? data.Content)) return false;
+            }
+        }
+        return true;
+    }
+
+    private static void UpdateNavItemInPlace(WinUI.NavigationViewItem nvi, NavigationViewItemData? oldData, NavigationViewItemData data)
+    {
+        if (!Equals(nvi.Content, data.Content)) nvi.Content = data.Content;
+
+        var newTag = data.Tag ?? data.Content;
+        if (!Equals(nvi.Tag, newTag)) nvi.Tag = newTag;
+
+        bool iconChanged = oldData is null
+            || !Equals(oldData.IconElement, data.IconElement)
+            || oldData.Icon != data.Icon;
+        if (iconChanged)
+        {
+            var icon = data.IconElement is not null
+                ? Reconciler.ResolveIconForDescriptor(data.IconElement)
+                : data.Icon is not null
+                    ? Reconciler.ResolveIconForDescriptor(new SymbolIconData(data.Icon))
+                    : null;
+            if (icon is not null) nvi.Icon = icon;
+            else if (nvi.Icon is not null) nvi.Icon = null;
+        }
+
+        if (data.Children is { Length: > 0 } children)
+            ReconcileMenuItems(nvi.MenuItems, oldData?.Children, children);
+        else if (nvi.MenuItems.Count > 0)
+            nvi.MenuItems.Clear();
+    }
+
     private static WinUI.NavigationViewItem CreateNavItem(NavigationViewItemData data)
     {
         var item = new WinUI.NavigationViewItem { Content = data.Content, Tag = data.Tag ?? data.Content };

tests/Reactor.AppTests.Host/FixtureRegistry.cs

@@ -61,9 +61,17 @@ internal static class FixtureRegistry
         // Collections
         "ListView_TypedRendering",
 
+        // TreeView expand/collapse (mirrors ReactorGallery Basic TreeView —
+        // E2E repro/guard for the item-body-collapse bug)
+        "TreeView_BasicTextTree",
+
         // Navigation
         "Navigation_TabSwitching",
 
+        // NavigationView hierarchical expand/collapse (E2E repro/guard for the
+        // re-render rebuild-clobber that collapsed expanded categories)
+        "NavigationView_Hierarchical",
+
         // Observable
         "Observable_UseObservable_Rerender",
         "Observable_UseObservable_ExternalMutation",
@@ -205,9 +213,15 @@ internal static class FixtureRegistry
         // Collections
         "ListView_TypedRendering" => CollectionFixtures.ListViewTyped(ctx),
 
+        // TreeView expand/collapse
+        "TreeView_BasicTextTree" => TreeViewE2EFixtures.BasicTextTree(ctx),
+
         // Navigation
         "Navigation_TabSwitching" => NavigationFixtures.TabSwitching(ctx),
 
+        // NavigationView hierarchical expand/collapse
+        "NavigationView_Hierarchical" => NavigationViewE2EFixtures.HierarchicalNav(ctx),
+
         // Observable
         "Observable_UseObservable_Rerender" => ObservableFixtures.UseObservable_Rerender(ctx),
         "Observable_UseObservable_ExternalMutation" => ObservableFixtures.UseObservable_ExternalMutation(ctx),

tests/Reactor.AppTests.Host/Fixtures/NavigationViewE2EFixtures.cs

@@ -0,0 +1,66 @@
+using Microsoft.UI.Reactor;
+using Microsoft.UI.Reactor.Core;
+using Microsoft.UI.Xaml.Controls;
+using static Microsoft.UI.Reactor.Factories;
+
+namespace Microsoft.UI.Reactor.AppTests.Host.Fixtures;
+
+/// <summary>
+/// E2E fixtures for hierarchical <c>NavigationView</c> expand/collapse, mirroring
+/// the ReactorGallery shell shape: a stateful component that holds the selected
+/// tag, re-renders on every selection, and rebuilds its <c>MenuItems</c> array
+/// fresh each render (the condition that made the old clear-and-rebuild update
+/// path collapse expanded categories on every re-render).
+///
+/// PaneDisplayMode is pinned to <c>Left</c> so the pane stays open with text
+/// labels (addressable by UIA Name) and children render inline when expanded,
+/// regardless of the host window size.
+/// </summary>
+internal static class NavigationViewE2EFixtures
+{
+    internal class HierarchicalNavComponent : Component
+    {
+        public override Element Render()
+        {
+            var (selectedTag, setSelectedTag) = UseState("alpha-1");
+
+            // Rebuilt fresh every render — exactly the pattern the Gallery uses
+            // and the one that exposed the rebuild-clobber bug.
+            var menuItems = new[]
+            {
+                NavItem("Alpha", tag: "alpha") with
+                {
+                    Children = new[]
+                    {
+                        NavItem("Alpha-1", tag: "alpha-1"),
+                        NavItem("Alpha-2", tag: "alpha-2"),
+                    }
+                },
+                NavItem("Bravo", tag: "bravo") with
+                {
+                    Children = new[]
+                    {
+                        NavItem("Bravo-1", tag: "bravo-1"),
+                    }
+                },
+            };
+
+            return NavigationView(
+                menuItems,
+                content: TextBlock($"Selected: {selectedTag}").AutomationId("NavSelectedTag")
+            ) with
+            {
+                SelectedTag = selectedTag,
+                PaneDisplayMode = NavigationViewPaneDisplayMode.Left,
+                IsSettingsVisible = false,
+                OnSelectedTagChanged = tag =>
+                {
+                    if (tag != null) setSelectedTag(tag);
+                },
+            };
+        }
+    }
+
+    internal static Element HierarchicalNav(RenderContext ctx) =>
+        Component<HierarchicalNavComponent>();
+}