From f0784284cfb64d2e7022b57c6412965973cdf781 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Wed, 9 Jul 2025 17:25:10 +0200
Subject: [PATCH 1/3] SSR streaming not started with custom router.

---
 .../Components/src/NavigationManager.cs       |   4 +-
 .../Components/src/PublicAPI.Unshipped.txt    |   5 +-
 .../src/Routing/NotFoundEventArgs.cs          |  13 +-
 .../Components/src/Routing/Router.cs          |   7 +-
 .../src/RazorComponentEndpointInvoker.cs      |  11 ++
 .../EndpointHtmlRenderer.EventDispatch.cs     |   2 +-
 .../src/Rendering/EndpointHtmlRenderer.cs     |   2 +
 .../test/EndpointHtmlRendererTest.cs          |   2 +-
 .../NoInteractivityTest.cs                    |  34 ++++
 .../RazorComponents/App.razor                 |  31 +++-
 .../RazorComponents/CustomRouter.cs           | 171 ++++++++++++++++++
 11 files changed, 256 insertions(+), 26 deletions(-)
 create mode 100644 src/Components/test/testassets/Components.TestServer/RazorComponents/CustomRouter.cs

diff --git a/src/Components/Components/src/NavigationManager.cs b/src/Components/Components/src/NavigationManager.cs
index 1900b91c629b..ff14da1adbb9 100644
--- a/src/Components/Components/src/NavigationManager.cs
+++ b/src/Components/Components/src/NavigationManager.cs
@@ -61,7 +61,7 @@ public event EventHandler<NotFoundEventArgs> OnNotFound
     // The URI. Always represented an absolute URI.
     private string? _uri;
     private bool _isInitialized;
-    internal string NotFoundPageRoute { get; set; } = string.Empty;
+    private readonly NotFoundEventArgs _notFoundEventArgs = new();
 
     /// <summary>
     /// Gets or sets the current base URI. The <see cref="BaseUri" /> is always represented as an absolute URI in string form with trailing slash.
@@ -211,7 +211,7 @@ private void NotFoundCore()
         }
         else
         {
-            _notFound.Invoke(this, new NotFoundEventArgs(NotFoundPageRoute));
+            _notFound.Invoke(this, _notFoundEventArgs);
         }
     }
 
diff --git a/src/Components/Components/src/PublicAPI.Unshipped.txt b/src/Components/Components/src/PublicAPI.Unshipped.txt
index dc07b8afddac..b69167202fee 100644
--- a/src/Components/Components/src/PublicAPI.Unshipped.txt
+++ b/src/Components/Components/src/PublicAPI.Unshipped.txt
@@ -8,8 +8,9 @@ Microsoft.AspNetCore.Components.NavigationManager.OnNotFound -> System.EventHand
 Microsoft.AspNetCore.Components.NavigationManager.NotFound() -> void
 Microsoft.AspNetCore.Components.Routing.IHostEnvironmentNavigationManager.Initialize(string! baseUri, string! uri, System.Func<string!, System.Threading.Tasks.Task!>! onNavigateTo) -> void
 Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs
-Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundEventArgs(string! url) -> void
-Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.Path.get -> string!
+Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundEventArgs() -> void
+Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.Path.get -> string?
+Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.Path.set -> void
 Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager.ComponentStatePersistenceManager(Microsoft.Extensions.Logging.ILogger<Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager!>! logger, System.IServiceProvider! serviceProvider) -> void
 Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager.SetPlatformRenderMode(Microsoft.AspNetCore.Components.IComponentRenderMode! renderMode) -> void
 Microsoft.AspNetCore.Components.Infrastructure.RegisterPersistentComponentStateServiceCollectionExtensions
diff --git a/src/Components/Components/src/Routing/NotFoundEventArgs.cs b/src/Components/Components/src/Routing/NotFoundEventArgs.cs
index e1e81e5cfc82..9263ed62a3ab 100644
--- a/src/Components/Components/src/Routing/NotFoundEventArgs.cs
+++ b/src/Components/Components/src/Routing/NotFoundEventArgs.cs
@@ -9,16 +9,7 @@ namespace Microsoft.AspNetCore.Components.Routing;
 public sealed class NotFoundEventArgs : EventArgs
 {
     /// <summary>
-    /// Gets the path of NotFoundPage.
+    /// Gets the path of NotFoundPage. If the path is set, it indicates that the router has handled the rendering of the NotFound contents.
     /// </summary>
-    public string Path { get; }
-
-    /// <summary>
-    /// Initializes a new instance of <see cref="NotFoundEventArgs" />.
-    /// </summary>
-    public NotFoundEventArgs(string url)
-    {
-        Path = url;
-    }
-
+    public string? Path { get; set; }
 }
diff --git a/src/Components/Components/src/Routing/Router.cs b/src/Components/Components/src/Routing/Router.cs
index eedff373f656..667583389f86 100644
--- a/src/Components/Components/src/Routing/Router.cs
+++ b/src/Components/Components/src/Routing/Router.cs
@@ -29,6 +29,7 @@ static readonly IReadOnlyDictionary<string, object> _emptyParametersDictionary
     string _locationAbsolute;
     bool _navigationInterceptionEnabled;
     ILogger<Router> _logger;
+    string _notFoundPageRoute;
 
     private string _updateScrollPositionForHashLastLocation;
     private bool _updateScrollPositionForHash;
@@ -159,7 +160,7 @@ public async Task SetParametersAsync(ParameterView parameters)
             var routeAttribute = (RouteAttribute)routeAttributes[0];
             if (routeAttribute.Template != null)
             {
-                NavigationManager.NotFoundPageRoute = routeAttribute.Template;
+                _notFoundPageRoute = routeAttribute.Template;
             }
         }
 
@@ -381,12 +382,14 @@ private void OnLocationChanged(object sender, LocationChangedEventArgs args)
         }
     }
 
-    private void OnNotFound(object sender, EventArgs args)
+    private void OnNotFound(object sender, NotFoundEventArgs args)
     {
         if (_renderHandle.IsInitialized)
         {
             Log.DisplayingNotFound(_logger);
             RenderNotFound();
+            // setting the path signals to the endpoint renderer that router handled rendering
+            args.Path = _notFoundPageRoute ?? "not-found-handled-by-router";
         }
     }
 
diff --git a/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs b/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
index 8e5338f54788..ee9385c6cc1a 100644
--- a/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
+++ b/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
@@ -168,6 +168,17 @@ await _renderer.InitializeStandardComponentServicesAsync(
             componentStateHtmlContent.WriteTo(bufferWriter, HtmlEncoder.Default);
         }
 
+        if (context.Response.StatusCode == StatusCodes.Status404NotFound &&
+            !isReExecuted &&
+            string.IsNullOrEmpty(_renderer.NotFoundEventArgs?.Path))
+        {
+            // Router did not handle the NotFound event, otherwise this would not be empty.
+            // Don't flush the response if we have an unhandled 404 rendering
+            // This will allow the StatusCodePages middleware to re-execute the request
+            context.Response.ContentType = null;
+            return;
+        }
+
         // Invoke FlushAsync to ensure any buffered content is asynchronously written to the underlying
         // response asynchronously. In the absence of this line, the buffer gets synchronously written to the
         // response as part of the Dispose which has a perf impact.
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index cdf17e376a00..f0060c25914f 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -109,7 +109,7 @@ internal async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs a
 
     private string GetNotFoundUrl(string baseUri, NotFoundEventArgs args)
     {
-        string path = args.Path;
+        string? path = args.Path;
         if (string.IsNullOrEmpty(path))
         {
             var pathFormat = _httpContext.Items[nameof(StatusCodePagesOptions)] as string;
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
index f5d0699e1efe..7214516348b0 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
@@ -63,6 +63,7 @@ public EndpointHtmlRenderer(IServiceProvider serviceProvider, ILoggerFactory log
     }
 
     internal HttpContext? HttpContext => _httpContext;
+    internal NotFoundEventArgs? NotFoundEventArgs { get; set; }
 
     internal void SetHttpContext(HttpContext httpContext)
     {
@@ -87,6 +88,7 @@ internal async Task InitializeStandardComponentServicesAsync(
 
         navigationManager?.OnNotFound += (sender, args) =>
         {
+            NotFoundEventArgs = args;
             _ = GetErrorHandledTask(SetNotFoundResponseAsync(navigationManager.BaseUri, args));
         };
 
diff --git a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
index 3c27365169cb..caa5d1c14c1c 100644
--- a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
+++ b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
@@ -944,7 +944,7 @@ public async Task Renderer_WhenNoNotFoundPathProvided_Throws()
         httpContext.Items[nameof(StatusCodePagesOptions)] = null; // simulate missing re-execution route
 
         var exception = await Assert.ThrowsAsync<InvalidOperationException>(async () =>
-            await renderer.SetNotFoundResponseAsync(httpContext, new NotFoundEventArgs(""))
+            await renderer.SetNotFoundResponseAsync(httpContext, new NotFoundEventArgs())
         );
         string expectedError = "The NotFoundPage route must be specified or re-execution middleware has to be set to render NotFoundPage when the response has started.";
 
diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index bbeb1ad4d9bb..c90ea78845e2 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -117,6 +117,16 @@ public void BrowserNavigationToNotExistingPathReExecutesTo404(bool streaming)
     private void AssertReExecutionPageRendered() =>
         Browser.Equal("Welcome On Page Re-executed After Not Found Event", () => Browser.Exists(By.Id("test-info")).Text);
 
+    private void AssertBrowserDefaultNotFoundViewRendered()
+    {
+        var mainMessage = Browser.FindElement(By.Id("main-message"));
+
+        Browser.True(
+            () => mainMessage.FindElement(By.CssSelector("p")).Text
+            .Contains("No webpage was found for the web address:", StringComparison.OrdinalIgnoreCase)
+        );
+    }
+
     private void AssertNotFoundPageRendered()
     {
         Browser.Equal("Welcome On Custom Not Found Page", () => Browser.FindElement(By.Id("test-info")).Text);
@@ -152,6 +162,30 @@ public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecuti
         AssertUrlNotChanged(testUrl);
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    // our custom router does not support NotFoundPage nor NotFound fragment to simulate most probable custom router behavior
+    public void NotFoundSetOnInitialization_ResponseNotStarted_CustomRouter_SSR(bool hasReExecutionMiddleware)
+    {
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr?useCustomRouter=true";
+        Navigate(testUrl);
+
+        if (hasReExecutionMiddleware)
+        {
+            AssertReExecutionPageRendered();
+        }
+        else
+        {
+            // Apps that don't support re-execution and don't have blazor's router,
+            // cannot render custom NotFound contents.
+            // The browser will display default 404 page.
+            AssertBrowserDefaultNotFoundViewRendered();
+        }
+        AssertUrlNotChanged(testUrl);
+    }
+
     [Theory]
     [InlineData(true, true)]
     [InlineData(true, false)]
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
index 2219da3955d2..457fcb518f4c 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
@@ -1,12 +1,17 @@
 @using Components.TestServer.RazorComponents.Pages.Forms
 @using Components.WasmMinimal.Pages.NotFound
 @using TestContentPackage.NotFound
+@using Components.TestServer.RazorComponents
 
 @code {
     [Parameter]
     [SupplyParameterFromQuery(Name = "useCustomNotFoundPage")]
     public string? UseCustomNotFoundPage { get; set; }
 
+    [Parameter]
+    [SupplyParameterFromQuery(Name = "useCustomRouter")]
+    public string? UseCustomRouter { get; set; }
+
     private Type? NotFoundPageType { get; set; }
 
     protected override void OnParametersSet()
@@ -30,13 +35,25 @@
     <HeadOutlet />
 </head>
 <body>
-    <Router AppAssembly="@typeof(App).Assembly" AdditionalAssemblies="new[] { typeof(TestContentPackage.NotFound.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
-        <Found Context="routeData">
-            <RouteView RouteData="@routeData" />
-            <FocusOnNavigate RouteData="@routeData" Selector="[data-focus-on-navigate]" />
-        </Found>
-        <NotFound><p id="not-found-fragment">There's nothing here</p></NotFound>
-    </Router>
+    @if(string.Equals(UseCustomRouter, "true", StringComparison.OrdinalIgnoreCase))
+    {
+        <CustomRouter AppAssembly="@typeof(App).Assembly">
+            <Found Context="routeData">
+                <RouteView RouteData="@routeData" />
+                <FocusOnNavigate RouteData="@routeData" Selector="[data-focus-on-navigate]" />
+            </Found>
+        </CustomRouter>
+    }
+    else
+    {
+        <Router AppAssembly="@typeof(App).Assembly" AdditionalAssemblies="new[] { typeof(TestContentPackage.NotFound.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
+            <Found Context="routeData">
+                <RouteView RouteData="@routeData" />
+                <FocusOnNavigate RouteData="@routeData" Selector="[data-focus-on-navigate]" />
+            </Found>
+            <NotFound><p id="not-found-fragment">There's nothing here</p></NotFound>
+        </Router>
+    }
     <script>
         // This script must come before blazor.web.js to test that
         // the framework does the right thing when an element is already focused.
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/CustomRouter.cs b/src/Components/test/testassets/Components.TestServer/RazorComponents/CustomRouter.cs
new file mode 100644
index 000000000000..0f6fcce2f931
--- /dev/null
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/CustomRouter.cs
@@ -0,0 +1,171 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.AspNetCore.Components;
+using Microsoft.AspNetCore.Components.Routing;
+using Microsoft.Extensions.Logging;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Threading.Tasks;
+
+namespace Components.TestServer.RazorComponents;
+
+/// <summary>
+/// A custom router implementation for testing NavigationManager.NotFound() with custom routers.
+/// This implementation matches the built-in Router component's behavior but with a different implementation
+/// to ensure custom routers can properly handle NotFound events.
+/// </summary>
+public class CustomRouter : IComponent, IHandleAfterRender, IDisposable
+{
+    private RenderHandle _renderHandle;
+    private bool _navigationInterceptionEnabled;
+    private ILogger<CustomRouter> _logger = default!;
+    private NavigationManager _navigationManager = default!;
+    private INavigationInterception _navigationInterception = default!;
+
+    [Inject] private IRoutingStateProvider? RoutingStateProvider { get; set; }
+
+    [Inject]
+    private NavigationManager NavigationManager
+    {
+        get => _navigationManager;
+        set
+        {
+            _navigationManager = value;
+            _navigationManager.LocationChanged += OnLocationChanged;
+        }
+    }
+
+    [Inject] private INavigationInterception NavigationInterception
+    {
+        get => _navigationInterception;
+        set => _navigationInterception = value;
+    }
+
+    [Inject] private ILoggerFactory LoggerFactory
+    {
+        get => throw new InvalidOperationException("This property should only be set via injection.");
+        set => _logger = value.CreateLogger<CustomRouter>();
+    }
+
+    [Parameter]
+    [EditorRequired]
+    public Assembly AppAssembly { get; set; } = default!;
+
+    [Parameter] public IEnumerable<Assembly> AdditionalAssemblies { get; set; } = default!;
+
+    [Parameter]
+    [EditorRequired]
+    public RenderFragment<Microsoft.AspNetCore.Components.RouteData> Found { get; set; } = default!;
+
+    [Parameter] public bool PreferExactMatches { get; set; }
+
+    public void Attach(RenderHandle renderHandle)
+    {
+        _renderHandle = renderHandle;
+    }
+
+    public Task SetParametersAsync(ParameterView parameters)
+    {
+        parameters.SetParameterProperties(this);
+
+        if (AppAssembly is null)
+        {
+            throw new InvalidOperationException($"The {nameof(CustomRouter)} component requires a value for the parameter {nameof(AppAssembly)}.");
+        }
+
+        if (Found is null)
+        {
+            throw new InvalidOperationException($"The {nameof(CustomRouter)} component requires a value for the parameter {nameof(Found)}.");
+        }
+
+        RefreshRouteTable();
+        RefreshComponent();
+
+        return Task.CompletedTask;
+    }
+
+    public void Dispose()
+    {
+        NavigationManager.LocationChanged -= OnLocationChanged;
+    }
+
+    private RouteTable? _routeTable;
+
+    [Inject] private IServiceProvider ServiceProvider { get; set; } = default!;
+
+    private void RefreshRouteTable()
+    {
+        var routeKey = new RouteKey(AppAssembly, AdditionalAssemblies);
+        _routeTable = RouteTableFactory.Instance.Create(routeKey, ServiceProvider);
+    }
+
+    private void OnLocationChanged(object? sender, LocationChangedEventArgs args)
+    {
+        RefreshComponent();
+    }
+
+    private void RefreshComponent()
+    {
+        if (_renderHandle.IsInitialized)
+        {
+            var uri = NavigationManager.Uri;
+            var locationPath = NavigationManager.ToBaseRelativePath(uri);
+            locationPath = StringUntilAny(locationPath, '?', '#');
+            var locationPathWithLeadingSlash = $"/{locationPath}";
+
+            // First check if the routing state provider has route data for us
+            if (RoutingStateProvider?.RouteData is { } routeData)
+            {
+                _logger.LogInformation($"CustomRouter: Using route from RoutingStateProvider for component {routeData.PageType}");
+
+                // Process parameters to ensure proper handling of route constraints and optional parameters
+                var processedRouteData = RouteTable.ProcessParameters(routeData);
+
+                _renderHandle.Render(Found(processedRouteData));
+                return;
+            }
+
+            // Otherwise, use our route table to find a matching route
+            var context = new Microsoft.AspNetCore.Components.Routing.RouteContext(locationPathWithLeadingSlash);
+            _routeTable?.Route(context);
+
+            if (context.Handler != null)
+            {
+                _logger.LogInformation($"CustomRouter: Navigating to component {context.Handler}");
+                var matchedRouteData = new Microsoft.AspNetCore.Components.RouteData(
+                    context.Handler,
+                    context.Parameters ?? new Dictionary<string, object?>());
+
+                // Set the template to maintain route information
+                if (context.Entry != null)
+                {
+                    matchedRouteData.Template = context.Entry.RoutePattern.RawText;
+                }
+
+                _renderHandle.Render(Found(matchedRouteData));
+            }
+        }
+    }
+
+    private static string StringUntilAny(string str, params char[] chars)
+    {
+        var firstIndex = str.IndexOfAny(chars);
+        return firstIndex < 0
+            ? str
+            : str.Substring(0, firstIndex);
+    }
+
+    Task IHandleAfterRender.OnAfterRenderAsync()
+    {
+        if (!_navigationInterceptionEnabled)
+        {
+            _navigationInterceptionEnabled = true;
+            return NavigationInterception.EnableNavigationInterceptionAsync();
+        }
+
+        return Task.CompletedTask;
+    }
+}

From 3ff48feffc6312e9a5e5d6ea625d88876d13d3e0 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 11 Jul 2025 19:30:48 +0200
Subject: [PATCH 2/3] Fix: move setting response dispatch to invoker.

---
 .../Components/src/Routing/Router.cs          |  6 +--
 .../src/RazorComponentEndpointInvoker.cs      |  9 ++++
 .../EndpointHtmlRenderer.EventDispatch.cs     | 41 ++++++++++---------
 .../src/Rendering/EndpointHtmlRenderer.cs     |  8 +---
 .../test/EndpointHtmlRendererTest.cs          |  2 +-
 .../NoInteractivityTest.cs                    |  2 +-
 6 files changed, 37 insertions(+), 31 deletions(-)

diff --git a/src/Components/Components/src/Routing/Router.cs b/src/Components/Components/src/Routing/Router.cs
index 667583389f86..ccab45f8e5f2 100644
--- a/src/Components/Components/src/Routing/Router.cs
+++ b/src/Components/Components/src/Routing/Router.cs
@@ -384,12 +384,12 @@ private void OnLocationChanged(object sender, LocationChangedEventArgs args)
 
     private void OnNotFound(object sender, NotFoundEventArgs args)
     {
-        if (_renderHandle.IsInitialized)
+        if (_renderHandle.IsInitialized && NotFoundPage != null)
         {
+            // setting the path signals to the endpoint renderer that router handled rendering
+            args.Path = _notFoundPageRoute;
             Log.DisplayingNotFound(_logger);
             RenderNotFound();
-            // setting the path signals to the endpoint renderer that router handled rendering
-            args.Path = _notFoundPageRoute ?? "not-found-handled-by-router";
         }
     }
 
diff --git a/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs b/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
index ee9385c6cc1a..c02595f6e454 100644
--- a/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
+++ b/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
@@ -136,6 +136,11 @@ await _renderer.InitializeStandardComponentServicesAsync(
             }
         }
 
+        if (_renderer.NotFoundEventArgs != null)
+        {
+            _renderer.SetNotFoundWhenResponseNotStarted();
+        }
+
         if (!quiesceTask.IsCompleted)
         {
             // An incomplete QuiescenceTask indicates there may be streaming rendering updates.
@@ -155,6 +160,10 @@ await _renderer.InitializeStandardComponentServicesAsync(
         if (!quiesceTask.IsCompletedSuccessfully)
         {
             await _renderer.SendStreamingUpdatesAsync(context, quiesceTask, bufferWriter);
+            if (_renderer.NotFoundEventArgs != null)
+            {
+                await _renderer.SetNotFoundWhenResponseHasStarted();
+            }
         }
         else
         {
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index f0060c25914f..b1c9621888b0 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -79,27 +79,28 @@ private Task ReturnErrorResponse(string detailedMessage)
             : Task.CompletedTask;
     }
 
-    internal async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs args)
+    internal void SetNotFoundWhenResponseNotStarted()
     {
-        if (_httpContext.Response.HasStarted ||
-            // POST waits for quiescence -> rendering the NotFoundPage would be queued for the next batch
-            // but we want to send the signal to the renderer to stop rendering future batches -> use client rendering
-            HttpMethods.IsPost(_httpContext.Request.Method))
-        {
-            if (string.IsNullOrEmpty(_notFoundUrl))
-            {
-                _notFoundUrl = GetNotFoundUrl(baseUri, args);
-            }
-            var defaultBufferSize = 16 * 1024;
-            await using var writer = new HttpResponseStreamWriter(_httpContext.Response.Body, Encoding.UTF8, defaultBufferSize, ArrayPool<byte>.Shared, ArrayPool<char>.Shared);
-            using var bufferWriter = new BufferedTextWriter(writer);
-            HandleNotFoundAfterResponseStarted(bufferWriter, _httpContext, _notFoundUrl);
-            await bufferWriter.FlushAsync();
-        }
-        else
+        _httpContext.Response.StatusCode = StatusCodes.Status404NotFound;
+
+        // When the application triggers a NotFound event, we continue rendering the current batch.
+        // However, after completing this batch, we do not want to process any further UI updates,
+        // as we are going to return a 404 status and discard the UI updates generated so far.
+        SignalRendererToFinishRendering();
+    }
+
+    internal async Task SetNotFoundWhenResponseHasStarted()
+    {
+        if (string.IsNullOrEmpty(_notFoundUrl))
         {
-            _httpContext.Response.StatusCode = StatusCodes.Status404NotFound;
+            var baseUri = $"{_httpContext.Request.Scheme}://{_httpContext.Request.Host}{_httpContext.Request.PathBase}/";
+            _notFoundUrl = GetNotFoundUrl(baseUri, NotFoundEventArgs);
         }
+        var defaultBufferSize = 16 * 1024;
+        await using var writer = new HttpResponseStreamWriter(_httpContext.Response.Body, Encoding.UTF8, defaultBufferSize, ArrayPool<byte>.Shared, ArrayPool<char>.Shared);
+        using var bufferWriter = new BufferedTextWriter(writer);
+        HandleNotFoundAfterResponseStarted(bufferWriter, _httpContext, _notFoundUrl);
+        await bufferWriter.FlushAsync();
 
         // When the application triggers a NotFound event, we continue rendering the current batch.
         // However, after completing this batch, we do not want to process any further UI updates,
@@ -107,9 +108,9 @@ internal async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs a
         SignalRendererToFinishRendering();
     }
 
-    private string GetNotFoundUrl(string baseUri, NotFoundEventArgs args)
+    private string GetNotFoundUrl(string baseUri, NotFoundEventArgs? args)
     {
-        string? path = args.Path;
+        string? path = args?.Path;
         if (string.IsNullOrEmpty(path))
         {
             var pathFormat = _httpContext.Items[nameof(StatusCodePagesOptions)] as string;
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
index 7214516348b0..5a7d38941b23 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
@@ -63,7 +63,7 @@ public EndpointHtmlRenderer(IServiceProvider serviceProvider, ILoggerFactory log
     }
 
     internal HttpContext? HttpContext => _httpContext;
-    internal NotFoundEventArgs? NotFoundEventArgs { get; set; }
+    internal NotFoundEventArgs? NotFoundEventArgs { get; private set; }
 
     internal void SetHttpContext(HttpContext httpContext)
     {
@@ -86,11 +86,7 @@ internal async Task InitializeStandardComponentServicesAsync(
         var navigationManager = httpContext.RequestServices.GetRequiredService<NavigationManager>();
         ((IHostEnvironmentNavigationManager)navigationManager)?.Initialize(GetContextBaseUri(httpContext.Request), GetFullUri(httpContext.Request), OnNavigateTo);
 
-        navigationManager?.OnNotFound += (sender, args) =>
-        {
-            NotFoundEventArgs = args;
-            _ = GetErrorHandledTask(SetNotFoundResponseAsync(navigationManager.BaseUri, args));
-        };
+        navigationManager?.OnNotFound += (sender, args) => NotFoundEventArgs = args;
 
         var authenticationStateProvider = httpContext.RequestServices.GetService<AuthenticationStateProvider>();
         if (authenticationStateProvider is IHostEnvironmentAuthenticationStateProvider hostEnvironmentAuthenticationStateProvider)
diff --git a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
index caa5d1c14c1c..d7c2e16e80b2 100644
--- a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
+++ b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
@@ -1823,7 +1823,7 @@ protected override void ProcessPendingRender()
         public async Task SetNotFoundResponseAsync(HttpContext httpContext, NotFoundEventArgs args)
         {
             SetHttpContext(httpContext);
-            await SetNotFoundResponseAsync(httpContext.Request.PathBase, args);
+            await SetNotFoundWhenResponseHasStarted();
         }
     }
 
diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index c90ea78845e2..1a7d74483b33 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -239,7 +239,7 @@ private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionM
     public void NotFoundSetOnFormSubmit_ResponseNotStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
-        string testUrl = $"{ServerPathBase}{reexecution}/post-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        string testUrl = $"{ServerPathBase}{reexecution}/post-not-found-ssr?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
         Navigate(testUrl);
         Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
 

From 842bc6c07e2f26a4add1a14bb9c64c1382129600 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 11 Jul 2025 19:55:48 +0200
Subject: [PATCH 3/3] Added tests from #62524 to fix it in one place.

---
 .../NoInteractivityTest.cs                    | 109 +++++++++++++++---
 .../NotFound/ComponentThatPostsNotFound.razor |   4 +-
 .../NotFound/ComponentThatSetsNotFound.razor  |   4 +-
 .../PageThatPostsNotFound-no-streaming.razor  |   7 +-
 .../PageThatPostsNotFound-streaming.razor     |   2 +-
 .../PageThatSetsNotFound-no-streaming.razor   |   7 +-
 .../PageThatSetsNotFound-streaming.razor      |   2 +-
 7 files changed, 111 insertions(+), 24 deletions(-)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index 1a7d74483b33..ca8f502f8472 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -151,21 +151,29 @@ public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecuti
         string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
         Navigate(testUrl);
 
-        if (hasCustomNotFoundPageSet)
-        {
-            AssertNotFoundPageRendered();
-        }
-        else
-        {
-            AssertNotFoundFragmentRendered();
-        }
+        AssertNotFoundRendered_ResponseNotStarted(hasCustomNotFoundPageSet);
+        AssertUrlNotChanged(testUrl);
+    }
+
+    [Theory]
+    [InlineData(true, true)]
+    [InlineData(true, false)]
+    [InlineData(false, true)]
+    [InlineData(false, false)]
+    public void NotFoundSetOnInitialization_AfterAsyncOperation_ResponseNotStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    {
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr?doAsync=true&useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        Navigate(testUrl);
+
+        AssertNotFoundRendered_ResponseNotStarted(hasCustomNotFoundPageSet);
         AssertUrlNotChanged(testUrl);
     }
 
     [Theory]
     [InlineData(true)]
     [InlineData(false)]
-    // our custom router does not support NotFoundPage nor NotFound fragment to simulate most probable custom router behavior
+    // our custom router does not support NotFoundPage to simulate the most probable custom router behavior
     public void NotFoundSetOnInitialization_ResponseNotStarted_CustomRouter_SSR(bool hasReExecutionMiddleware)
     {
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
@@ -196,7 +204,7 @@ public void NotFoundSetOnInitialization_ResponseStarted_SSR(bool hasReExecutionM
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
         string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
         Navigate(testUrl);
-        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        AssertNotFoundRendered_ResponseStarted(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
         AssertUrlNotChanged(testUrl);
     }
 
@@ -210,11 +218,47 @@ public void NotFoundSetOnInitialization_ResponseStarted_EnhancedNavigationDisabl
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
         string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
         Navigate(testUrl);
-        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        AssertNotFoundRendered_ResponseStarted(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
         AssertUrlChanged(testUrl);
     }
 
-    private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet, string testUrl)
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    // our custom router does not support NotFoundPage to simulate the most probable custom router behavior
+    public void NotFoundSetOnInitialization_ResponseStarted_CustomRouter_SSR(bool hasReExecutionMiddleware)
+    {
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr-streaming?useCustomRouter=true";
+        Navigate(testUrl);
+
+        if (hasReExecutionMiddleware)
+        {
+            AssertReExecutionPageRendered();
+        }
+        else
+        {
+            // Apps that don't support re-execution and don't have blazor's router,
+            // cannot render custom NotFound contents.
+            // The browser will display default 404 page.
+            AssertBrowserDefaultNotFoundViewRendered();
+        }
+        AssertUrlNotChanged(testUrl);
+    }
+
+    private void AssertNotFoundRendered_ResponseNotStarted(bool hasCustomNotFoundPageSet)
+    {
+        if (hasCustomNotFoundPageSet)
+        {
+            AssertNotFoundPageRendered();
+        }
+        else
+        {
+            AssertNotFoundContentNotRendered();
+        }
+    }
+
+    private void AssertNotFoundRendered_ResponseStarted(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet, string testUrl)
     {
         if (hasCustomNotFoundPageSet)
         {
@@ -243,7 +287,33 @@ public void NotFoundSetOnFormSubmit_ResponseNotStarted_SSR(bool hasReExecutionMi
         Navigate(testUrl);
         Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
 
-        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        AssertNotFoundRendered_ResponseNotStarted(hasCustomNotFoundPageSet);
+        AssertUrlNotChanged(testUrl);
+    }
+
+    [Theory]
+    [InlineData(true, true)]
+    [InlineData(true, false)]
+    [InlineData(false, true)]
+    [InlineData(false, false)]
+    public void NotFoundSetOnFormSubmit_AfterAsyncOperation_ResponseNotStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    {
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/post-not-found-ssr?doAsync=true&useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        Navigate(testUrl);
+        Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
+
+        AssertNotFoundRendered_ResponseNotStarted(hasCustomNotFoundPageSet);
+        AssertUrlNotChanged(testUrl);
+    }
+
+    [Fact]
+    public void NotFoundSetOnFormSubmit_ResponseNotStarted_CustomRouter_SSR()
+    {
+        string testUrl = $"{ServerPathBase}/reexecution/post-not-found-ssr?useCustomRouter=true";
+        Navigate(testUrl);
+
+        AssertReExecutionPageRendered();
         AssertUrlNotChanged(testUrl);
     }
 
@@ -259,12 +329,19 @@ public void NotFoundSetOnFormSubmit_ResponseStarted_SSR(bool hasReExecutionMiddl
         Navigate(testUrl);
         Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
 
-        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        AssertNotFoundRendered_ResponseStarted(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
         AssertUrlNotChanged(testUrl);
     }
 
-    private void AssertNotFoundFragmentRendered() =>
-        Browser.Equal("There's nothing here", () => Browser.FindElement(By.Id("not-found-fragment")).Text);
+    [Fact]
+    public void NotFoundSetOnFormSubmit_ResponseStarted_CustomRouter_SSR()
+    {
+        string testUrl = $"{ServerPathBase}/reexecution/post-not-found-ssr-streaming?useCustomRouter=true";
+        Navigate(testUrl);
+
+        AssertReExecutionPageRendered();
+        AssertUrlNotChanged(testUrl);
+    }
 
     private void AssertNotFoundContentNotRendered() =>
         Browser.Equal("Any content", () => Browser.FindElement(By.Id("test-info")).Text);
diff --git a/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatPostsNotFound.razor b/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatPostsNotFound.razor
index f9b6cd9c7a3c..dc3aaaae5868 100644
--- a/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatPostsNotFound.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatPostsNotFound.razor
@@ -16,14 +16,14 @@
 
 @code{
     [Parameter]
-    public bool StartStreaming { get; set; } = false;
+    public bool DoAsyncOperationBeforeSettingNotFound { get; set; } = false;
 
     [Parameter]
     public bool WaitForInteractivity { get; set; } = false;
 
     private async Task HandleSubmit()
     {
-        if (StartStreaming)
+        if (DoAsyncOperationBeforeSettingNotFound)
         {
             await Task.Yield();
         }
diff --git a/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatSetsNotFound.razor b/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatSetsNotFound.razor
index fe024d3bb8d0..b06007c907b8 100644
--- a/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatSetsNotFound.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatSetsNotFound.razor
@@ -10,14 +10,14 @@
 
 @code{
     [Parameter]
-    public bool StartStreaming { get; set; } = false;
+    public bool DoAsyncOperationBeforeSettingNotFound { get; set; } = false;
 
     [Parameter]
     public bool WaitForInteractivity { get; set; } = false;
 
     protected async override Task OnInitializedAsync()
     {
-        if (StartStreaming)
+        if (DoAsyncOperationBeforeSettingNotFound)
         {
             await Task.Yield();
         }
diff --git a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-no-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-no-streaming.razor
index 4aa3a245b047..ea91b922cb82 100644
--- a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-no-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-no-streaming.razor
@@ -8,4 +8,9 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<TestContentPackage.NotFound.ComponentThatPostsNotFound />
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatPostsNotFound DoAsyncOperationBeforeSettingNotFound="@DoAsync" />
+
+@code{
+    [SupplyParameterFromQuery(Name = "doAsync")]
+    public bool DoAsync { get; set; } = false;
+}
\ No newline at end of file
diff --git a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-streaming.razor
index a542c35c52f4..834c977d943b 100644
--- a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-streaming.razor
@@ -8,4 +8,4 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<TestContentPackage.NotFound.ComponentThatPostsNotFound StartStreaming="true"/>
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatPostsNotFound DoAsyncOperationBeforeSettingNotFound="true"/>
\ No newline at end of file
diff --git a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-no-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-no-streaming.razor
index b99ed19711d7..4dcdbebf16e2 100644
--- a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-no-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-no-streaming.razor
@@ -8,4 +8,9 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<TestContentPackage.NotFound.ComponentThatSetsNotFound />
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatSetsNotFound DoAsyncOperationBeforeSettingNotFound="@DoAsync" />
+
+@code{
+    [SupplyParameterFromQuery(Name = "doAsync")]
+    public bool DoAsync { get; set; } = false;
+}
\ No newline at end of file
diff --git a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-streaming.razor
index e3124758ce65..7574e5920396 100644
--- a/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-streaming.razor
@@ -8,4 +8,4 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<TestContentPackage.NotFound.ComponentThatSetsNotFound StartStreaming="true"/>
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatSetsNotFound DoAsyncOperationBeforeSettingNotFound="true"/>
\ No newline at end of file