Fix standalone Blazor WASM hot reload in Gateway

Use the full web application builder when dynamic code is available so dotnet watch and IDE hosting startup middleware can run. Keep the slim builder for NativeAOT, and prevent conditional SPA fallback responses from bypassing browser-refresh script injection.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 25710030-aafc-4ad2-9163-82e077b402a7

Author: lewing

src/Components/Gateway/src/BlazorGateway.cs

@@ -1,8 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Runtime.CompilerServices;
 using Microsoft.AspNetCore.Diagnostics.HealthChecks;
+using Microsoft.AspNetCore.StaticAssets;
 using Microsoft.Extensions.Diagnostics.HealthChecks;
+using Microsoft.Net.Http.Headers;
 using OpenTelemetry;
 using OpenTelemetry.Metrics;
 using OpenTelemetry.Trace;
@@ -19,7 +22,9 @@ public static class BlazorGateway
     /// Reads ClientApps config section for endpoint manifests and YARP reverse proxy configuration.
     /// </summary>
     public static WebApplication BuildWebHost(string[] args) =>
-        BuildWebHost(WebApplication.CreateSlimBuilder(args));
+        BuildWebHost(RuntimeFeature.IsDynamicCodeSupported
+            ? WebApplication.CreateBuilder(args)
+            : WebApplication.CreateSlimBuilder(args));
 
     internal static WebApplication BuildWebHost(WebApplicationBuilder builder)
     {
@@ -113,13 +118,57 @@ internal static WebApplication BuildWebHost(WebApplicationBuilder builder)
 
             if (!string.IsNullOrEmpty(appConfig.EndpointsManifest))
             {
-                app.MapGroup(appConfig.PathPrefix ?? "").MapStaticAssets(appConfig.EndpointsManifest);
+                var staticAssets = app.MapGroup(appConfig.PathPrefix ?? "").MapStaticAssets(appConfig.EndpointsManifest);
+                if (app.Environment.IsDevelopment())
+                {
+                    staticAssets.Add(DisableSpaFallbackCaching);
+                }
             }
         }
 
         return app;
     }
 
+    private static void DisableSpaFallbackCaching(EndpointBuilder endpointBuilder)
+    {
+        if (endpointBuilder is not RouteEndpointBuilder { Order: int.MaxValue, RequestDelegate: { } next } ||
+            GetStaticAssetDescriptor(endpointBuilder) is not { Route: var route } ||
+            !route.StartsWith("{**", StringComparison.Ordinal))
+        {
+            return;
+        }
+
+        endpointBuilder.RequestDelegate = context =>
+        {
+            // The dotnet-watch middleware injects its browser refresh script into the response body.
+            // A conditional response has no body, so preserve the old DevServer's no-store behavior.
+            context.Request.Headers.Remove(HeaderNames.IfNoneMatch);
+            context.Request.Headers.Remove(HeaderNames.IfModifiedSince);
+            context.Response.OnStarting(static state =>
+            {
+                var response = (HttpResponse)state;
+                response.Headers[HeaderNames.CacheControl] = "no-store";
+
+                return Task.CompletedTask;
+            }, context.Response);
+
+            return next(context);
+        };
+    }
+
+    private static StaticAssetDescriptor? GetStaticAssetDescriptor(EndpointBuilder endpointBuilder)
+    {
+        foreach (var metadata in endpointBuilder.Metadata)
+        {
+            if (metadata is StaticAssetDescriptor descriptor)
+            {
+                return descriptor;
+            }
+        }
+
+        return null;
+    }
+
     private static IHostApplicationBuilder ConfigureOpenTelemetry(this IHostApplicationBuilder builder, BlazorGatewayOptions.TelemetryOptions telemetry)
     {
         builder.Logging.AddOpenTelemetry(logging =>

src/Components/Gateway/test/BlazorGatewayTests.cs

@@ -10,10 +10,14 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 
+[assembly: HostingStartup(typeof(Microsoft.AspNetCore.Components.Gateway.BlazorGatewayTests.TestHostingStartup))]
+
 namespace Microsoft.AspNetCore.Components.Gateway;
 
 public class BlazorGatewayTests
 {
+    private const string HostingStartupHeaderName = "X-Test-Hosting-Startup";
+
     [Fact]
     public async Task HealthChecks_ReturnsOk_InDevelopment_WithDefaultOptions()
     {
@@ -216,6 +220,51 @@ public async Task BuildWebHost_StartsWithHttpsUrl_WhenKestrelCertificateConfigur
         Assert.Contains(app.Urls, address => address.StartsWith("https://127.0.0.1:", StringComparison.Ordinal));
     }
 
+    [Fact]
+    public async Task BuildWebHost_RunsConfiguredHostingStartup()
+    {
+        var hostingStartupAssembly = typeof(TestHostingStartup).Assembly.GetName().Name!;
+        await using var app = BlazorGateway.BuildWebHost(
+        [
+            "--environment", Environments.Development,
+            "--hostingStartupAssemblies", hostingStartupAssembly,
+            "--urls", "http://127.0.0.1:0",
+        ]);
+
+        await app.StartAsync();
+
+        using var client = new HttpClient
+        {
+            BaseAddress = new Uri(app.Urls.Single()),
+        };
+        var response = await client.GetAsync("/health");
+
+        Assert.Equal("true", response.Headers.GetValues(HostingStartupHeaderName).Single());
+    }
+
+    [Fact]
+    public async Task SpaFallback_DisablesDocumentCachingInDevelopment()
+    {
+        await using var gateway = await StartSpaFallbackGatewayAsync(Environments.Development);
+
+        var response = await gateway.Client.SendAsync(CreateConditionalDocumentRequest());
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.True(response.Headers.CacheControl?.NoStore);
+        Assert.Contains("Gateway", await response.Content.ReadAsStringAsync());
+    }
+
+    [Fact]
+    public async Task SpaFallback_PreservesDocumentCachingInProduction()
+    {
+        await using var gateway = await StartSpaFallbackGatewayAsync(Environments.Production);
+
+        var response = await gateway.Client.SendAsync(CreateConditionalDocumentRequest());
+
+        Assert.Equal(HttpStatusCode.NotModified, response.StatusCode);
+        Assert.False(response.Headers.CacheControl?.NoStore);
+    }
+
     private static bool IsRedirect(HttpStatusCode status) =>
         status is HttpStatusCode.MovedPermanently
             or HttpStatusCode.Found
@@ -229,4 +278,55 @@ private static Task<GatewayUnderTest> StartGatewayAsync(
         string environment,
         Dictionary<string, string?> configuration) =>
         GatewayTestHelpers.StartGatewayAsync(environment, configuration);
+
+    private static async Task<GatewayUnderTest> StartSpaFallbackGatewayAsync(string environment)
+    {
+        var webRoot = Path.Combine(AppContext.BaseDirectory, "TestAssets");
+        var manifest = Path.Combine(webRoot, "test.staticwebassets.endpoints.json");
+        var builder = WebApplication.CreateSlimBuilder(new WebApplicationOptions
+        {
+            EnvironmentName = environment,
+            WebRootPath = webRoot,
+        });
+        builder.Configuration.AddInMemoryCollection(new Dictionary<string, string?>
+        {
+            ["ClientApps:app:EndpointsManifest"] = manifest,
+        });
+        builder.WebHost.UseTestServer();
+
+        var app = BlazorGateway.BuildWebHost(builder);
+        await app.StartAsync();
+
+        return new GatewayUnderTest(app);
+    }
+
+    private static HttpRequestMessage CreateConditionalDocumentRequest()
+    {
+        var request = new HttpRequestMessage(HttpMethod.Get, "/");
+        request.Headers.Accept.ParseAdd("text/html");
+        request.Headers.Add("Sec-Fetch-Dest", "document");
+        request.Headers.TryAddWithoutValidation("If-None-Match", "\"test-etag\"");
+
+        return request;
+    }
+
+    public sealed class TestHostingStartup : IHostingStartup
+    {
+        public void Configure(IWebHostBuilder builder) =>
+            builder.ConfigureServices(services => services.AddSingleton<IStartupFilter, TestStartupFilter>());
+    }
+
+    public sealed class TestStartupFilter : IStartupFilter
+    {
+        public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next) =>
+            app =>
+            {
+                app.Use(async (context, next) =>
+                {
+                    context.Response.Headers[HostingStartupHeaderName] = "true";
+                    await next(context);
+                });
+                next(app);
+            };
+    }
 }

src/Components/Gateway/test/Microsoft.AspNetCore.Components.Gateway.Tests.csproj

@@ -22,6 +22,7 @@
     <Reference Include="Microsoft.Extensions.ServiceDiscovery.Yarp" />
     <Reference Include="Yarp.ReverseProxy" />
     <Content Include="$(SharedSourceRoot)TestCertificates\*.pfx" LinkBase="shared\TestCertificates" CopyToOutputDirectory="PreserveNewest" />
+    <Content Include="TestAssets\**" CopyToOutputDirectory="PreserveNewest" />
     <PackageReference Include="OpenTelemetry.Exporter.OpenTelemetryProtocol" Version="$(OpenTelemetryExporterOpenTelemetryProtocolVersion)" />
     <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="$(OpenTelemetryExtensionsHostingVersion)" />
     <PackageReference Include="OpenTelemetry.Instrumentation.AspNetCore" Version="$(OpenTelemetryInstrumentationAspNetCoreVersion)" />

src/Components/Gateway/test/TestAssets/index.html

@@ -0,0 +1 @@
+<!DOCTYPE html><body>Gateway</body>

src/Components/Gateway/test/TestAssets/test.staticwebassets.endpoints.json

@@ -0,0 +1,35 @@
+{
+  "Version": 1,
+  "ManifestType": "Build",
+  "Endpoints": [
+    {
+      "Route": "{**fallback:nonfile}",
+      "AssetFile": "index.html",
+      "Order": "2147483647",
+      "Selectors": [],
+      "EndpointProperties": [],
+      "ResponseHeaders": [
+        {
+          "Name": "Cache-Control",
+          "Value": "no-cache"
+        },
+        {
+          "Name": "Content-Length",
+          "Value": "36"
+        },
+        {
+          "Name": "Content-Type",
+          "Value": "text/html"
+        },
+        {
+          "Name": "ETag",
+          "Value": "\"test-etag\""
+        },
+        {
+          "Name": "Last-Modified",
+          "Value": "Fri, 28 Aug 2026 00:00:00 GMT"
+        }
+      ]
+    }
+  ]
+}