Fix form actions for apps deployed behind reverse proxy (#51403)

# Fix form actions for apps deployed behind reverse proxy

Makes forms work for apps deployed behind a reverse proxy.

## Description

Apps deployed behind a reverse proxy (e.g., container apps) should not try to emit absolute URLs by default because the scheme/hostname/port may differ from what is reachable from the outside world. The fix is to emit root-relative URLs.

Fixes #51380

## Customer Impact

Without this fix, apps deployed behind a reverse proxy (e.g., in ACA) would not support form posts.

## Regression?

- [ ] Yes
- [x] No

No because this only affects SSR forms, which is a new feature in .NET 8.

## Risk

- [ ] High
- [ ] Medium
- [x] Low

Low because this is only a change to how we generate the URL for a form's `action` attribute. Previously we used an absolute URL, but now we use a root-relative one. There is no other runtime change. Everything else in this PR is extra tests and updating existing tests.

## Verification

- [x] Manual (required)
- [x] Automated

## Packaging changes reviewed?

- [ ] Yes
- [ ] No
- [x] N/A

src/Components/Web/src/HtmlRendering/StaticHtmlRenderer.HtmlWriting.cs

@@ -351,12 +351,29 @@ void EmitFormActionIfNotExplicit(TextWriter output, bool isForm, bool hasExplici
                 output.Write("action");
                 output.Write('=');
                 output.Write('\"');
-                _htmlEncoder.Encode(output, _navigationManager.Uri);
+                _htmlEncoder.Encode(output, GetRootRelativeUrlForFormAction(_navigationManager));
                 output.Write('\"');
             }
         }
     }
 
+    private static string GetRootRelativeUrlForFormAction(NavigationManager navigationManager)
+    {
+        // We want a root-relative URL because:
+        // - if we used a base-relative one, then if currentUrl==baseHref, that would result
+        //   in an empty string, but forms have special handling for action="" (it means "submit
+        //   to the current URL, but that would be wrong if there's an uncommitted navigation in
+        //   flight, e.g., after the user clicking 'back' - it would go to whatever's now in the
+        //   address bar, ignoring where the form was rendered)
+        // - if we used an absolute URL, then it creates a significant extra pit of failure for
+        //   apps hosted behind a reverse proxy (e.g., container apps), because the server's view
+        //   of the absolute URL isn't usable outside the container
+        //   - of course, sites hosted behind URL rewriting that modifies the path will still be
+        //     wrong, but developers won't do that often as it makes things like <a href> really
+        //     difficult to get right. In that case, developers must emit an action attribute manually.
+        return new Uri(navigationManager.Uri, UriKind.Absolute).PathAndQuery;
+    }
+
     private int RenderChildren(int componentId, TextWriter output, ArrayRange<RenderTreeFrame> frames, int position, int maxElements)
     {
         if (maxElements == 0)

src/Components/Web/test/HtmlRendering/HtmlRendererTest.cs

@@ -3,6 +3,7 @@
 
 using System.Globalization;
 using System.Text;
+using System.Web;
 using Microsoft.AspNetCore.Components.Forms;
 using Microsoft.AspNetCore.Components.Forms.Mapping;
 using Microsoft.AspNetCore.Components.Rendering;
@@ -1106,15 +1107,25 @@ await htmlRenderer.Dispatcher.InvokeAsync(async () =>
         });
     }
 
-    [Fact]
-    public async Task RenderComponentAsync_AddsActionAttributeWithCurrentUrlToFormWithoutAttributes_WhenNoActionSpecified()
+    [Theory]
+    [InlineData("https://example.com/", "https://example.com", "/")]
+    [InlineData("https://example.com/", "https://example.com/", "/")]
+    [InlineData("https://example.com/", "https://example.com/page", "/page")]
+    [InlineData("https://example.com/", "https://example.com/a/b/c", "/a/b/c")]
+    [InlineData("https://example.com/", "https://example.com/a/b/c?q=1&p=hello%20there", "/a/b/c?q=1&p=hello%20there")]
+    [InlineData("https://example.com/subdir/", "https://example.com/subdir", "/subdir")]
+    [InlineData("https://example.com/subdir/", "https://example.com/subdir/", "/subdir/")]
+    [InlineData("https://example.com/a/b/", "https://example.com/a/b/c?q=1&p=2", "/a/b/c?q=1&p=2")]
+    [InlineData("http://user:pass@xyz.example.com:1234/a/b/", "http://user:pass@xyz.example.com:1234/a/b/c&q=1&p=2", "/a/b/c&q=1&p=2")]
+    public async Task RenderComponentAsync_AddsActionAttributeWithCurrentUrlToFormWithoutAttributes_WhenNoActionSpecified(
+        string baseUrl, string currentUrl, string expectedAction)
     {
         // Arrange
         var serviceProvider = GetServiceProvider(collection => collection.AddSingleton(new RenderFragment(rtb =>
         {
             rtb.OpenElement(0, "form");
             rtb.CloseElement();
-        })).AddScoped<NavigationManager, TestNavigationManager>());
+        })).AddScoped<NavigationManager>(_ => new TestNavigationManager(baseUrl, currentUrl)));
 
         var htmlRenderer = GetHtmlRenderer(serviceProvider);
         await htmlRenderer.Dispatcher.InvokeAsync(async () =>
@@ -1123,7 +1134,7 @@ await htmlRenderer.Dispatcher.InvokeAsync(async () =>
             var result = await htmlRenderer.RenderComponentAsync<TestComponent>();
 
             // Assert
-            Assert.Equal("<form action=\"https://www.example.com/page\"></form>", result.ToHtmlString());
+            Assert.Equal($"<form action=\"{HttpUtility.HtmlAttributeEncode(expectedAction)}\"></form>", result.ToHtmlString());
         });
     }
 
@@ -1145,7 +1156,7 @@ await htmlRenderer.Dispatcher.InvokeAsync(async () =>
             var result = await htmlRenderer.RenderComponentAsync<TestComponent>();
 
             // Assert
-            Assert.Equal("<form method=\"post\" action=\"https://www.example.com/page\"></form>", result.ToHtmlString());
+            Assert.Equal("<form method=\"post\" action=\"/page\"></form>", result.ToHtmlString());
         });
     }
 
@@ -1382,7 +1393,21 @@ public void Map(FormValueMappingContext context)
 
     private class TestNavigationManager : NavigationManager
     {
-        protected override void EnsureInitialized() => Initialize("https://www.example.com/", "https://www.example.com/page");
+        private string _baseUrl;
+        private string _currentUrl;
+
+        public TestNavigationManager()
+            : this("https://www.example.com/", "https://www.example.com/page")
+        {
+        }
+
+        public TestNavigationManager(string baseUrl, string currentUrl)
+        {
+            _baseUrl = baseUrl;
+            _currentUrl = currentUrl;
+        }
+
+        protected override void EnsureInitialized() => Initialize(_baseUrl, _currentUrl);
     }
 
     private IServiceProvider GetServiceProvider(Action<IServiceCollection> configure = null)