Add E2E test

Author: javiercn

src/Components/test/E2ETest/ServerExecutionTests/StreamingRenderingTest.cs

@@ -64,4 +64,20 @@ public void CanPerformStreamingRendering()
         Browser.FindElement(By.Id("end-response-link")).Click();
         Browser.Equal("Finished", () => getStatusText().Text);
     }
+
+    [Fact]
+    public void CanPerformFormPostWithStreamedResponses()
+    {
+        Navigate($"{ServerPathBase}/form-streaming");
+
+        // Initial "waiting" state
+        var submit = Browser.Exists(By.CssSelector("input[type=submit]"));
+        var getStatusText = () => Browser.Exists(By.Id("status"));
+        Assert.Equal("", getStatusText().Text);
+
+        submit.Click();
+
+        Assert.Equal("Processing form...", getStatusText().Text);
+        Assert.Equal("Completed", getStatusText().Text);
+    }
 }

src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/FormStreamingRendering.razor

@@ -0,0 +1,36 @@
+@page "/form-streaming"
+@using Microsoft.AspNetCore.Components.Forms
+@attribute [StreamRendering(true)]
+
+<h1>Streaming Rendering</h1>
+
+@* This just sets the context for the binding, this happens normally inside RouteView *@
+<CascadingModelBinder Name="/form-streaming">
+    <EditForm id="process" method="POST" OnValidSubmit="ProcessForm">
+        <input type="submit" value="Process form" />
+    </EditForm>
+</CascadingModelBinder>
+
+<p id="status">
+    @if (_processing)
+    {
+        <text>Processing form...</text>
+    }
+    else if (_done)
+    {
+        <text>Completed</text>
+    }
+</p>
+
+@code {
+    bool _processing = false;
+    bool _done = false;
+
+    private async Task ProcessForm()
+    {
+        _processing = true;
+        await Task.Yield();
+        _processing = false;
+        _done = true;
+    }
+}