fix "Blazor enhanced form handling doesn't honor button formmethod attribute" (#51942)

src/Components/Web.JS/src/Services/NavigationEnhancement.ts

@@ -132,6 +132,10 @@ function onDocumentSubmit(event: SubmitEvent) {
         // Replicate the normal behavior of overriding action attribute of form element
         url = new URL(submitter.formAction);
       }
+      if (submitter.formMethod) {
+        // Replicate the normal behavior of overriding method attribute of form element
+        fetchOptions.method = submitter.formMethod;
+      }
     }
 
     if (fetchOptions.method === 'get') { // method is always returned as lowercase

src/Components/test/E2ETest/ServerRenderingTests/FormHandlingTests/FormWithParentBindingContextTest.cs

@@ -1368,6 +1368,17 @@ public class FormWithParentBindingContextTest : ServerTestBase<BasicTestAppServe
         Browser.Equal("Formaction url", () => Browser.Exists(By.TagName("html")).Text);
     }
 
+    [Fact]
+    public void SubmitButtonFormmethodAttributeOverridesEnhancedFormMethod()
+    {
+        GoTo("forms/form-submit-button-with-formmethod");
+        Browser.DoesNotExist(By.Id("submitted"));
+
+        Browser.Exists(By.Id("submit-button")).Click();
+
+        Browser.Equal("Form submitted!", () => Browser.Exists(By.Id("submitted")).Text);
+    }
+
     // Can't just use GetAttribute or GetDomAttribute because they both auto-resolve it
     // to an absolute URL. We want to be able to assert about the attribute's literal value.
     private string ReadFormActionAttribute(IWebElement form)

src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/Forms/FormSubmitButtonWithFormmethod.razor

@@ -0,0 +1,18 @@
+@page "/forms/form-submit-button-with-formmethod"
+@using Microsoft.AspNetCore.Components.Forms
+
+<h1>Form Submit Button with formaction</h1>
+
+<form data-enhance method="get" @onsubmit="() => _submitted = true" @formname="someform">
+    <AntiforgeryToken />
+    <input id="submit-button" type="submit" formmethod="post" />
+</form>
+
+@if (_submitted)
+{
+    <p id="submitted">Form submitted!</p>
+}
+
+@code {
+    bool _submitted = false;
+}