fix(blazor): preserve ValidationSummary fixed class when AdditionalAttributes contains class

src/Components/Web/src/Forms/ValidationSummary.cs

@@ -73,8 +73,9 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
                 first = false;
 
                 builder.OpenElement(0, "ul");
-                builder.AddAttribute(1, "class", "validation-errors");
-                builder.AddMultipleAttributes(2, AdditionalAttributes);
+                builder.AddMultipleAttributes(1, AdditionalAttributes);
+                builder.AddAttributeIfNotNullOrEmpty(2, "class",
+                    AttributeUtilities.CombineClassNames(AdditionalAttributes, "validation-errors"));
             }
 
             builder.OpenElement(3, "li");

src/Components/test/E2ETest/Tests/FormsTest.cs

@@ -1096,4 +1096,32 @@ private bool ElementHasAttribute(IWebElement webElement, string attribute)
         var jsExecutor = (IJavaScriptExecutor)Browser;
         return (bool)jsExecutor.ExecuteScript($"return arguments[0].attributes['{attribute}'] !== undefined;", webElement);
     }
+
+    [Fact]
+    public void ValidationSummaryCombinesAdditionalAttributesClassWithDefault()
+    {
+        var appElement = Browser.MountTestComponent<ValidationSummaryWithCssClassComponent>();
+        var nameInput = appElement.FindElement(By.ClassName("name")).FindElement(By.TagName("input"));
+        var submitButton = appElement.FindElement(By.CssSelector("button[type=submit]"));
+
+        // Make the form invalid to trigger rendering of the ValidationSummary elements
+        nameInput.SendKeys("01234567890123456789\t");
+        submitButton.Click();
+
+        // Default ValidationSummary renders with only "validation-errors" class.
+        Browser.Equal("validation-errors",
+            () => appElement.FindElement(By.ClassName("all-errors")).FindElement(By.TagName("ul")).GetDomAttribute("class"));
+
+        // ValidationSummary with class="pt-2" via AdditionalAttributes combines both:
+        // splatted class + fixed "validation-errors"
+        Browser.Equal("pt-2 validation-errors",
+            () => appElement.FindElement(By.ClassName("all-errors-with-class")).FindElement(By.TagName("ul")).GetDomAttribute("class"));
+
+        // Additional non-class attributes (e.g. data-testid) are also splatted
+        Browser.Equal("validation-errors",
+            () => appElement.FindElement(By.ClassName("all-errors-with-attr")).FindElement(By.TagName("ul")).GetDomAttribute("class"));
+        Browser.Equal("my-summary",
+            () => appElement.FindElement(By.ClassName("all-errors-with-attr")).FindElement(By.TagName("ul")).GetDomAttribute("data-testid"));
+    }
+
 }

src/Components/test/testassets/BasicTestApp/FormsTest/ValidationSummaryWithCssClassComponent.razor

@@ -0,0 +1,40 @@
+@using System.ComponentModel.DataAnnotations
+@using Microsoft.AspNetCore.Components.Forms
+
+<EditForm EditContext="@editContext" OnValidSubmit="@HandleValidSubmit">
+    <DataAnnotationsValidator />
+
+    <p class="name">
+        Name: <InputText @bind-Value="person.Name" placeholder="Enter your name" />
+    </p>
+
+    <button type="submit">Submit</button>
+
+    <p class="all-errors">
+        <ValidationSummary />
+    </p>
+    <p class="all-errors-with-class">
+        <ValidationSummary class="pt-2" />
+    </p>
+    <p class="all-errors-with-attr">
+        <ValidationSummary data-testid="my-summary" />
+    </p>
+</EditForm>
+
+@code {
+    private EditContext editContext = default!;
+    private Person person = new();
+
+    protected override void OnInitialized()
+    {
+        editContext = new EditContext(person);
+    }
+
+    private void HandleValidSubmit() { }
+
+    private class Person
+    {
+        [Required, MaxLength(10)]
+        public string Name { get; set; } = string.Empty;
+    }
+}

src/Components/test/testassets/BasicTestApp/Index.razor

@@ -50,6 +50,7 @@
         <option value="BasicTestApp.FormsTest.SimpleValidationComponentUsingExperimentalValidator">Simple validation using experimental validator</option>
         <option value="BasicTestApp.FormsTest.ModelLevelValidationComponent">Model-level validation</option>
         <option value="BasicTestApp.FormsTest.TypicalValidationComponent">Typical validation</option>
+        <option value="BasicTestApp.FormsTest.ValidationSummaryWithCssClassComponent">Validation summary with CssClass</option>
         <option value="BasicTestApp.MediaTest.ImageTestComponent">Image component tests</option>
         <option value="BasicTestApp.MediaTest.VideoTestComponent">Video component tests</option>
         <option value="BasicTestApp.MediaTest.FileDownloadTestComponent">File download component tests</option>