Make AppendAriaDescribedby case-insensitive and split on all HTML whitespace

- Look up the existing aria-describedby attribute with OrdinalIgnoreCase to match the casing rules used by HTML and TagHelperAttributeList
- Tokenize the existing value on all ASCII whitespace (space, tab, newline, carriage return, form feed) instead of just the literal space
- Cover the whitespace-separated case with a new test

framework/src/Volo.Abp.AspNetCore.Mvc.UI.Bootstrap/TagHelpers/Extensions/TagHelperOutputExtensions.cs

@@ -8,6 +8,10 @@ namespace Volo.Abp.AspNetCore.Mvc.UI.Bootstrap.TagHelpers.Extensions;
 
 public static class TagHelperOutputExtensions
 {
+    // ASCII whitespace per the HTML5 spec, used to tokenize space-separated
+    // attribute values such as aria-describedby.
+    private static readonly char[] HtmlAsciiWhitespace = { ' ', '\t', '\n', '\r', '\f' };
+
     public static string Render(this TagHelperOutput output, HtmlEncoder htmlEncoder)
     {
         using (var writer = new StringWriter())
@@ -30,15 +34,16 @@ public static class TagHelperOutputExtensions
         }
 
         var existing = output.Attributes
-            .FirstOrDefault(a => a.Name == "aria-describedby")?.Value?.ToString();
+            .FirstOrDefault(a => string.Equals(a.Name, "aria-describedby", StringComparison.OrdinalIgnoreCase))
+            ?.Value?.ToString();
 
-        if (string.IsNullOrEmpty(existing))
+        if (string.IsNullOrWhiteSpace(existing))
         {
             output.Attributes.SetAttribute("aria-describedby", token);
             return;
         }
 
-        var tokens = existing.Split(' ', StringSplitOptions.RemoveEmptyEntries);
+        var tokens = existing.Split(HtmlAsciiWhitespace, StringSplitOptions.RemoveEmptyEntries);
         if (tokens.Any(t => t == token))
         {
             return;

framework/test/Volo.Abp.AspNetCore.Mvc.UI.Tests/Volo/Abp/AspNetCore/Mvc/UI/Bootstrap/TagHelpers/Form/AbpSelectTagHelperService_Tests.cs

@@ -97,6 +97,24 @@ public class AbpSelectTagHelperService_Tests
         service.LastGroupHtml.ShouldContain("aria-describedby=\"custom-id TestSelectInfoText\"");
     }
 
+    [Fact]
+    public async Task Aria_describedby_should_split_on_html_whitespace_separators()
+    {
+        var service = new TestAbpSelectTagHelperService(existingAriaDescribedby: "id1\tid2");
+        var tagHelper = new AbpSelectTagHelper(service)
+        {
+            AspFor = CreateModelExpression(),
+            InfoText = "Description"
+        };
+
+        var output = CreateOutput();
+
+        await tagHelper.ProcessAsync(CreateContext(), output);
+
+        service.LastSelectTag.ShouldNotBeNull();
+        service.LastSelectTag!.Attributes["aria-describedby"].Value.ToString().ShouldBe("id1\tid2 TestSelectInfoText");
+    }
+
     [Fact]
     public async Task InputInfoText_attribute_should_render_info_text_with_single_aria_describedby()
     {