Fix ScriptTagHelper importmap regression by checking for existing content

Co-authored-by: javiercn <6995051+javiercn@users.noreply.github.com>

Author: Copilot

src/Mvc/Mvc.TagHelpers/src/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+~override Microsoft.AspNetCore.Mvc.TagHelpers.ScriptTagHelper.ProcessAsync(Microsoft.AspNetCore.Razor.TagHelpers.TagHelperContext context, Microsoft.AspNetCore.Razor.TagHelpers.TagHelperOutput output) -> System.Threading.Tasks.Task

src/Mvc/Mvc.TagHelpers/src/ScriptTagHelper.cs

@@ -236,36 +236,40 @@ private StringWriter StringWriter
 
     /// <inheritdoc />
     public override void Process(TagHelperContext context, TagHelperOutput output)
+    {
+        ProcessAsync(context, output).GetAwaiter().GetResult();
+    }
+
+    /// <inheritdoc />
+    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
     {
         ArgumentNullException.ThrowIfNull(context);
         ArgumentNullException.ThrowIfNull(output);
 
         if (string.Equals(Type, "importmap", StringComparison.OrdinalIgnoreCase))
         {
-            // This is an importmap script, we'll write out the import map and
-            // stop processing.
-            var importMap = ImportMap ?? ViewContext.HttpContext.GetEndpoint()?.Metadata.GetMetadata<ImportMapDefinition>();
-            if (importMap == null)
+            // This is an importmap script, check if there's existing content first
+            var childContent = await output.GetChildContentAsync();
+            if (!childContent.IsEmptyOrWhiteSpace)
             {
-                // No importmap found. Only suppress output if this was intended to be
-                // an automatically generated importmap (i.e., when asp-importmap was used).
-                // If the user provided explicit content without asp-importmap, let it render as-is.
-                if (ImportMap != null || context.AllAttributes.ContainsName(ImportMapAttributeName))
-                {
-                    output.SuppressOutput();
-                    return;
-                }
-                // Let the tag render as-is by continuing with normal processing
-                // Don't return here, let normal attribute copying happen
+                // User provided explicit content, preserve it
+                return;
             }
-            else
+
+            // No existing content, so we can apply import map logic
+            var importMap = ImportMap ?? ViewContext.HttpContext.GetEndpoint()?.Metadata.GetMetadata<ImportMapDefinition>();
+            if (importMap == null)
             {
-                output.TagName = "script";
-                output.TagMode = TagMode.StartTagAndEndTag;
-                output.Attributes.SetAttribute("type", "importmap");
-                output.Content.SetHtmlContent(importMap.ToString());
+                // No importmap found, nothing to do.
+                output.SuppressOutput();
                 return;
             }
+
+            output.TagName = "script";
+            output.TagMode = TagMode.StartTagAndEndTag;
+            output.Attributes.SetAttribute("type", "importmap");
+            output.Content.SetHtmlContent(importMap.ToString());
+            return;
         }
 
         // Pass through attribute that is also a well-known HTML attribute.

src/Mvc/Mvc.TagHelpers/test/ScriptTagHelperTest.cs

@@ -795,55 +795,56 @@ public void ScriptTagHelper_RendersImportMap_FromEndpoint()
     }
 
     [Fact]
-    public void ScriptTagHelper_PreservesExplicitImportMapContent_WhenNoImportMapDefinition()
+    public async Task ScriptTagHelper_PreservesExplicitImportMapContent_WhenUserProvidesContent()
     {
         // Arrange - this simulates the user's scenario where they provide explicit importmap content
-        // without using asp-importmap attribute
         var context = MakeTagHelperContext(
             attributes: new TagHelperAttributeList
             {
                 new TagHelperAttribute("type", "importmap"),
             });
 
         var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
+        // Simulate user providing explicit content
+        output.Content.SetHtmlContent(@"{""imports"":{""jquery"":""https://code.jquery.com/jquery.js""}}");
 
         var helper = GetHelper();
         helper.Type = "importmap";
-        // No endpoint with ImportMapDefinition and no asp-importmap attribute
-        // This should NOT suppress the output, allowing user content to render
+        // No endpoint with ImportMapDefinition - this should NOT suppress the output
+        // since user provided explicit content
 
         // Act
-        helper.Process(context, output);
+        await helper.ProcessAsync(context, output);
 
         // Assert
         Assert.Equal("script", output.TagName); // Tag should not be suppressed
         Assert.Equal("importmap", output.Attributes["type"].Value);
-        // The output should not be suppressed, allowing user's explicit content to render
-        Assert.False(output.IsContentModified); // Content should remain as user provided
+        // The user's explicit content should be preserved
+        Assert.Equal(@"{""imports"":{""jquery"":""https://code.jquery.com/jquery.js""}}", output.Content.GetContent());
     }
 
     [Fact]
-    public void ScriptTagHelper_SuppressesOutput_WhenAspImportMapAttributeUsedButNoDefinition()
+    public async Task ScriptTagHelper_SuppressesOutput_WhenNoContentAndNoImportMapDefinition()
     {
-        // Arrange - this simulates using asp-importmap attribute but having no ImportMapDefinition
+        // Arrange - this simulates an empty importmap script with no definition
         var context = MakeTagHelperContext(
             attributes: new TagHelperAttributeList
             {
                 new TagHelperAttribute("type", "importmap"),
-                new TagHelperAttribute("asp-importmap", null), // asp-importmap used but no value
             });
 
         var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
+        // No content provided
 
         var helper = GetHelper();
         helper.Type = "importmap";
-        // No endpoint with ImportMapDefinition but asp-importmap attribute is present
-        // This should suppress the output since it was intended to be auto-generated
+        // No endpoint with ImportMapDefinition and no explicit content
+        // This should suppress the output since there's nothing to render
 
         // Act
-        helper.Process(context, output);
+        await helper.ProcessAsync(context, output);
 
-        // Assert - output should be suppressed when asp-importmap is used but no definition found
+        // Assert - output should be suppressed when no content and no definition
         Assert.Null(output.TagName); // Tag should be suppressed
     }