[release/10.0] Fix ScriptTagHelper regression by checking for existing content before processing importmaps (#63631)

* Initial plan

* Fix script tag importmap regression - preserve user content when no asp-importmap

Co-authored-by: MackinnonBuck <[email protected]>

* Fix ScriptTagHelper importmap regression by checking for existing content

Co-authored-by: javiercn <[email protected]>

* Manual fixes

* Handle other tag helper setting content

* Update PublicAPI.Unshipped.txt

* Use `ProcessAsync()` in tests

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: MackinnonBuck <[email protected]>
Co-authored-by: javiercn <[email protected]>
Co-authored-by: Mackinnon Buck <[email protected]>

Author: 4 people

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

@@ -1 +1,3 @@
 #nullable enable
+*REMOVED*~override Microsoft.AspNetCore.Mvc.TagHelpers.ScriptTagHelper.Process(Microsoft.AspNetCore.Razor.TagHelpers.TagHelperContext context, Microsoft.AspNetCore.Razor.TagHelpers.TagHelperOutput output) -> void
+~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

@@ -235,15 +235,29 @@ private StringWriter StringWriter
     }
 
     /// <inheritdoc />
-    public override void Process(TagHelperContext context, TagHelperOutput output)
+    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.
+            // Do not update the content if another tag helper targeting this element has already done so.
+            if (output.IsContentModified)
+            {
+                return;
+            }
+
+            // This is an importmap script, check if there's existing content first.
+            var childContent = await output.GetChildContentAsync();
+            if (!childContent.IsEmptyOrWhiteSpace)
+            {
+                // User provided existing content; preserve it.
+                output.Content.SetHtmlContent(childContent);
+                return;
+            }
+
+            // No existing content, so we can apply import map logic.
             var importMap = ImportMap ?? ViewContext.HttpContext.GetEndpoint()?.Metadata.GetMetadata<ImportMapDefinition>();
             if (importMap == null)
             {
@@ -252,10 +266,10 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 return;
             }
 
+            output.Content.SetHtmlContent(importMap.ToString());
             output.TagName = "script";
             output.TagMode = TagMode.StartTagAndEndTag;
             output.Attributes.SetAttribute("type", "importmap");
-            output.Content.SetHtmlContent(importMap.ToString());
             return;
         }