Fix ScriptTagHelper regression by checking for existing content before processing importmaps

[Copilot]

The ScriptTagHelper was incorrectly suppressing <script type="importmap"> tags that contained explicit user-provided content, causing them to render as nothing in ASP.NET Core 9.0.

Problem

When users wrote importmap scripts with explicit content like:

<script type="importmap">
{
  "imports": {
    "jquery": "https://code.jquery.com/jquery.js"
  }
}
</script>

The ScriptTagHelper would completely suppress the output instead of preserving the user's content. This regression broke existing applications that relied on explicit importmap definitions.

Root Cause

The tag helper was being overly aggressive - it processed all script tags with type="importmap" and when no ImportMapDefinition was found in endpoint metadata, it would call output.SuppressOutput() regardless of whether the user had provided explicit content.

Solution

Modified the ScriptTagHelper to check for existing content first using await output.GetChildContentAsync():

• If existing content is found (not empty/whitespace) → preserve it and don't process further
• If no existing content → apply normal importmap logic (generate from definition or suppress if none found)

This approach ensures that user-provided content is never replaced, while maintaining all existing functionality for automatic importmap generation.

Implementation Details

• Added ProcessAsync method that properly handles the async content checking
• Maintained backward compatibility by keeping the original Process method
• Updated public API to reflect the new async method

Testing

Added comprehensive tests to verify:

• Explicit importmap content is preserved when users provide it
• Output is still suppressed when no content and no definition exists
• All existing functionality remains intact

Fixes #58973.

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[javiercn]

I don't think this is the right fix.

If the goal is to do nothing if there is already content on the script, then what should happen is:

• Call var originalContent = await output.GetChildContentAsync().
• Check for content.IsEmptyOrWhitespace
• Set the import map only in that situation.

[javiercn]

Note that we still want to process all <script type="importmap"></script> but the fix is only to avoid replacing existing content if there's already data the developer put it.

[javiercn]

@copilot @MackinnonBuck

[Copilot]

Fixed in commit 15776b3. I've implemented the approach you suggested:

1. Call await output.GetChildContentAsync() to get existing content
2. Check if content is empty or whitespace using IsEmptyOrWhiteSpace
3. If content exists, preserve it and return early
4. If no content exists, apply the normal importmap logic

The fix now properly processes all <script type="importmap"></script> tags but avoids replacing existing content when developers have provided their own importmap data. I've maintained backward compatibility by keeping the original Process method that calls the new ProcessAsync method.

[Copilot]

Pull Request Overview

This PR fixes a regression in the ScriptTagHelper where it was incorrectly suppressing <script type="importmap"> tags that contained explicit user-provided content. The fix ensures user-provided importmap content is preserved while maintaining automatic importmap generation functionality.

• Modified ScriptTagHelper to check for existing content before processing importmaps
• Added async ProcessAsync method to properly handle content checking
• Updated test infrastructure to support child content testing

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/Mvc/Mvc.TagHelpers/src/ScriptTagHelper.cs	Changed Process to ProcessAsync and added logic to preserve existing content in importmap scripts
src/Mvc/Mvc.TagHelpers/test/ScriptTagHelperTest.cs	Added comprehensive tests for content preservation and updated helper methods to support child content
src/Mvc/Mvc.TagHelpers/src/PublicAPI.Unshipped.txt	Added new public API entry for the ProcessAsync method override

[javiercn]

Looks good to me!

[MackinnonBuck]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[MackinnonBuck]

Also manually confirmed the fix.

[MackinnonBuck]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/aspnetcore/actions/runs/17628140064