Preserve TagHelper child content when appending output content

[Wezylnia]

Fixes #29835

Summary

• initialize TagHelperOutput.Content from cached child content so AppendHtml appends instead of replacing unmaterialized child markup
• keep plain Content reads from marking content as modified
• preserve replace semantics for SetContent/SetHtmlContent, suppress output behavior, and reinitialization reuse
• add focused TagHelperOutput regression tests

Testing

• Not run: local repo restore artifacts are unavailable without running restore.cmd; dotnet test with the installed .NET 10 SDK reaches MSBuild but fails on missing artifacts/bin/GenerateFiles/Directory.Build.props.

[dotnet-policy-service]

Thanks for your PR, @Wezylnia. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR changes TagHelperOutput.Content to surface existing child content via a new wrapper that avoids marking content as modified on read, and adds tests to validate the new semantics.

Changes:

• Update TagHelperOutput.Content to initialize from GetChildContentAsync() via a new TagHelperOutputContent wrapper.
• Adjust Reinitialize / SuppressOutput behavior around _content lifecycle.
• Add unit tests covering Content read/append/set/suppress/reinitialize scenarios.

Reviewed changes

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

File	Description
src/Razor/Razor/test/TagHelpers/TagHelperOutputTest.cs	Adds tests validating new Content behavior (append/read/set/suppress/reinitialize).
src/Razor/Razor/src/TagHelpers/TagHelperOutput.cs	Changes Content initialization and introduces TagHelperOutputContent wrapper to track modifications and preserve initial child content.

Comments suppressed due to low confidence (1)

src/Razor/Razor/src/TagHelpers/TagHelperOutput.cs:110

• Content now eagerly blocks on GetChildContentAsync() via GetAwaiter().GetResult(). This is sync-over-async and also forces child-content evaluation even when callers only intend to overwrite content (e.g., output.Content.SetContent(...)), which can introduce unnecessary work and potential deadlocks in environments with a synchronization context. Consider making the child-content retrieval lazy inside TagHelperOutputContent (store the Task/factory and only block when the initial content is actually needed for reading or for append-merge), so SetContent/Clear can avoid evaluating child content entirely.

    public TagHelperContent Content
    {
        get
        {
            if (_content == null)
            {
                _content = new TagHelperOutputContent(GetChildContentAsync().GetAwaiter().GetResult());
            }

            return _content;
        }

[Wezylnia]

@dotnet-policy-service agree