[Regression] Codex Desktop rich preview renders valid YAML frontmatter as body content

What version of the Codex App are you using (From “About Codex” dialog)?

26.715.31925 (build 5551)

What subscription do you have?

Pro

What platform is your computer?

Darwin 25.5.0 arm64 arm

What issue are you seeing?

Codex Desktop's Markdown rich preview no longer recognizes valid YAML frontmatter at the very beginning of a local .md file as document metadata.

Instead, the opening and closing --- delimiters are rendered as Markdown horizontal rules, and the YAML key/value lines between them are displayed as ordinary body content. Markdown below the frontmatter, such as headings and lists, is otherwise rendered normally.

This appears to be a regression. Earlier Desktop builds displayed the same frontmatter using the metadata presentation.

Minimal example:

---
title: Frontmatter Preview Reproduction
description: Valid YAML frontmatter should render as document metadata.
tags: [codex, markdown, frontmatter]
draft: false
---

# Frontmatter Preview Reproduction

This paragraph is ordinary Markdown body content.

What steps can reproduce the bug?

1. Save the minimal example above as a local .md file.
2. Ensure the file is UTF-8 without a BOM and uses LF line endings.
3. Open the file in Codex Desktop.
4. Switch the file viewer to View preview / rich preview.
5. Inspect the YAML frontmatter at the top of the document.

What is the expected behavior?

The leading YAML block should be recognized as frontmatter and displayed using the metadata presentation.

The raw --- delimiters and YAML source should not be rendered as ordinary Markdown body content. Markdown following the frontmatter should continue to render normally.

Additional information

_No response_