Markdown table of contents doesn't handle YAML frontmatter

[bollwyvl]

Description

When editing a Markdown document with YAML front-matter, the Table of Contents shows only the last item of frontmatter.

Reproduce

• Open the Launcher

• Click on Markdown File

• Open the Table of Contents sidebar

• Add a header

  # Hello World

• See Hello World in the Table of Contents

• Add some YAML front matter before the header

  ---
  foo: bar
  boo: baz
  ---
  
  # Hello World

• See only boo: baz in the Table of Contents

This was found while working on a JEP in JupyterLab.

By adding a newline above the second ---, the problem also goes away, so presumably it's treating that last item as an h2 which counterfeits everything below it.

Expected behavior

• Definitely see all the headers
• Maybe see a collapsible table above/below the rest of the items
  • but definitely not worth adding/faking a YAML parser
  • maybe just a chunk of YAML-formatted code (e.g. if the first line starts with [^:]+:

Context

• Operating System and version: Linux Ubuntu 21.04
• Browser and version: Chromium 110
• JupyterLab version: 4.0.0a39 (+ Update to marked 7.0.2, add mermaidjs 10.3.1 #14102)

[JasonWeill]

Does this happen with 3.6.x or older versions of JupyterLab? Is this a regression?

[bollwyvl]

Cheers. I never shook this down in 3.*, but I'd wager it exhibits the condition there was all.