Markdown header sections are NOT collapsible in (new) Positron notebooks

System details:

Positron Version: 2025.12.0 build 35
Code - OSS Version: 1.105.0
Commit: 5418e79f67133ab96a97fc1b3b80e9d3314032e7
Date: 2025-11-06T07:19:04.909Z
Electron: 37.6.0
Chromium: 138.0.7204.251
Node.js: 22.19.0
V8: 13.8.258.32-electron.0
OS: Darwin arm64 25.0.0

Session details:

Python 3.13

Describe the issue:

• In old notebooks, Markdown cells with headers (e.g., #, ##, ###) create collapsible sections that allow users to hide or expand the code and output cells nested within that section.
• In new notebooks, this collapsible functionality no longer works. There is NO triangle next to the header, and users cannot hide cells belonging to that section.

Expected or desired behavior:

As a notebooks user, I want to be able to collapse and expand cells within a Markdown header section so that I can organize long notebooks efficiently and focus only on relevant parts of the workflow.

This feature is useful for:

• Structuring complex analysis notebooks
• Teaching materials with sections and subsections
• Hiding setup code or large output blocks

Steps to reproduce the issue:

1. Open an old notebook and insert the following:

   # Section 1
   

   Add a few code cells below it.

2. Collapse the section by clicking the triangle next to "Section 1".
   → Works: cells hide correctly.

3. Open a new notebook and repeat the same steps.

   # Section 1
   

   Add a few code cells below it.

4. Try to collapse the section. Notice that there is no triangle next to the markdown cell. It's impossible to collapse the section (in order to have cells hidden underneath.
   → Nothing happens: cells cannot be hidden.

Note:

• This regression affects notebook readability and structure management.
• Old notebooks correctly interpret Markdown headers as collapsible regions, while new notebooks do not.
• Feature parity with the old notebook format and JupyterLab-style collapsible sections is expected.

No source files, tests, or entry points are identified in the issue. Compare the old and new notebook implementations while reproducing a Markdown # Section 1 followed by code cells; done means new notebooks show a triangle and correctly collapse and expand the cells under the header.