HTML header contents in markdown notebook cells not correctly populated in breadcrumbs

[joyceerhl]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: github.dev
• OS Version: Windows 10

Steps to Reproduce:

1. Visit https://github.dev/lostintangent/python-tuples
2. Select any of the markdown cells
3. 🐛 breadcrumb preview says 'empty cell', this is apparently because the cell headers are created with HTML; if I rewrite them to be Markdown the breadcrumbs are accurate
   

[mjbvz]

Here's the relevant code:

vscode/src/vs/workbench/contrib/notebook/browser/contrib/outline/notebookOutline.ts

Line 416 in 5c8ba90

for (const token of marked.lexer(content, { gfm: true })) {

We currently only look for markdown headers. Instead, we'd likely want to get the headers from the rendered markup. That would nicely support other types of rendered markup or extensions to the markdown rendering

[IanMatthewHuff]

This would also be a big improvement for Outline view I believe. Jupyter picks up HTML headers and adds them to the table of contents view, but in VS Code we miss out on them so the structure looks funny with some notebooks.