Fix #10436: YAML delimiter detection in mid-document blocks

[kompre]

Summary

Fixes #10436 - Mid-document YAML blocks are incorrectly parsed when the closing delimiter --- is followed by a blank line.

Problem

After commit 86122d8 (which fixed #8998 for slide separators), the YAML delimiter detection logic required a non-empty line after ---. This broke mid-document YAML blocks because the closing delimiter wasn't recognized when followed by blank lines, causing:

• Cell boundaries to be incorrect in .quarto_ipynb
• Opening --- from mid-document YAML appended to previous cell
• Spurious blank line appearing after the opening delimiter
• YAML content misparsed as markdown

This particularly affected .qmd files included via {{< include >}} with YAML frontmatter and code cells.

Solution

Modified the lookahead check in src/core/jupyter/jupyter.ts:459 to differentiate between opening and closing delimiters:

(inYaml || contentLines[currentLine + 1]?.trim() !== "")

• Opening ---: Requires non-empty next line (preserves --- as slide separation markers confuses Jupyter's cell detection #8998 fix for slide separators)
• Closing ---: Always recognized when inYaml=true (fixes including multiple files which contain jupyter: python3 in the frontmatter block AND some actual code blocks, will print the content of subsequent the frontmatter block #10436)

Changes

1. Fix: src/core/jupyter/jupyter.ts - Updated YAML delimiter detection logic
2. Test: tests/unit/core/jupyter-yaml-delimiter.test.ts - Unit tests for the fix

Testing

Added comprehensive unit tests:

1. ✅ Mid-document YAML with blank line - verifies the bug fix
2. ✅ Multiple mid-document YAML blocks - tests edge cases
3. ✅ Slide separators not treated as YAML - regression test for --- as slide separation markers confuses Jupyter's cell detection #8998

All tests pass:

ok | 3 passed | 0 failed (420ms)

Verification

To test manually:

1. Create a .qmd with mid-document YAML and code cells
2. Render it: quarto render test.qmd
3. Check .quarto_ipynb - mid-document YAML should be in a proper raw cell
4. Check output - no spurious blank lines in YAML blocks

Co-Authored-By: Claude Sonnet 4.5 noreply@anthropic.com

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[cscheid]

Thanks for the PR. This part of the code base is quite susceptible to regressions and I'm a bit concerned about that here for 1.9, and I'm not sure I want blank lines to be allowed around metadata (because that causes syntax confusion with HorizontalRule nodes). I think we need a further discussion of this, (and I think I'd like to consider this in the context of our new upcoming parser).

[kompre]

The fix proposed in the PR is not meant to allow blank lines around metadata delimiter.

What it does is to make sure that if the parser is in a yaml block (inYaml===true), then the first closing delimiter --- will properly close the yaml block.

As it stands now the closing delimiter will never be recognized as such, because it is always followed by a blank line, i.e. there is a whole part of the conditional statement that is unreachable. This is true even with just the yaml at the top of the document, but it just generally not noticeable because:

• when the qmd is converted to the ipynb, only code cell matter, the malformed raw/markdown cell is ignored
• when the ipynb is converted back to the intermediate md file, the malformed raw/markdown cell is written as is, with just one more blank line between it and the next adjson code cell.

So it is mostly an happy accident that generally it works fine.

By the way, I specifically tested for --- as horizontal rule and they work correctly with the proposed PR.