fix(format/yaml): move overflowing multiline flow scalars below their key

[dyc3]

Summary

Before, a value always started after the key:

  description: A long first line that exceeds the configured width
    followed by another line

The fix gives multiline scalars two layouts:

  description: A short first line
    followed by another line

or, when the first line would overflow:

  description:
    A long first line that exceeds the configured width
    followed by another line

Found this while manually stress testing with some huge yaml files.

implemented by fable 5/opus 5

Test Plan

added tests

Docs

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Latest commit: 30cac32

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[coderabbitai]

Walkthrough

The YAML formatter now detects line breaks in flow scalars and checks available width before placing them. Multiline scalars remain beside the key when they fit; otherwise, they move to an indented line. Single-line scalars keep their existing placement. New fixtures cover quoted, anchored, nested, and exact-width cases.

Possibly related PRs

• biomejs/biome#10989: Changes related YAML mapping-value placement and fitting logic in the same formatter module.
• biomejs/biome#11050: Extends related flow-scalar placement logic in block_map_implicit_entry.rs.
• biomejs/biome#11057: Changes YAML property attachment and lookahead for related multiline scalar cases.

Suggested reviewers: siketyan, ematipico

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes moving overflowing multiline YAML flow scalars below their key.
Description check	✅ Passed	The description explains the YAML formatting change and identifies the added test coverage.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dyc3/yaml-fmt-multiline-scalar-break

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/biome_yaml_formatter/tests/specs/yaml/scalar/multiline_overflow.yaml`:
- Around line 13-15: Update the exact_fit fixture’s scalar text so the complete
line is exactly 80 characters wide, preserving its purpose of exercising the
equal-to-width formatting branch. Keep the existing YAML structure and add or
adjust only the necessary text; retain appropriate coverage for this exact-width
case.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between f399d3a and 30cac32.

⛔ Files ignored due to path filters (1)

• crates/biome_yaml_formatter/tests/specs/yaml/scalar/multiline_overflow.yaml.snap is excluded by !**/*.snap and included by **

📒 Files selected for processing (2)

• crates/biome_yaml_formatter/src/yaml/auxiliary/block_map_implicit_entry.rs
• crates/biome_yaml_formatter/tests/specs/yaml/scalar/multiline_overflow.yaml

[coderabbitai]

🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Make the exact-width fixture exactly 80 columns.

Line 15 is 78 characters long. It does not test the equal-to-width branch.

Proposed fix

-    - name: first line is exactly eighty columns wide, so the scalar stays put
+    - name: first line is precisely eighty columns wide, so the scalar stays put

As per coding guidelines, all code changes must include appropriate tests.

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

	exact_fit:
	deeply:
	- name: first line is exactly eighty columns wide, so the scalar stays put
	exact_fit:
	deeply:
	- name: first line is precisely eighty columns wide, so the scalar stays put

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/biome_yaml_formatter/tests/specs/yaml/scalar/multiline_overflow.yaml`
around lines 13 - 15, Update the exact_fit fixture’s scalar text so the complete
line is exactly 80 characters wide, preserving its purpose of exercising the
equal-to-width formatting branch. Keep the existing YAML structure and add or
adjust only the necessary text; retain appropriate coverage for this exact-width
case.

Source: Coding guidelines