🐛 Keep a nested final-block foot comment at its indent on edit

[frenck]

Breaking change

None. Fixes a round-trip fidelity bug; no API change.

Proposed change

Fixes a foot-comment fidelity bug exposed while adding comment_after (#213). A comment at the end of a nested final block round-trips byte-for-byte when untouched, but the moment the document is edited it re-emits at column 0 instead of the block's indent:

parent:
  a: 1
  b: 2
  # end of parent      <- edit `b`, and this jumps to column 0

The cause was in the composer: every trailing comment left after the walk was attached to the outermost (root) node's foot, which the emitter renders at indent 0. On the AST re-emit path (taken after an edit) the comment therefore detached from the block it belongs to.

The fix routes each trailing comment to the deepest block it is indented into, by descending the last-child spine while the child is a block collection no more indented than the comment. A comment aligned with a nested block's contents now lands on that block (and re-emits at its indent); a comment at the document's own indent still stays on the outermost node. Multi-level trailing comments each attach to their own level. The descent is bounded by the document's nesting depth and stack-guarded, consistent with the other composer walks.

Because unmodified documents re-emit their trailing comment verbatim (independent of which node owns it), this changes only the post-edit path: the full compliance suite and the entire real-world round-trip corpus stay byte-for-byte. New tests cover the edit-keeps-indent case, the nested read-back attribution, and multi-level routing.

Type of change

• Dependency or tooling upgrade
• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Deprecation (replaces or removes a feature, with a migration path)
• Breaking change (a fix or feature that changes existing behavior)
• Code quality, refactor, or test-only change
• Documentation only

Additional information

• This PR fixes or closes issue: None
• This PR is related to: ✨ Add writable foot comments via node.comment_after #213 (follow-up to writable foot comments)
• Link to a separate documentation pull request:

Checklist

• I have read the AI Policy, and this pull request was not created by an autonomous agent.
• I fully understand the code in this pull request and can explain every line, including any AI-assisted changes.
• The change is covered by tests, and uv run pytest passes locally. A pull request cannot be merged unless CI is green.
• uv run ruff check . and uv run ruff format --check . pass.
• cargo fmt --check and cargo clippy --all-targets -- -D warnings pass.
• Round-trip fidelity is preserved: an unmodified document still re-emits byte-for-byte.
• No commented-out or dead code is left in the pull request.

If the change is user-facing:

• Documentation under docs/ is added or updated, and docs/verify_examples.py still passes.

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Review details

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 75918a7d-fb58-4dec-8071-da01eb7a619d

📥 Commits

Reviewing files that changed from the base of the PR and between 11b61e6 and a830d98.

📒 Files selected for processing (3)

• src/roundtrip/composer.rs
• tests/roundtrip/test_node.py
• tests/roundtrip/test_roundtrip.py

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[Copilot]

Pull request overview

This PR fixes a round-trip fidelity bug in the composer's trailing-comment attachment. Previously, all comments left after the AST walk were attached to the outermost (root) node's foot, which the emitter renders at column 0. For an unmodified document this was hidden by the verbatim source cache, but once the document was edited (forcing the AST re-emit path), a comment at the end of a nested final block would flatten to column 0 instead of keeping the block's indent. The fix introduces a bounded descent (foot_target) that routes each trailing comment to the deepest block collection it is indented into, so it re-emits at the correct indent. This is a follow-up to #213 (writable foot comments).

Changes:

• Added foot_target/foot_target_inner to descend the last-child spine and attribute each trailing comment to the deepest block collection whose column is <= the comment's column.
• Added is_block_collection helper to identify non-empty block mappings/sequences that can carry a foot comment.
• Added tests for the edit-keeps-indent case, nested read-back attribution, and multi-level trailing-comment routing.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/roundtrip/composer.rs	Routes trailing comments to the deepest indented block via a new bounded, stack-guarded descent instead of always the root node.
tests/roundtrip/test_roundtrip.py	Adds a test that an edited nested final-block foot comment keeps its indent.
tests/roundtrip/test_node.py	Adds tests that nested/multi-level trailing comments read back as the owning block's comment_after.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks

---

_{Comparing frenck/foot-final-block-indent (a830d98) with main (11b61e6)}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.64%. Comparing base (11b61e6) to head (a830d98).

Files with missing lines	Patch %	Lines
src/roundtrip/composer.rs	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
+ Coverage   92.62%   92.64%   +0.01%     
==========================================
  Files          40       40              
  Lines       12155    12188      +33     
==========================================
+ Hits        11259    11291      +32     
- Misses        896      897       +1     

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