fix: preserve hash lines in block scalars as content - #6287

Summary

kustomize edit previously collected all lines starting with # as field comments, regardless of indentation, causing lines inside YAML block scalars (|, >) to be misinterpreted as comments. This caused those lines to be duplicated on each run and inserted before the following field. This PR introduces indentation tracking for block scalar headers so lines inside a block scalar are recognized as content rather than field comments.

Rationale

Editing a kustomization.yaml that contains multiline scripts or configs inside block scalars should be idempotent. Previously, running kustomize edit multiple times duplicated # lines exponentially, changing unrelated transformer content and growing the file without limit.

Related issue

Fixes #6225

While #4481 addresses the broader problem of comment placement and movement across fields, this fix specifically addresses content corruption caused by mistaking block scalar content for field comments.

Assignment and coordination

Assigned via /assign on 2026-09-22 in #6225 (comment) prior to starting the implementation.

Testing

• Commit 1: Added a regression test in kustomizationfile_test.go that pins the current incorrect output, where # lines inside a block scalar are moved above the field and duplicated.
• Commit 2: Implemented indentation-aware block scalar handling in kustomizationfile.go and updated the test to expect the input unchanged after a write, which also makes repeated edits idempotent.
• Ran unit tests locally:
  go test ./kustomize/commands/internal/kustfile/... -v (all tests passed).
• golangci-lint with the repo config reports 0 issues, and the reproduction script from the issue keeps the marker count at 1 across 5 runs.

AI disclosure

I used Anthropic Claude Code (Claude Opus 5) to trace the root cause, write the fix and the regression test, and run the local checks. It also helped with light edits to this description. I reviewed the full diff and the test results, and wrote the commit messages and this description myself.