From acebb00b50ab9dd8af12bfeec2c9117f067ed21e Mon Sep 17 00:00:00 2001
From: ssweber <57631333+ssweber@users.noreply.github.com>
Date: Wed, 15 Apr 2026 14:00:46 -0400
Subject: [PATCH] fix(csv): downgrade trailing blank row mismatch to warning in
 writer validation

Rungs with blank rows beyond a tall instruction's visual height caused
WriterError because the stripped continuation absorbed the extra blank
row on read-back. Warn instead of failing when the lost rows are pure
padding.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 CHANGELOG.md                  |  8 ++++++++
 src/laddercodec/csv/writer.py | 28 ++++++++++++++++++++++------
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 358bc4a..231de31 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,13 @@
 # Changelog
 
+## 0.1.8
+
+### Fixed
+- **CSV writer**: downgrade trailing blank row mismatch from error to warning
+  when a tall instruction's stripped continuation absorbs extra blank rows on
+  read-back — fixes `WriterError` on rungs with blank rows beyond an
+  instruction's visual height
+
 ## 0.1.7
 
 ### Fixed
diff --git a/src/laddercodec/csv/writer.py b/src/laddercodec/csv/writer.py
index 80b9ce2..568cc2a 100644
--- a/src/laddercodec/csv/writer.py
+++ b/src/laddercodec/csv/writer.py
@@ -365,10 +365,26 @@ def _validate_roundtrip(rung: Rung, rows: Sequence[Sequence[str]]) -> None:
     rebuilt = _rebuild_rung_from_rows(rows)
 
     if rebuilt.logical_rows != rung.logical_rows:
-        raise WriterError(
-            "CSV round-trip validation failed: "
-            f"logical row count mismatch: expected {rung.logical_rows}, got {rebuilt.logical_rows}"
-        )
+        # Trailing blank rows (no conditions, no AF) can be lost when
+        # a tall instruction's stripped continuation absorbs them on
+        # read-back.  This is cosmetic — warn instead of failing.
+        extra = rung.logical_rows - rebuilt.logical_rows
+        if extra > 0 and all(
+            _is_padding_row(rung.conditions[r]) and rung.instructions[r] == ""
+            for r in range(rebuilt.logical_rows, rung.logical_rows)
+        ):
+            import warnings
+
+            warnings.warn(
+                f"CSV round-trip lost {extra} trailing blank row(s) "
+                f"(logical_rows {rung.logical_rows} → {rebuilt.logical_rows})",
+                stacklevel=2,
+            )
+        else:
+            raise WriterError(
+                "CSV round-trip validation failed: "
+                f"logical row count mismatch: expected {rung.logical_rows}, got {rebuilt.logical_rows}"
+            )
 
     if rebuilt.comment != rung.comment:
         raise WriterError(
@@ -377,7 +393,7 @@ def _validate_roundtrip(rung: Rung, rows: Sequence[Sequence[str]]) -> None:
         )
 
     for row_idx, (expected_row, actual_row) in enumerate(
-        zip(rung.conditions, rebuilt.conditions, strict=True),
+        zip(rung.conditions[: rebuilt.logical_rows], rebuilt.conditions, strict=True),
         start=1,
     ):
         for col_idx, (expected, actual) in enumerate(zip(expected_row, actual_row, strict=True)):
@@ -389,7 +405,7 @@ def _validate_roundtrip(rung: Rung, rows: Sequence[Sequence[str]]) -> None:
                 )
 
     for row_idx, (expected, actual) in enumerate(
-        zip(rung.instructions, rebuilt.instructions, strict=True),
+        zip(rung.instructions[: rebuilt.logical_rows], rebuilt.instructions, strict=True),
         start=1,
     ):
         if not _af_tokens_match(expected, actual):