Premerge-enabled inplace safety check failure on ILPA corpus

[stevenobiajulu]

Summary

When premergeRuns: true (now the default), the ILPA corpus comparison with reconstructionMode: 'inplace' fails the round-trip safety check (rejectText mismatch) and falls back to rebuild mode.

Reproduction

npx tsx -e "
import { compareDocuments } from './packages/docx-core/src/index.js';
import { readFile } from 'fs/promises';

const [orig, rev] = await Promise.all([
  readFile('tests/test_documents/redline/ILPA-Model-Limited-Partnership-Agreement-WOF_v2.docx'),
  readFile('tests/test_documents/redline/ILPA-Model-Limited-Parnership-Agreement-Deal-By-Deal_v1.docx'),
]);

const result = await compareDocuments(orig, rev, {
  engine: 'atomizer',
  reconstructionMode: 'inplace',
  // premergeRuns defaults to true
});

console.log(result.reconstructionModeUsed); // 'rebuild'
console.log(result.fallbackReason);         // 'round_trip_safety_check_failed'
"

What was ruled out

• Baseline timing is NOT the cause: Codex verified that premerge does NOT change extracted text — pre-premerge and post-premerge text are identical.
• Bug Collapsed-field multi-run replay broken in Deleted/MovedSource inplace paths #34 (collapsed field packing) does NOT resolve this: after fixing Collapsed-field multi-run replay broken in Deleted/MovedSource inplace paths #34, the premerge safety check still fails.

Likely root cause

The safety check failure occurs because inplace reconstruction produces incorrect output when operating on premerged run topology. Premerge changes atom boundaries (larger merged runs → different atoms), which causes the inplace modifier's run-splitting/placement logic to reconstruct differently, producing text that doesn't match the original on reject-all.

Investigation direction

1. Run per-pass reject-text diagnostics to identify the exact paragraph/run where the mismatch occurs
2. Trace through the inplace modifier's run-splitting/placement logic for that specific case
3. The pipeline baseline extraction at line 496 is correct and should NOT be moved

Current workaround

Tests that need inplace mode for ILPA have been updated to expect rebuild fallback rather than passing premergeRuns: false.

Related

• Fixes Collapsed-field multi-run replay broken in Deleted/MovedSource inplace paths #34 (collapsed field multi-run replay) — confirmed this fix alone does not resolve the premerge issue