Copy/move assignment kept the destination's line index, so closing a
buffer (vector erase move-assigns later buffers down) left the shifted
buffer with its predecessor's line offsets and out-of-bounds line views.
Appends that merged into an existing piece also skipped invalidation.
Audit findings M1, B12.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB
Claude