d4679a7543
When a file does not end with a newline, appending content extends the last logical line's text and thus its visual height. The incremental extend path in handle_file_appended only computed heights for newly created logical lines, missing the old last line whose content changed. Add VisualHeightIndex::replace_last_line_height() — an O(1) method that rewrites the final prefix sum entry and total. Called before extend_from_heights so the correct line is targeted. Changes: - progressive_reader.rs: add replace_last_line_height, pub with_params, 7 VHI unit tests - app.rs: save old_reader_line_count before update, recompute last old line height in extend path, 2 integration regression tests