Preserve cell formatting through decode→encode round-trip

[ben-milanko]

Problem

Loading a styled .xlsx, mutating one cell, then re-encoding silently rebuilds the styles table and drops every untouched cell's font / fill / border / alignment. End-to-end on a real Numbers-exported template (~95 cellXfs, dataValidations, conditionalFormatting), styles.xml grows to 117 cellXfs (22 generic borderId="0" fillId="0" fontId="0" entries appended) and every cell's <c s="N"> index is remapped to point into the new table — so headers render as plain text, dropdowns vanish visually, etc.

Root cause

Four compounding bugs:

1. Sheet._putData clobbers existing styles. Every value write unconditionally did cell._cellStyle = CellStyle(numberFormat: NumFormat.defaultFor(value)), wiping the cell's font/fill/border/alignment.
2. Always-true guard. The follow-up if (cell._cellStyle != NumFormat.standard_0) compares a CellStyle? to a NumFormat (different types — always true), so _styleChanges flipped on every value write and triggered a styles-table rebuild.
3. Parser flips the change flags. The parser drives every cell through Sheet.updateCell while loading the file, so _styleChanges / _mergeChanges / _rtlChanges were already true after a no-op decode. Those flags should reflect post-decode user mutations, not the act of decoding itself.
4. StandardNumericNumFormat.accepts(TextCellValue()) returned true only for numFmtId == 0 (General), not 49 (@/text). Writing text into a text-formatted column was therefore deemed format-incompatible and triggered a numberFormat overwrite.

Fix

Targeted, minimal — preserves the existing public contract (writing a Double into a General cell still upgrades to defaultFloat; writing null resets to General):

• Sheet._putData keeps the existing CellStyle and only patches numberFormat via copyWith. Promotes General to a value-specific default; resets to General on null; otherwise leaves explicit formats alone unless accepts(value) is false.
• Excel._ resets _styleChanges / _mergeChanges / _rtlChanges after parser._startParsing() so a no-op decode→encode skips the rebuild paths.
• StandardNumericNumFormat.accepts(TextCellValue()) accepts both numFmtId == 0 and numFmtId == 49.

Tests

All 34 existing tests still pass. Two new tests in Style preservation on round-trip cover the regression on borders.xlsx:

• writing into one cell keeps an untouched cell's style intact — writes Z50, asserts A1's bold + medium-border header survives.
• overwriting a styled cell's value preserves font/fill/border — writes a new value into A1, asserts every visual attribute is intact after round-trip.

Verification on a real template

On a Numbers-exported template with the bulk-import write pattern (5 text writes into the valueList sheet + setColumnAutoFit on 11 columns):

	Before fix	After fix
<fonts count>	7 (was 10)	10 ✓
<fills count>	9 (was 8)	8 ✓
<cellXfs count>	117 (was 95)	95 ✓
styles.xml diff vs original	1610 bytes added	0 bytes ✓
Header A1 <c s="N">	s="76" (was 60)	s="60" ✓
<dataValidations>	preserved	preserved
<conditionalFormatting>	preserved	preserved

[gonojuarez]

hello @ben-milanko I created a fork of Flutter Excel. The name is excel_community you can find it here link