Replace SixLabors.Fonts with SkiaSharp for text measurement

[Copilot]

SixLabors.Fonts 1.0.1 has unfixed bugs with certain fonts (e.g. NullReferenceException in the GSUB substitution engine for fonts like "B Nazanin"), and upgrading to 2.x is blocked because 2.x dropped .NET Standard 2.0/2.1 support.

Replacement: SkiaSharp 2.88.9

• SkiaSharp replaces SixLabors.Fonts for all text measurement and font resolution
• SkiaSharp.NativeAssets.Linux.NoDependencies added explicitly — SkiaSharp auto-includes Win32 and macOS native assets as transitive deps but deliberately omits Linux; this variant requires only standard glibc components and works on all Linux distributions including minimal containers

Key API changes

SheetUtil.IFont2Font(IFont) now returns SKFont (disposable — callers must use using). Font size is stored in pixels (pointSize × dpi / 72); typeface resolution is cached separately keyed by (fontName, bold, italic).

// Before
Font wfont = SheetUtil.IFont2Font(font);
float width = TextMeasurer.MeasureSize(txt, new TextOptions(wfont) { Dpi = 96 }).Width;

// After
using SKFont wfont = SheetUtil.IFont2Font(font);
using var paint = new SKPaint { Typeface = wfont.Typeface, TextSize = wfont.Size };
float width = paint.MeasureText(txt);

Files changed

• SheetUtil.cs — IFont2Font → SKFont; TextMeasurer → SKPaint.MeasureText; height uses SKFont.Size (em-square) to match prior MeasureAdvance line-height semantics; empty strings return 0 height
• EscherGraphics.cs — Font/FontFamily/SystemFonts → SKFont/SKTypeface; Font property type updated
• HSSFSheet.cs — HSSFFont2Font() delegates to SheetUtil.IFont2Font
• XSSFSheet.cs, XSSFClientAnchor.cs — inline TextMeasurer.MeasureSize("0", …) → SKPaint.MeasureText("0")
• FormulaParser.cs — SixLabors.Fonts.Unicode.CodePoint removed; replaced with char.IsLetter/IsDigit/IsLetterOrDigit via char.ConvertFromUtf32

Original prompt

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This section details on the original issue you should resolve

<issue_title>Problem with font "BNazanin.ttf", System.NullReferenceException</issue_title>
<issue_description>### NPOI Version
2.7.4 (or whichever version uses SixLabors.Fonts 1.0.1)

File Type

• XLSX
• XLS
• DOCX
• XLSM
• OTHER

Upload the Excel File

No file attached — issue can be reproduced with the code below.

Reproduce Steps

var workbook = new SXSSFWorkbook();
var sheet = (SXSSFSheet)workbook.CreateSheet("test");

var cell = sheet.CreateRow(0).CreateCell(0);
cell.SetCellValue("نام");

IFont font = workbook.CreateFont();
font.FontName = "B Nazanin"; 
cell.CellStyle = workbook.CreateCellStyle();
cell.CellStyle.SetFont(font);

sheet.TrackColumnForAutoSizing(0);
sheet.AutoSizeColumn(0);

1. Create a new SXSSFWorkbook and sheet.
2. Add a cell with the value "نام".
3. Set the font to "B Nazanin".
4. Track column 0 for auto-sizing and call AutoSizeColumn(0).
5. Observe crash.

Stack trace:

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at SixLabors.Fonts.Tables.AdvancedTypographic.GSub.LookupType5Format2SubTable.TrySubstitution(FontMetrics fontMetrics, GSubTable table, GlyphSubstitutionCollection collection, Tag feature, Int32 index, Int32 count)
   at SixLabors.Fonts.Tables.AdvancedTypographic.GSub.LookupTable.TrySubstitution(FontMetrics fontMetrics, GSubTable table, GlyphSubstitutionCollection collection, Tag feature, Int32 index, Int32 count)
   at SixLabors.Fonts.Tables.AdvancedTypographic.GSubTable.ApplyFeature(FontMetrics fontMetrics, GlyphSubstitutionCollection collection, SkippingGlyphIterator& iterator, Tag& featureTag, ScriptClass current, Int32 index, Int32& count, Int32& i, Int32& collectionCount, Int32 maxCount, Int32 maxOperationsCount, Int32& currentOperations)
   at SixLabors.Fonts.Tables.AdvancedTypographic.GSubTable.ApplySubstitution(FontMetrics fontMetrics, GlyphSubstitutionCollection collection)
   at SixLabors.Fonts.StreamFontMetrics.ApplySubstitution(GlyphSubstitutionCollection collection)
   at SixLabors.Fonts.FileFontMetrics.ApplySubstitution(GlyphSubstitutionCollection collection)
   at SixLabors.Fonts.TextLayout.DoFontRun(ReadOnlySpan`1 text, Int32 start, IReadOnlyList`1 textRuns, Int32& textRunIndex, Int32& codePointIndex, Int32& bidiRunIndex, Boolean isFallbackRun, Font font, BidiRun[] bidiRuns, Dictionary`2 bidiMap, GlyphSubstitutionCollection substitutions, GlyphPositioningCollection positionings)
   at SixLabors.Fonts.TextLayout.ProcessText(ReadOnlySpan`1 text, TextOptions options)
   at SixLabors.Fonts.TextLayout.GenerateLayout(ReadOnlySpan`1 text, TextOptions options)
   at SixLabors.Fonts.TextMeasurer.MeasureSize(ReadOnlySpan`1 text, TextOptions options)
   at SixLabors.Fonts.TextMeasurer.MeasureSize(String text, TextOptions options)
   at NPOI.SS.Util.SheetUtil.GetCellWidth(Int32 defaultCharWidth, Int32 colspan, ICellStyle style, Double width, String str, Font windowsFont, ICell cell)
   at NPOI.SS.Util.SheetUtil.GetCellWidth(ICell cell, Int32 defaultCharWidth, DataFormatter formatter, Boolean useMergedCells)
   at NPOI.SS.Util.SheetUtil.GetColumnWidthForRow(IRow row, Int32 column, Int32 defaultCharWidth, DataFormatter formatter, Boolean useMergedCells)
   at NPOI.SS.Util.SheetUtil.GetColumnWidth(ISheet sheet, Int32 column, Boolean useMergedCells, Int32 firstRow, Int32 lastRow, Int32 maxRows)
   at NPOI.SS.Util.SheetUtil.GetColumnWidth(ISheet sheet, Int32 column, Boolean useMergedCells, Int32 maxRows)
   at NPOI.XSSF.Streaming.SXSSFSheet.AutoSizeColumn(Int32 column, Boolean useMergedCells)
   at NPOI.XSSF.Streaming.SXSSFSheet.AutoSizeColumn(Int32 column)

Issue Description

When using B Nazanin font with SXSSFWorkbook and calling AutoSizeColumn, NPOI crashes because it depends on SixLabors.Fonts 1.0.1, which contains a bug with this font.

This bug is fixed in SixLabors.Fonts 2.0.1: SixLabors/Fonts#364

Proposed fix: update NPOI’s dependency on SixLabors.Fonts to >= 2.0.1.</issue_description>

<agent_instructions>Help me check if this issue still exists in lastest master branch</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@tonyqus NPOI cannot upgrade SixLabors.Fonts to 2.x because SixLabors.Fonts 2.x only supports .NET 6. It doesn't even support .NET Standard 2.0 and .NET Standard 2.1

SixLabors/Fonts#361</comment_new>

• Fixes Problem with font "BNazanin.ttf", System.NullReferenceException #1630

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