Updating size-adjust calculation (vercel#41406)

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Updating size-adjust calc to use azAvgWidth instead of xAvgCharWidth

Co-authored-by: Hannes Bornö <hannes.borno@vercel.com>
Co-authored-by: Balázs Orbán <info@balazsorban.com>
Co-authored-by: JJ Kasper <jj@jjsweb.site>

packages/font/src/local/loader.ts

@@ -6,7 +6,7 @@ import fontFromBuffer from '@next/font/dist/fontkit'
 import type { AdjustFontFallback, FontLoader } from 'next/font'
 
 import { promisify } from 'util'
-import { validateData } from './utils'
+import { calcAzWidth, validateData } from './utils'
 
 const fetchFonts: FontLoader = async ({
   functionName,
@@ -58,6 +58,7 @@ const fetchFonts: FontLoader = async ({
       lineGap: fontMetadata.lineGap,
       unitsPerEm: fontMetadata.unitsPerEm,
       xAvgCharWidth: (fontMetadata as any)['OS/2']?.xAvgCharWidth,
+      azAvgWidth: calcAzWidth(fontMetadata),
     })
     adjustFontFallbackMetrics = {
       fallbackFont,

packages/font/src/local/utils.ts

@@ -1,3 +1,5 @@
+import type { Font } from 'fontkit'
+
 const allowedDisplayValues = ['auto', 'block', 'swap', 'fallback', 'optional']
 
 const formatValues = (values: string[]) =>
@@ -91,3 +93,12 @@ export function validateData(functionName: string, data: any): FontOptions {
     declarations,
   }
 }
+
+// Calculating the a-z average width
+export function calcAzWidth(font: Font) {
+  const widths = font
+    .glyphsForString('abcdefghijklmnopqrstuvwxyz')
+    .map((glyph) => glyph.advanceWidth)
+  const totalWidth = widths.reduce((sum, width) => sum + width, 0)
+  return totalWidth / widths.length
+}

packages/next/server/font-utils.ts

@@ -119,14 +119,14 @@ export function calculateOverrideValues(fontMetrics: any) {
 }
 
 export function calculateSizeAdjustValues(fontMetrics: any) {
-  let { category, ascent, descent, lineGap, unitsPerEm, xAvgCharWidth } =
+  let { category, ascent, descent, lineGap, unitsPerEm, azAvgWidth } =
     fontMetrics
   const fallbackFont =
     category === 'serif' ? DEFAULT_SERIF_FONT : DEFAULT_SANS_SERIF_FONT
 
-  let sizeAdjust = xAvgCharWidth
-    ? xAvgCharWidth / fallbackFont.xAvgCharWidth
-    : 1
+  const mainFontAvgWidth = azAvgWidth / unitsPerEm
+  const fallbackFontAvgWidth = fallbackFont.azAvgWidth / fallbackFont.unitsPerEm
+  let sizeAdjust = azAvgWidth ? mainFontAvgWidth / fallbackFontAvgWidth : 1
 
   ascent = formatOverrideValue(ascent / (unitsPerEm * sizeAdjust))
   descent = formatOverrideValue(descent / (unitsPerEm * sizeAdjust))