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Why My Fonts Caused Layout Shift: Field Notes on next/font, size-adjust, and Arabic Fallback Metrics

Font-driven layout shift is a metrics mismatch, not a download-speed problem — the fallback font the browser paints first occupies a different box than the webfont that replaces it.

  • Font-driven layout shift caused by fallback font occupying different box than webfont.
  • next/font generates size-adjusted fallback @font-face for Latin text, but not for Arabic.
  • font-display: optional reduces swap-driven layout shift, but download speed is irrelevant.

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