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0012. Bundled fonts, real measurement and font policies

Status: accepted (text-measurement-and-fonts change, 10 October 2026). The sketch.* themes use Inter until a hand font is approved and bundled, by decision 0058.

Context

Section titled “Context”

Text widths were estimated from character classes, so labels could overflow the boxes the layout sized, and exported SVGs depended on the fonts installed on each viewer’s machine. The proposal asks the engine to measure text before layout, keep the result across formats, and ship fonts only with distribution and embedding permissions.

Decision

Section titled “Decision”
  • Bundle Inter, IBM Plex Sans and Cascadia Mono (OFL-1.1), read with fontkit (MIT).
  • Measure and outline with the complete fonts where a package provides them: inter-ui and @ibm/plex-sans carry every glyph, including arrows. Fontsource’s Google-style subsets drop arrows.
  • Embed the small Fontsource script subsets, limited with unicode-range. fontkit cannot write the cmap table browsers need, so glyph-level subsets are not an option.
  • Characters a family lacks fall back to the other bundled families, as in browsers.
  • The layout depends on a TextMeasurer interface and the renderer on a FontProvider. Both stay host-neutral; @meltemi/diagrams-fonts implements them.
  • Boxes without size grow to fit their text (16 px each side). An explicit size with less than 6 px of margin is layout.text-overflow. Text is never scaled down.
  • Three SVG font policies: embedded (CLI default), outlined and system (renderSvg default).
  • A license inventory is generated from the production dependencies, and CI checks that it is current.

Alternatives considered

Section titled “Alternatives considered”
  • Precomputed width tables: no outlines or embedding, and large kerning tables.
  • opentype.js: no WOFF2 support.
  • Glyph-level TrueType subsets: rejected by browsers without cmap, name, OS/2 and post tables.
  • Always outlining: exact everywhere, but text is no longer selectable and files are larger.
  • A 16 px overflow threshold: flagged tight designs (such as the first conversion) whose text clearly fits.

Consequences

Section titled “Consequences”
  • Corpus and example snapshots are rendered with the bundled fonts and the system policy, so they show real geometry and stay readable.
  • The VS Code extension and the playground must ship or load the font files (roadmap changes 8 and 22).
  • Raster and PDF exports reuse the same policies (roadmap change 16).