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Layout and rendering research synthesis

Layout is not geometry

NUIF separates authored constraints from resolved boxes. Fixed x/y/width/height are valid authored values for freeform content, but they are not the universal layout representation.

The initial layout vocabulary contains families rather than one universal algorithm:

  • freeform — transforms/anchors and explicit geometry.
  • stack — one-dimensional flow with intrinsic sizing, distribution, alignment and gaps.
  • flex — web-compatible flexible layout semantics.
  • grid — bounded explicit fixed/fr tracks, spans and deterministic no-implicit-track placement in profile 0; broader CSS Grid features remain capability-reported adapter input.
  • constraint — relational linear constraints for editor/native-layout cases.
  • custom — extension/dialect-defined evaluator with declared fallback/resolved geometry.

Common sizing primitives are normalized across families: fixed, intrinsic-min, intrinsic-max, fit-content, fill/available, percentage, min/max clamps, aspect ratio and content measurement.

Taffy is the recommended first evaluator for CSS-compatible block/flex/grid behavior because it implements web algorithms in Rust. SwiftUI’s proposal-response model and Cassowary-style constraints demonstrate why the canonical schema must remain a superset rather than serializing Taffy’s Style directly.

Evaluation context

Resolved layout is keyed by an explicit context including viewport/container size, pixel ratio, locale, writing direction, font set, token/theme selection and feature/dialect capabilities. Multiple resolved snapshots may coexist as caches or conformance fixtures.

Rendering semantics

The draft specification defines the visual meaning of paths, fills, strokes, transforms, clipping, masks, gradients, compositing, images, text and supported effects. It does not specify GPU command buffers or a renderer implementation.

The reference renderer uses a backend trait. Vello/wgpu is the leading interactive experiment, but conformance requires deterministic raster comparisons and must permit CPU reference rendering where GPU differences would make tests unstable.

Text

Canonical text remains Unicode text + style runs + semantic annotations + font references. Shaping produces resolved glyph IDs, clusters, advances and offsets using pinned font data and a declared Unicode/shaping version. A glyph cache never replaces semantic text.

Portability reports must distinguish font substitution, missing glyphs, line-break differences and rasterization differences from document-model loss.

Canonical source.