svelte-kit-vice/docs/rfcs/rfc-structure.md

title	type	audience	status	source
RFC — Eidos structural systems (space · density · scale)	rfc	human + agent	partially implemented — phase 1 (space builder) pending	migrated from src/uix/eidos/STRUCTURE_ENGINE_RFC.md (2026-07-02, docs-book F7.4)

RFC — Eidos structural systems (space · density · scale)

Sibling of rfc-color-engine.md, rfc-typography.md, rfc-depth.md and rfc-shape.md. Takes the structural systems to reference-grade. Unlike the expressive channels (the book's 8), the structural is state-only — the stage, not the happening. So the novelty here is not eventful: it is rhythm, fluidity and axis composition, under the open cage.

0. Thesis

Space is not a lookup table of arbitrary pixels; it is a rhythm — derived from a base unit, fluid (it breathes with the viewport) and composed with density and zoom from a minimal seed.

Everybody ships a flat space scale (4 · 8 · 12 · 16 · 24…), arbitrary, static and detached from typography. Eidos already has the other two structural axes —density (compactness) and scaling (zoom)— above the average; what's missing is for space itself to be rhythm: modular, fluid and with a runtime builder, as typography already did.

1. The survey — how the references do it and where they stop

Framework	Space	Limit
Tailwind	fixed scale (0.25rem × N)	flat, arbitrary, static
Material	8dp grid	multiples of 8, static, no fluidity
Radix / Chakra / Mantine	space tokens	flat static scale; density (if any) = global preset
Bootstrap / Ant / Carbon / Fluent	spacer scale	same — flat + static
Utopia.fyi	fluid space (technique)	an external calculator, not a token system integrated with density + zoom

Common limit: space is a flat px scale, static (doesn't breathe with the viewport), arbitrary (derives from nothing), and disconnected from density / zoom as a system. Utopia proved fluid space but as a spreadsheet, not as a token engine.

2. Where Eidos stands today (strong on 2 of 3 axes)

• Density — 3 levels (compact · comfortable · spacious) × 2 axes (spaceScale + controlScale). Tightens layout without touching text legibility. ✓ (above average)
• Scaling — global zoom 90–110 that scales the px including typography (Radix parity), composing with density. ✓
• Layout — containers + padding + breakpoints + aspect-ratios. ✓
• Composition — --space-{key} is emitted as calc(value · var(--density-space-scale) · var(--scaling)): density × zoom already compose. ✓
• BUT space ITSELF (STATIC_SPACE) is flat, arbitrary px (base 4, hand-placed half-steps), static (doesn't breathe) and builder-less — unlike type, which has buildTypeScale (modular + fluid) + applyTypeScale (runtime). It is the lagging axis.

3. The novel model — space as rhythm

1. Modular — every step = base unit × N (a coherent ladder), not loose px.
2. Fluid — clamp(): space breathes with the viewport (like fluid type — almost no framework does it for space). Reuses the type scale's fluidClamp.
3. Three orthogonal axes — rhythm (the scale) × density (compactness) × scaling (zoom), composed multiplicatively. One minimal seed governs them.
4. Runtime builder — buildSpaceScale(seed) (pure) + applySpacing(seed) (DOM), sibling of applyColorScheme / applyTypeScale / applyDepth / applyShape. Completes the quintet.

3.bis Structural = state-only (no two moments)

Unlike motion / depth / shape, space does not "happen": it is the stage, not the happening. The two-moments model (state vs event) belongs to the expressive channels. Forcing an "eventful space" would be a costume — the doctrinally honest position is that the novelty here is rhythm + fluidity + axis composition, not eventfulness. (Same rigor: don't invent a moment that doesn't exist.)

4. Doctrine — strong default, open cage

Piece	Strong default	Open door
space scale	authored STATIC_SPACE (stable, curated)	buildSpaceScale / applySpacing = modular + fluid opt-in alternative (same stance as applyTypeScale over the authored scale)
density	3 levels × 2 axes	config-driven + runtime ([data-density])
scaling	90–110, universal factors	runtime ([data-scaling]); composes with density
composition	calc(value · density · scaling)	the raw --space-* primitives always reachable; the builder preserves the composition
whole system	canonical theme	runtime applySpacing(seed)

5. Token contract

--space-{key}   value · var(--density-space-scale) · var(--scaling)   (existing scale — kept)

The builder rewrites the value (managed block) with a modular/fluid one, preserving the calc(… · density · scaling) so density and zoom keep composing. Zero renames → zero breakage.

6. Phases

1. Space builder — buildSpaceScale(seed) (pure: base unit × ladder, fluid via fluidClamp) + ActiveEidos.applySpacing / clearSpacing (managed block that preserves · density · scaling) + export + test. Opt-in; STATIC_SPACE untouched.
2. Showcase + docs — /temas/estructura (density × scaling × fluid space, live) + theming reference §structure + this RFC.
3. ⏸️ (future) applyTheme(seed) — one seed composing type + space (shared rhythm).

7. Composition with the existing

• fluidClamp (from the type scale) → fluid space (not reinvented).
• calc(value · density · scaling) → preserved (density + zoom keep composing).
• buildTypeScale → the exact pattern buildSpaceScale mirrors (seed → fluid ladder).
• Box/Flex/Grid/Stack/Container → consume --space-*; untouched.

8. Doctrine (parallel to color / typography / depth / shape)

• Authored scale = stable canon; the builder = opt-in mathematical alternative (same as typography). The theme retunes, the builder recomposes.
• Density and scaling = axes orthogonal to rhythm; the three compose.
• Open cage: raw --space-* always one step away.

9. Out of scope

• Rigid baseline grid (pixel-perfect vertical rhythm): modular + fluid rhythm gives cadence without imposing a rigid grid that fights real content.
• Reinventing layout: Box · Flex · Grid · Stack · Container · AutoGrid already cover composition; here we elevate space, not the layout primitives.