WORK-428
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ID:WORK-428Status:done

bg guest body + engine relocation + bare-surface guardrail

The gate for SPEC-104: teach bg to host a live sandbox as a bare, full-bleed backdrop. Covers §1–§4 + §7 — the constrained body, the engine relocation that mirrors bg-video, the backdrop posture (mounted-but-inert), the bare-surface guardrail, boot-frame layering, and reduced-motion/off-screen gating. Everything downstream (the preset, the showcase) builds on the data-bg-guest contract this establishes.

Priority:highComplexity:complexMilestone:v0.24.0Source:SPEC-104
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Criteria completion

Criteria completion: 7 of 7 (100%) checked; tracking started on Jun 15, no incremental history yet0%25%50%75%100%Jun 15Jun 16

Tracking started Jun 15 — check back for trends.

Relationships

Blocks 1
Implements 1
Related 6
Branches 6
main current done
changeset-release/main donemain doneclaude/v024-work428-bg-guest doneclaude/milestone-v0.24.0-split readyclaude/v023-bg-motion-breakdown readyclaude/defer-work-417 ready

Scope

  • Constrained body (§1) — bg (today a directive with no body, packages/runes/src/tags/bg.ts) gains an optional body holding a single bare guest. bg transforms it normally (so the real sandbox rune runs with file resolution + sanitisation via config.variables.__sandboxReadFile), tags the rendered output data-bg-guest, forces height="fill" (the SPEC-101 host-owned-height mode), and gives it the backdrop posture. The tagged element is emitted alongside the existing bg-* metas.
  • Engine relocation (§2) — engine.ts §1f, when it raises the bg layer, collects any data-bg-guest descendant into the data-name="bg" div — above the --bg-image boot frame, below overlay/scrim, a sibling of the bg-video branch — and marks it consumed so it is not also rendered in content flow.
  • Backdrop posture (§2) — the guest gets data-guest-posture="backdrop", distinct from SPEC-090 presentational: the enhancement layer mounts it and forces eager activation (the scene runs), while suppressing interaction only (pointer-events: none, no controls, out of tab order). NOT presentational (which skips enhancement → a dead backdrop).
  • Bare-surface guardrail (§3) — the body accepts a bare guest only; a chromed content rune (video/audio/figure) emits a SPEC-084-style build warning redirecting to the media-guest slot or bg video="…". Bare image/video backdrops keep their existing bg src=/video= attribute path, unchanged.
  • Boot-frame composition (§4) — a --bg-image (image/gradient) paints behind the guest; overlay/scrim above it; a host's positioned image still flows to og:image / structured-data image via the host's normal media extraction.
  • Reduced motion & performance (§7) — under prefers-reduced-motion: reduce the live scene is not mounted (the boot frame stands in); the backdrop suspends its render loop off-screen / on a hidden tab; no-JS/crawler get the boot frame. Consistent with the SPEC-105 baseline.

Acceptance Criteria

  • bg accepts an optional body holding one bare guest (sandbox); bg transforms it (real sandbox rune — file resolution + sanitisation), tags it data-bg-guest, and forces height="fill" + the backdrop posture.
  • The engine (§1f) relocates a data-bg-guest element into the data-name="bg" layer — above the --bg-image boot frame, below overlay/scrim, sibling to bg-video — and marks it consumed so it does not render in content flow.
  • Backdrop posture: the guest gets data-guest-posture="backdrop" — the enhancement layer mounts it + forces eager activation (it runs), suppressing interaction only (pointer-events:none, no controls, out of tab order). Explicitly not SPEC-090 presentational; a test covers that a backdrop guest mounts while a presentational one does not.
  • A chromed content rune (video/audio/figure) in the bg body produces a build warning redirecting to the media-guest slot or bg video="…"; bare image/video backdrops keep their existing attribute path (unchanged).
  • Boot frame composes: a gradient/image paints behind the guest; overlay/scrim above it; a host's positioned image still flows to og:image / structured-data image.
  • Reduced motion & performance: under prefers-reduced-motion the live scene is not mounted (boot frame stands in); the backdrop suspends off-screen / on a hidden tab; no-JS/crawler render the boot frame.
  • Unit tests cover the body transform + tagging, the engine relocation + consumption, the guardrail warning, the boot-frame layering, and the backdrop-vs-presentational mount distinction; contracts regenerated.

Dependencies

  • None within the milestone — this is the SPEC-104 gate. WORK-429 (preset) and WORK-430 (docs) depend on the data-bg-guest contract here.

References

  • SPEC-104 §1–§4 · packages/runes/src/tags/bg.ts · packages/transform/src/engine.ts (§1f bg resolution) · SPEC-101 (height="fill", boot frame; packages/behaviors/src/elements/sandbox.ts) · SPEC-090 (presentational posture) · SPEC-084 (validation).

Resolution

Completed: 2026-06-16

Branch: claude/v024-work428-bg-guest.

What was done

  • bg body + guest tagging (packages/runes/src/tags/bg.ts): bg gained an optional body. It transforms the body normally (the real sandbox rune runs, with file resolution + sanitisation), finds the guest, tags it data-bg-guest + data-guest-posture="backdrop", forces height="fill" + eager activation, and emits it alongside the bg-* metas. A chromed guest (video/audio/figure) is rejected with a build warning (the bare-surface guardrail).
  • Hoist (packages/runes/src/lib/index.ts): injectBgMetasFrom now hoists the data-bg-guest element (not just metas) from the {% bg %} body up to the host.
  • Engine relocation (packages/transform/src/engine.ts §1f): captures the hoisted guest, relocates it into the data-name="bg" layer as a sibling of the bg-video branch (above the --bg-image boot frame, below overlay/scrim), and drops the flow copy so it isn't rendered twice. A guest alone raises the bg layer.
  • Backdrop posture (packages/skeleton/styles/{runes/bg.css,dimensions/guest-posture.css}): the guest fills the bg layer (position/inset/sizing); [data-guest-posture="backdrop"] is pointer-events:none. Distinct from presentational — the behaviours layer still mounts a backdrop.
  • Reduced motion & off-screen (packages/behaviors/src/elements/sandbox.ts): a data-guest-posture="backdrop" sandbox never mounts under prefers-reduced-motion (the bg boot frame stands in), mounts only while on-screen, and suspends (tears the iframe down) off-screen / on a hidden tab via IntersectionObserver + visibilitychange — re-mounting on return.

Tests

  • transform/test/bg-guest.test.ts (5) — relocation, consumption, overlay layering, gradient boot-frame composition, guest-alone trigger.
  • runes/test/bg-guest.test.ts (3) — body transform + tagging + posture, metas-alongside-guest, chromed-guest guardrail warning.
  • behaviors/test/bg-backdrop.test.ts (4) — reduced-motion skip, on/off-screen mount-suspend cycle, no-IO fallback, backdrop-enhanced-vs-presentational-skipped at the enhancement layer.

Notes

  • The sandbox is an autonomous custom element (rf-sandbox) that mounts via its own connectedCallback, independent of initRuneBehaviors — so for a sandbox the backdrop posture's job is interaction-suppression + reduced-motion/off-screen gating, not "ensure it mounts" (it mounts regardless).
  • bg is a directive Schema with no config-derived contract surface, so structure contracts are unchanged. The failing packages/lumina/test/contracts.test.ts drift is pre-existing and unrelated — the committed contracts/structures.json tracks refrakt contracts --site main (which includes the plan plugin), while the test regenerates from fullConfig (8 plugins, no plan); they have diverged (~525 lines + the chart tick-* modifiers). It fails identically on main. Flagged for a separate cleanup (reconcile the test's generation with the committed-file generation method).
  • Full suite otherwise green (3397); the audio-reactive showcase (WORK-430) remains deferred on SPEC-006.