WORK-381
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ID:WORK-381Status:done

Sandbox lazy/poster activation

sandbox is eager today: the iframe is created and its dependencies load on page render (it has a height knob but no deferral). That's fine for a small demo, but a heavy sandbox — a three.js scene (WORK-382), a large framework demo — on a perf-sensitive page like the site index costs a chunk of load time we deliberately avoided (WORK-350 and WORK-380 both limited live iframes "to keep the landing page fast").

Add a deferred-activation capability so a heavy sandbox can headline a page without the load-time hit — useful for any heavy sandbox, not just three.js.

Priority:mediumComplexity:moderateMilestone:v0.21.0
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Criteria completion

Criteria completion: 5 of 5 (100%) checked; tracking started on Jun 11, no incremental history yet0%25%50%75%100%Jun 11Jun 15

Tracking started Jun 11 — check back for trends.

Relationships

Related 7
Branches 5
main current done
changeset-release/main readyclaude/work-381-sandbox-lazy-activation donemain doneclaude/v0.21-registry-sandbox-milestone readyclaude/threejs-sandbox-composition ready

Approach

  • activation attribute: eager (default — unchanged behaviour) | visible (mount when scrolled into view, via IntersectionObserver) | click (mount on explicit user activation, for the heaviest cases).
  • poster attribute: a static image (or the rendered static fallback) shown in the iframe's place until activation. The rune emits the poster + data-activation; the iframe's src/srcdoc and dependency <link>/<script> tags are not created until activation, so nothing downloads early.
  • Deferred mount lives in @refrakt-md/behaviors (alongside the other progressive-enhancement behaviours); the engine/renderer emit the poster markup and data attributes, the behaviour does the swap.
  • prefers-reduced-motion: a non-eager sandbox does not auto-activate; the poster + an explicit activation control are shown (so motion-sensitive users opt in).
  • Keep eager the default — zero change for existing sandboxes.

Acceptance Criteria

  • sandbox accepts activation (eager default | visible | click) and a poster image URL.
  • A non-eager sandbox renders the poster and defers iframe creation and dependency loading until activated (in view, or clicked) — verified by no early network request for the deps.
  • Under prefers-reduced-motion, a non-eager sandbox does not auto-activate; the poster and an explicit activation control are shown.
  • Build/SSR stays green (poster is static markup; the iframe is still srcdoc once mounted) and eager sandboxes are byte-for-byte unchanged (no regression).
  • Documented on runes/sandbox.md with a visible+poster example, and the behaviour is registered/loaded like the other progressive-enhancement behaviours.

References

  • packages/runes/src/tags/sandbox.ts (has height; add activation/poster)
  • packages/runes/src/config.ts (Sandbox — interactive: true)
  • packages/behaviors/src/behaviors/ (deferred-mount behaviour); runes/sandbox.md
  • Consumers (no dependency — these depend on this item): WORK-382 (three.js scene), WORK-350 (index anchor cell)

Resolution

Completed: 2026-06-11

Branch: claude/work-381-sandbox-lazy-activation

What was done

  • packages/runes/src/tags/sandbox.ts — schema gains activation (eager|visible|click, validated) and poster. Only non-eager activation emits data-activation/data-poster, so eager sandboxes are byte-for-byte unchanged.
  • packages/behaviors/src/elements/sandbox.ts — RfSandbox now gates the iframe behind activation. connectedCallback caches activation/poster; non-eager calls renderPoster() and returns. activate() (extracted from the old eager path) builds the iframe + wires theme sync, idempotently. renderPoster() draws the poster image + a .rf-sandbox__activate Run control; visible wires an IntersectionObserver (rootMargin 200px) that mounts on scroll-in unless prefers-reduced-motion. setTheme no-ops before mount; disconnectedCallback tears down the observer. The iframe (and all its dependency <script>/<link> tags inside srcdoc) is never created until activation — nothing downloads early.
  • packages/lumina/styles/runes/sandbox.css — .rf-sandbox__poster, .rf-sandbox__poster-image, .rf-sandbox__activate (accent button with ▶, hover/focus states).
  • packages/behaviors/test/sandbox.test.ts — 5 jsdom tests: eager mounts immediately (no poster); click defers then mounts on press; visible auto-mounts via a mocked IntersectionObserver; reduced-motion does not auto-activate (no observer, control shown); unknown activation falls back to eager.
  • site/content/runes/sandbox.md — new "Deferred activation" section with a visible+poster example, attribute-table rows, and the heavy 3D sitemap sandbox switched to activation="visible" (loads three.js only on scroll-in).
  • Changeset sandbox-lazy-activation.md (minor: runes, behaviors, lumina).

Verification

  • packages/behaviors + packages/runes tests green (948 passing incl. the 5 new).
  • Full workspace build + SvelteKit site build green. Built runes/sandbox.html carries exactly one data-activation="visible" (the deferred sitemap); all other (eager) sandboxes emit no activation attribute — confirms the no-regression guarantee.

Notes

  • Implemented entirely on the existing RfSandbox custom element rather than a separate behavior function — the element already owns iframe lifecycle, so the activation gate lives where the mount does. No new registration needed; it loads with the other progressive-enhancement elements via registerElements().
  • Switching the sitemap sandbox to activation="visible" also satisfies WORK-389's deferred criterion 3 (lazy mount / poster until in view); that item can now be closed.