ビューポート侵入で遅延ロード

<%# Lazy-frame demo: in production the controller goes on a <turbo-frame> and writes its
    held url to src to start the Turbo load. The catalog runs Turbo, so to avoid a real
    fetch this demo puts the controller on a plain <div> (writing src to a div is inert)
    and demo.js swaps the placeholder on the load event — demonstrating the lazy *trigger*
    (it fires only once the frame scrolls into view, thanks to the spacer above it). The
    library only writes src and reflects data-lazy-loaded. %>
<div class="lazy-frame-demo">
  <p class="lazy-frame-demo__hint"><%= t("components.lazy_frame.demo.hint") %></p>
  <div class="lazy-frame-demo__spacer" aria-hidden="true"></div>
  <div
    class="lazy-frame-demo__frame"
    data-controller="stimeo--lazy-frame"
    data-stimeo--lazy-frame-url-value="/posts/1/comments"
    data-stimeo--lazy-frame-root-margin-value="0px"
    data-loaded-label="<%= t("components.lazy_frame.demo.loaded") %>"
    data-loading-label="<%= t("components.lazy_frame.demo.loading") %>">
    <%= t("components.lazy_frame.demo.placeholder") %>
  </div>
</div>

/*
 * Presentation-only styles for the lazy-frame demo. The library writes src and reflects
 * data-lazy-loaded; this CSS lays out the placeholder frame, pushes it below the fold with
 * a spacer so the lazy trigger is visible on scroll, and tints it once loaded.
 */

.lazy-frame-demo {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  max-width: 28rem;
}

.lazy-frame-demo__hint {
  margin: 0;
  color: var(--color-text-muted);
}

/* Pushes the frame below the fold so it loads on scroll, not on initial render. */
.lazy-frame-demo__spacer {
  height: 60vh;
}

.lazy-frame-demo__frame {
  padding: 1.5rem;
  border: 1px dashed var(--border);
  border-radius: 0.5rem;
  text-align: center;
  color: var(--color-text-muted);
}

/*
 * The visible state is driven by a demo-owned `data-demo-state` (set by demo.js),
 * not the library's `data-lazy-loaded`: the library marks the load *triggered* the
 * instant the frame enters the viewport, but the demo simulates Turbo's fetch
 * latency so the transition is perceptible — loading (amber) then loaded (green).
 */
.lazy-frame-demo__frame[data-demo-state="loading"] {
  border-style: solid;
  border-color: var(--amber-500);
  background: var(--amber-50);
  color: var(--amber-500);
}

.lazy-frame-demo__frame[data-demo-state="loaded"] {
  border-style: solid;
  border-color: var(--leaf-500);
  background: var(--leaf-50);
  color: var(--leaf-500);
}

// Lazy-frame demo (consumer-side JS).
//
// In production the lazy-frame controller writes its held url to a <turbo-frame>'s src and
// Turbo fetches it. The catalog has no backend for that, so the demo runs the controller
// on a <div> (writing src to a div does nothing) and reflects the load event into the
// placeholder — proving the lazy *trigger* fired only once the frame scrolled into view.
//
// The library marks the load *triggered* the instant the frame enters the viewport, so a
// naive swap would flip to "loaded" before it can be perceived. To mirror real-world fetch
// latency (and keep the lazy transition visible), show a brief "loading" state first and
// only then the "loaded" content.
document.querySelectorAll(".lazy-frame-demo").forEach((root) => {
  const frame = root.querySelector('[data-controller~="stimeo--lazy-frame"]');
  if (!frame) return;
  // Idempotent: Turbo can re-run this inline module on navigation; wire each root once. The
  // marker is a property, not an attribute: Turbo copies attributes into its page snapshot,
  // so an attribute one comes back set on a restored page whose elements carry no listeners.
  if (root.demoWired) return;
  root.demoWired = true;

  const loadedLabel = frame.dataset.loadedLabel ?? "Loaded:";
  const loadingLabel = frame.dataset.loadingLabel ?? "Loading…";
  frame.addEventListener("stimeo--lazy-frame:load", (event) => {
    // Trigger fired (the library wrote src + set data-lazy-loaded). Simulate Turbo's
    // network fetch so the placeholder -> loaded transition is perceptible on scroll.
    frame.dataset.demoState = "loading";
    frame.textContent = loadingLabel;
    window.setTimeout(() => {
      frame.dataset.demoState = "loaded";
      frame.textContent = `${loadedLabel} ${event.detail.url}`;
    }, 800);
  });
});