最下部追従

<%# Stick-to-bottom demo: the scrollable log follows new messages while you're at the
    bottom, but holds and shows the "new messages" jump button if you've scrolled up. This
    catalog has no Action Cable / Turbo Stream, so the Add button appends a <li> — exactly
    the mutation a Turbo Stream broadcast would make — and the controller reacts. The jump
    button lives inside the log (a descendant, wired with a plain data-action) and CSS
    reveals it only while data-has-new. pin-on-connect starts the log at the bottom so the
    first message is followed rather than flagged — a fresh scroll container renders at the
    top, and the controller does this on every connect, which a once-per-document page event
    cannot. The library only follows/flags and reflects data-pinned / data-has-new; demo.css
    owns the look.

    The log scrolls but its only control — the jump button — is hidden until there is
    something to jump to, so most of the time a keyboard user cannot reach it at all
    (WCAG 2.1.1). Rather than hand-write a tab stop, compose stimeo--scroll-area over it:
    that controller exists for exactly this and gives the viewport tabindex="0" only while
    it holds no rendered focusable content, so the tab stop appears and disappears in step
    with the jump button instead of doubling up with it. It goes on a wrapper because
    stick-to-bottom must sit on the scrolling element itself, and it takes the log as its
    viewport target. role="region" comes free with the label below. %>
<div class="stb-demo">
  <div class="stb-demo__log-wrap" data-controller="stimeo--scroll-area">
    <div
      class="stb-demo__log"
      data-controller="stimeo--stick-to-bottom"
      data-stimeo--stick-to-bottom-behavior-value="smooth"
      data-stimeo--stick-to-bottom-pin-on-connect-value="true"
      data-stimeo--scroll-area-target="viewport"
      aria-label="<%= t("components.stick_to_bottom.demo.viewport_label") %>">
      <ul class="stb-demo__messages" data-stimeo--stick-to-bottom-target="content">
        <% (1..8).each do |i| %>
          <li><%= t("components.stick_to_bottom.demo.message") %> <%= i %></li>
        <% end %>
      </ul>
      <button
        type="button"
        class="stb-demo__jump"
        data-action="click->stimeo--stick-to-bottom#scrollToBottom">
        <%= t("components.stick_to_bottom.demo.jump") %>
      </button>
    </div>
  </div>
  <button
    type="button"
    class="demo-trigger"
    data-stb-demo-add
    data-message-label="<%= t("components.stick_to_bottom.demo.message") %>">
    <%= t("components.stick_to_bottom.demo.add") %>
  </button>
</div>

/*
 * Presentation-only styles for the stick-to-bottom demo. The library follows/flags new
 * content and reflects data-pinned / data-has-new; this CSS gives the log a fixed height
 * so it scrolls, and reveals the floating jump button only while data-has-new is set.
 */

.stb-demo {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  max-width: 24rem;
  align-items: flex-start;
}

/* Wrapper for the composed stimeo--scroll-area (it needs an element of its own, and
   stick-to-bottom must stay on the scrolling log). Layout-neutral. */
.stb-demo__log-wrap {
  width: 100%;
}

.stb-demo__log {
  position: relative;
  width: 100%;
  height: 9rem;
  overflow: auto;
  border: 1px solid var(--border);
  border-radius: 0.5rem;
  padding: 0.5rem;
}

.stb-demo__messages {
  margin: 0;
  padding: 0;
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
}

.stb-demo__messages li {
  padding: 0.25rem 0.5rem;
  border-radius: 0.25rem;
  background: var(--surface-subtle);
}

/* The "new messages" button floats at the bottom, shown only while scrolled up. */
.stb-demo__jump {
  position: sticky;
  bottom: 0;
  display: none;
  margin-left: auto;
  padding: 0.25rem 0.625rem;
  border: 0;
  border-radius: 999px;
  background: var(--color-primary);
  color: var(--white);
  cursor: pointer;
}

.stb-demo__log[data-has-new] .stb-demo__jump {
  display: block;
}

// Stick-to-bottom demo (consumer-side JS).
//
// No Action Cable / Turbo Stream here, so the Add button appends a <li> to the log —
// the same mutation a broadcast would make — and the controller follows it (while pinned)
// or flags it (while scrolled up). The starting scroll position is not wired here: the
// log declares pin-on-connect, so it comes up at the bottom on its own.
document.querySelectorAll(".stb-demo").forEach((root) => {
  const list = root.querySelector('[data-stimeo--stick-to-bottom-target="content"]');
  const add = root.querySelector("[data-stb-demo-add]");
  if (!list || !add) return;

  let count = list.children.length;
  const label = add.dataset.messageLabel ?? "Message";
  add.addEventListener("click", () => {
    const li = document.createElement("li");
    li.textContent = `${label} ${(count += 1)}`;
    list.appendChild(li);
  });
});