FLIP Layout Animation

:root {
  --bg: #0c0d10;
  --surface: #15161b;
  --surface-2: #1d1f27;
  --ink: #e9eaf0;
  --muted: #9a9cab;
  --accent: #8b5cf6;
  --accent-2: #22d3ee;
  --ok: #34d399;
  --warn: #fbbf24;
  --danger: #f87171;
  --line: rgba(255, 255, 255, 0.1);
  --line-2: rgba(255, 255, 255, 0.18);
  --r-sm: 8px;
  --r-md: 12px;
  --r-lg: 16px;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  -webkit-text-size-adjust: 100%;
}

body {
  margin: 0;
  background: radial-gradient(1200px 600px at 80% -10%, rgba(139, 92, 246, 0.12), transparent 60%),
    radial-gradient(900px 500px at -10% 10%, rgba(34, 211, 238, 0.08), transparent 55%), var(--bg);
  color: var(--ink);
  font-family: "Inter", system-ui, -apple-system, "Segoe UI", sans-serif;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  min-height: 100vh;
}

code {
  font-family: "JetBrains Mono", ui-monospace, "SF Mono", monospace;
  font-size: 0.9em;
  background: var(--surface-2);
  border: 1px solid var(--line);
  padding: 0.05em 0.4em;
  border-radius: 6px;
  color: var(--accent-2);
}

.wrap {
  max-width: 980px;
  margin: 0 auto;
  padding: clamp(20px, 4vw, 44px) clamp(16px, 4vw, 32px) 64px;
}

/* ---------- Header ---------- */
.head {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 20px;
  flex-wrap: wrap;
  margin-bottom: 26px;
}

.head h1 {
  margin: 0 0 8px;
  font-size: clamp(1.5rem, 4vw, 2rem);
  font-weight: 700;
  letter-spacing: -0.02em;
}

.sub {
  margin: 0;
  color: var(--muted);
  max-width: 60ch;
  font-size: 0.95rem;
}

.sub strong {
  color: var(--ink);
  font-weight: 600;
}

/* ---------- Reduced-motion switch ---------- */
.switch {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  cursor: pointer;
  user-select: none;
  flex-shrink: 0;
  padding: 8px 12px;
  border: 1px solid var(--line);
  border-radius: 999px;
  background: var(--surface);
}

.switch input {
  position: absolute;
  opacity: 0;
  width: 1px;
  height: 1px;
}

.switch__track {
  width: 40px;
  height: 22px;
  border-radius: 999px;
  background: var(--surface-2);
  border: 1px solid var(--line-2);
  position: relative;
  transition: background 0.2s ease, border-color 0.2s ease;
}

.switch__thumb {
  position: absolute;
  top: 2px;
  left: 2px;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  background: var(--muted);
  transition: transform 0.2s ease, background 0.2s ease;
}

.switch input:checked + .switch__track {
  background: rgba(139, 92, 246, 0.35);
  border-color: var(--accent);
}

.switch input:checked + .switch__track .switch__thumb {
  transform: translateX(18px);
  background: var(--accent);
}

.switch input:focus-visible + .switch__track {
  outline: 2px solid var(--accent-2);
  outline-offset: 2px;
}

.switch__label {
  font-size: 0.85rem;
  color: var(--muted);
  font-weight: 500;
}

/* ---------- Toolbar ---------- */
.toolbar {
  display: flex;
  flex-wrap: wrap;
  gap: 14px 22px;
  align-items: center;
  padding: 14px 16px;
  background: var(--surface);
  border: 1px solid var(--line);
  border-radius: var(--r-lg);
  margin-bottom: 14px;
}

.toolbar__group {
  display: flex;
  align-items: center;
  gap: 8px;
  flex-wrap: wrap;
}

.toolbar__label {
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--muted);
  font-weight: 600;
}

.btn {
  appearance: none;
  border: 1px solid var(--line-2);
  background: var(--surface-2);
  color: var(--ink);
  font: inherit;
  font-weight: 600;
  font-size: 0.88rem;
  padding: 9px 14px;
  border-radius: var(--r-sm);
  cursor: pointer;
  transition: background 0.15s ease, border-color 0.15s ease, transform 0.08s ease;
}

.btn:hover {
  background: #262933;
  border-color: var(--line-2);
}

.btn:active {
  transform: translateY(1px);
}

.btn:focus-visible {
  outline: 2px solid var(--accent-2);
  outline-offset: 2px;
}

.btn--accent {
  background: linear-gradient(180deg, #9b6cf8, var(--accent));
  border-color: transparent;
  color: #fff;
}

.btn--accent:hover {
  background: linear-gradient(180deg, #a880fa, #7c4fee);
}

.btn--ghost {
  background: transparent;
}

/* ---------- Category chips ---------- */
.chips {
  display: flex;
  gap: 6px;
  flex-wrap: wrap;
}

.chip {
  appearance: none;
  border: 1px solid var(--line);
  background: transparent;
  color: var(--muted);
  font: inherit;
  font-weight: 600;
  font-size: 0.82rem;
  padding: 6px 12px;
  border-radius: 999px;
  cursor: pointer;
  transition: background 0.15s ease, color 0.15s ease, border-color 0.15s ease;
}

.chip:hover {
  color: var(--ink);
  border-color: var(--line-2);
}

.chip[aria-selected="true"] {
  background: rgba(34, 211, 238, 0.14);
  border-color: var(--accent-2);
  color: var(--accent-2);
}

.chip:focus-visible {
  outline: 2px solid var(--accent-2);
  outline-offset: 2px;
}

/* ---------- Status ---------- */
.status {
  margin: 0 0 18px;
  font-size: 0.85rem;
  color: var(--muted);
  font-family: "JetBrains Mono", ui-monospace, monospace;
  min-height: 1.2em;
}

.status b {
  color: var(--ok);
  font-weight: 600;
}

/* ---------- Grid ---------- */
.grid {
  list-style: none;
  margin: 0 0 34px;
  padding: 0;
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(150px, 1fr));
  gap: 12px;
}

.card {
  position: relative;
  background: linear-gradient(180deg, var(--surface-2), var(--surface));
  border: 1px solid var(--line);
  border-radius: var(--r-md);
  padding: 14px;
  min-height: 108px;
  display: flex;
  flex-direction: column;
  gap: 8px;
  overflow: hidden;
  will-change: transform, opacity;
  cursor: default;
}

.card::before {
  content: "";
  position: absolute;
  inset: 0 auto 0 0;
  width: 3px;
  background: var(--cat, var(--accent));
}

.card__top {
  display: flex;
  align-items: center;
  gap: 9px;
}

.card__dot {
  width: 26px;
  height: 26px;
  border-radius: 8px;
  display: grid;
  place-items: center;
  font-size: 0.9rem;
  background: color-mix(in srgb, var(--cat) 22%, transparent);
  color: var(--cat, var(--accent));
  flex-shrink: 0;
  border: 1px solid color-mix(in srgb, var(--cat) 40%, transparent);
}

.card__name {
  font-weight: 600;
  font-size: 0.95rem;
  letter-spacing: -0.01em;
}

.card__cat {
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--muted);
  font-weight: 600;
}

.card__meta {
  margin-top: auto;
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 8px;
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.78rem;
  color: var(--muted);
}

.card__weight {
  color: var(--ink);
  font-weight: 600;
}

/* enter / exit states (transform + opacity only) */
.card.is-enter {
  opacity: 0;
  transform: scale(0.86);
}

.card.is-exit {
  opacity: 0;
  transform: scale(0.86);
}

/* ---------- FLIP note ---------- */
.note {
  background: var(--surface);
  border: 1px solid var(--line);
  border-radius: var(--r-lg);
  padding: clamp(18px, 3vw, 26px);
}

.note__title {
  margin: 0 0 16px;
  font-size: 1.05rem;
  font-weight: 700;
  letter-spacing: -0.01em;
}

.flip-steps {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 12px;
  grid-template-columns: repeat(auto-fit, minmax(210px, 1fr));
}

.flip-step {
  display: flex;
  gap: 12px;
  align-items: flex-start;
  padding: 14px;
  background: var(--surface-2);
  border: 1px solid var(--line);
  border-radius: var(--r-md);
}

.flip-step__k {
  flex-shrink: 0;
  width: 34px;
  height: 34px;
  border-radius: 10px;
  display: grid;
  place-items: center;
  font-family: "JetBrains Mono", monospace;
  font-weight: 700;
  font-size: 1.1rem;
  color: #fff;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
}

.flip-step__body {
  font-size: 0.88rem;
  color: var(--muted);
}

.flip-step__body strong {
  color: var(--ink);
  font-weight: 600;
}

.note__foot {
  margin: 16px 0 0;
  font-size: 0.85rem;
  color: var(--muted);
  border-top: 1px solid var(--line);
  padding-top: 14px;
}

/* ---------- Responsive ---------- */
@media (max-width: 520px) {
  .head {
    flex-direction: column;
    gap: 14px;
  }
  .toolbar {
    gap: 12px;
  }
  .toolbar__group {
    width: 100%;
  }
  .grid {
    grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
  }
  .btn {
    flex: 1 1 auto;
  }
}

@media (prefers-reduced-motion: reduce) {
  .card {
    will-change: auto;
  }
}

/* FLIP Layout Animation — First · Last · Invert · Play
 * Vanilla JS, no dependencies. Animates only transform + opacity. */
(function () {
  "use strict";

  var grid = document.getElementById("grid");
  var status = document.getElementById("status");
  var chipsWrap = document.getElementById("chips");
  var reduceToggle = document.getElementById("reduce");

  var DURATION = 460; // ms
  var EASING = "cubic-bezier(0.22, 0.61, 0.36, 1)";

  // Respect OS-level reduced-motion by default.
  var mq = window.matchMedia("(prefers-reduced-motion: reduce)");
  reduceToggle.checked = mq.matches;
  try {
    mq.addEventListener("change", function (e) {
      reduceToggle.checked = e.matches;
    });
  } catch (_) {
    /* older Safari — ignore */
  }

  var CATEGORIES = {
    core: { label: "Core", color: "#8b5cf6", icon: "◆" },
    data: { label: "Data", color: "#22d3ee", icon: "▲" },
    ui: { label: "UI", color: "#34d399", icon: "●" },
    net: { label: "Net", color: "#fbbf24", icon: "◇" }
  };
  var CAT_KEYS = Object.keys(CATEGORIES);

  var NAMES = [
    "Aurora", "Basalt", "Cobalt", "Dune", "Ember", "Flux", "Glint", "Halo",
    "Ionic", "Jade", "Krypton", "Lumen", "Mistral", "Nimbus", "Onyx",
    "Prism", "Quartz", "Ripple", "Sable", "Talon", "Umbra", "Vertex",
    "Willow", "Xenon", "Yonder", "Zephyr"
  ];

  var uid = 0;
  function makeItem(name, cat) {
    return {
      id: ++uid,
      name: name,
      cat: cat || CAT_KEYS[(Math.random() * CAT_KEYS.length) | 0],
      weight: Math.round((Math.random() * 9.4 + 0.5) * 10) / 10
    };
  }

  // Seed model
  var items = [];
  for (var i = 0; i < 10; i++) {
    items.push(makeItem(NAMES[i], CAT_KEYS[i % CAT_KEYS.length]));
  }

  var activeFilter = "all";
  var nextName = 10;

  /* ---------- Rendering ---------- */
  function visibleItems() {
    if (activeFilter === "all") return items.slice();
    return items.filter(function (it) {
      return it.cat === activeFilter;
    });
  }

  function cardEl(it) {
    var meta = CATEGORIES[it.cat];
    var li = document.createElement("li");
    li.className = "card";
    li.dataset.id = String(it.id);
    li.style.setProperty("--cat", meta.color);
    li.innerHTML =
      '<div class="card__top">' +
      '<span class="card__dot" aria-hidden="true">' + meta.icon + "</span>" +
      '<span class="card__name"></span>' +
      "</div>" +
      '<span class="card__cat">' + meta.label + "</span>" +
      '<div class="card__meta">' +
      "<span>weight</span>" +
      '<span class="card__weight"></span>' +
      "</div>";
    li.querySelector(".card__name").textContent = it.name;
    li.querySelector(".card__weight").textContent = it.weight.toFixed(1) + " kg";
    return li;
  }

  // Map id -> element currently in DOM
  function currentEls() {
    var map = new Map();
    var nodes = grid.children;
    for (var i = 0; i < nodes.length; i++) {
      map.set(nodes[i].dataset.id, nodes[i]);
    }
    return map;
  }

  var animating = false;

  /* ---------- The FLIP pass ---------- */
  function render(reason) {
    var reduce = reduceToggle.checked;
    var wanted = visibleItems();
    var wantedIds = new Set(
      wanted.map(function (it) {
        return String(it.id);
      })
    );

    var existing = currentEls();

    // ---- FIRST: record positions of elements already on screen ----
    var firstRects = new Map();
    existing.forEach(function (el, id) {
      firstRects.set(id, el.getBoundingClientRect());
    });

    // ---- Determine exits (present now, not wanted) ----
    var exits = [];
    existing.forEach(function (el, id) {
      if (!wantedIds.has(id)) exits.push(el);
    });

    // ---- LAST: build the target DOM order ----
    // Remove exiting nodes from flow immediately for measurement,
    // but keep them for the exit animation as absolutely-positioned ghosts.
    if (reduce) {
      // No animation: just rebuild directly.
      grid.replaceChildren();
      wanted.forEach(function (it) {
        grid.appendChild(cardEl(it));
      });
      report(reason, wanted.length, 0, exits.length, 0);
      return;
    }

    // Freeze exiting elements as ghosts so they can fade without holding space.
    exits.forEach(function (el) {
      var r = firstRects.get(el.dataset.id);
      var host = grid.getBoundingClientRect();
      el.style.position = "absolute";
      el.style.zIndex = "0";
      el.style.margin = "0";
      el.style.width = r.width + "px";
      el.style.height = r.height + "px";
      el.style.left = r.left - host.left + grid.scrollLeft + "px";
      el.style.top = r.top - host.top + grid.scrollTop + "px";
    });

    // Reorder / create the kept & new elements into wanted order.
    var enters = [];
    var moved = 0;
    var frag = document.createDocumentFragment();

    wanted.forEach(function (it) {
      var id = String(it.id);
      var el = existing.get(id);
      if (el) {
        frag.appendChild(el); // moving a live node preserves it
      } else {
        el = cardEl(it);
        el.classList.add("is-enter");
        enters.push(el);
        frag.appendChild(el);
      }
    });

    // Put ghosts back so they render during the transition.
    grid.appendChild(frag);
    exits.forEach(function (el) {
      grid.appendChild(el);
    });

    // ---- LAST measure + INVERT for kept nodes ----
    wanted.forEach(function (it) {
      var id = String(it.id);
      var first = firstRects.get(id);
      if (!first) return; // newly entered, handled separately
      var el = grid.querySelector('.card[data-id="' + id + '"]');
      if (!el) return;
      var last = el.getBoundingClientRect();
      var dx = first.left - last.left;
      var dy = first.top - last.top;
      if (dx || dy) {
        moved++;
        el.style.transition = "none";
        el.style.transform = "translate(" + dx + "px," + dy + "px)";
      }
    });

    // Force a reflow so the inverted transforms are committed before we play.
    void grid.offsetWidth;

    // ---- PLAY ----
    animating = true;
    requestAnimationFrame(function () {
      requestAnimationFrame(function () {
        // Kept nodes slide to zero.
        wanted.forEach(function (it) {
          var id = String(it.id);
          if (!firstRects.has(id)) return;
          var el = grid.querySelector('.card[data-id="' + id + '"]');
          if (!el) return;
          el.style.transition =
            "transform " + DURATION + "ms " + EASING;
          el.style.transform = "translate(0,0)";
        });

        // Entering nodes scale+fade in.
        enters.forEach(function (el) {
          el.style.transition =
            "transform " + DURATION + "ms " + EASING +
            ", opacity " + DURATION + "ms ease";
          el.classList.remove("is-enter");
        });

        // Exiting ghosts fade + shrink, then are removed.
        exits.forEach(function (el) {
          el.style.transition =
            "transform " + DURATION * 0.7 + "ms ease, opacity " +
            DURATION * 0.7 + "ms ease";
          el.classList.add("is-exit");
        });
      });
    });

    // Cleanup after the transition finishes.
    window.setTimeout(function () {
      wanted.forEach(function (it) {
        var el = grid.querySelector('.card[data-id="' + it.id + '"]');
        if (el) {
          el.style.transition = "";
          el.style.transform = "";
        }
      });
      exits.forEach(function (el) {
        if (el.parentNode) el.parentNode.removeChild(el);
      });
      animating = false;
    }, DURATION + 40);

    report(reason, wanted.length, moved, exits.length, enters.length);
  }

  function report(reason, total, moved, removed, added) {
    var parts = [];
    if (reason) parts.push(reason);
    parts.push(total + " shown");
    if (moved) parts.push("<b>" + moved + " moved</b>");
    if (added) parts.push(added + " added");
    if (removed) parts.push(removed + " removed");
    status.innerHTML = parts.join("  ·  ");
  }

  /* ---------- Actions ---------- */
  function shuffle() {
    for (var i = items.length - 1; i > 0; i--) {
      var j = (Math.random() * (i + 1)) | 0;
      var t = items[i];
      items[i] = items[j];
      items[j] = t;
    }
    render("Shuffled");
  }

  function sortByName() {
    items.sort(function (a, b) {
      return a.name.localeCompare(b.name);
    });
    render("Sorted A–Z");
  }

  function sortByWeight() {
    items.sort(function (a, b) {
      return b.weight - a.weight;
    });
    render("Sorted by weight");
  }

  function addCard() {
    var name = NAMES[nextName % NAMES.length] +
      (nextName >= NAMES.length ? " " + (Math.floor(nextName / NAMES.length) + 1) : "");
    nextName++;
    var it = makeItem(name);
    // Insert near the front so the move is visible.
    items.splice((Math.random() * (items.length + 1)) | 0, 0, it);
    render("Added a card");
  }

  function removeLast() {
    if (!items.length) {
      report("Nothing to remove", 0, 0, 0, 0);
      return;
    }
    items.pop();
    render("Removed a card");
  }

  /* ---------- Wiring ---------- */
  var ACTIONS = {
    shuffle: shuffle,
    "sort-name": sortByName,
    "sort-weight": sortByWeight,
    add: addCard,
    remove: removeLast
  };

  document.querySelector(".toolbar").addEventListener("click", function (e) {
    var btn = e.target.closest("[data-act]");
    if (!btn) return;
    if (animating) return; // avoid overlapping FLIP passes
    var fn = ACTIONS[btn.dataset.act];
    if (fn) fn();
  });

  // Build filter chips.
  function buildChips() {
    var defs = [{ id: "all", label: "All" }].concat(
      CAT_KEYS.map(function (k) {
        return { id: k, label: CATEGORIES[k].label };
      })
    );
    defs.forEach(function (d) {
      var b = document.createElement("button");
      b.type = "button";
      b.className = "chip";
      b.role = "tab";
      b.dataset.filter = d.id;
      b.textContent = d.label;
      b.setAttribute("aria-selected", d.id === activeFilter ? "true" : "false");
      chipsWrap.appendChild(b);
    });
  }

  chipsWrap.addEventListener("click", function (e) {
    var chip = e.target.closest(".chip");
    if (!chip || animating) return;
    activeFilter = chip.dataset.filter;
    Array.prototype.forEach.call(chipsWrap.children, function (c) {
      c.setAttribute(
        "aria-selected",
        c.dataset.filter === activeFilter ? "true" : "false"
      );
    });
    render("Filter: " + (activeFilter === "all" ? "All" : CATEGORIES[activeFilter].label));
  });

  reduceToggle.addEventListener("change", function () {
    render(reduceToggle.checked ? "Reduced motion on" : "Reduced motion off");
  });

  /* ---------- Init ---------- */
  buildChips();
  // Initial paint (no FLIP needed — nothing to invert from).
  visibleItems().forEach(function (it) {
    grid.appendChild(cardEl(it));
  });
  report("Ready", items.length, 0, 0, 0);
})();

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>FLIP Layout Animation</title>
  <link rel="preconnect" href="https://fonts.googleapis.com" />
  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
  <link
    href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500;600&display=swap"
    rel="stylesheet"
  />
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <main class="wrap">
    <header class="head">
      <div class="head__title">
        <h1>FLIP Layout Animation</h1>
        <p class="sub">
          A reflowing grid where every item glides between layouts using the
          <strong>First&nbsp;·&nbsp;Last&nbsp;·&nbsp;Invert&nbsp;·&nbsp;Play</strong> technique —
          only <code>transform</code> and <code>opacity</code> are animated.
        </p>
      </div>
      <label class="switch" title="Snap instantly, no motion">
        <input type="checkbox" id="reduce" />
        <span class="switch__track" aria-hidden="true"><span class="switch__thumb"></span></span>
        <span class="switch__label">Reduced motion</span>
      </label>
    </header>

    <section class="toolbar" aria-label="Layout controls">
      <div class="toolbar__group" role="group" aria-label="Rearrange">
        <button class="btn btn--accent" data-act="shuffle" type="button">Shuffle</button>
        <button class="btn" data-act="sort-name" type="button">Sort A–Z</button>
        <button class="btn" data-act="sort-weight" type="button">Sort by weight</button>
      </div>

      <div class="toolbar__group" role="group" aria-label="Filter by category">
        <span class="toolbar__label">Filter</span>
        <div class="chips" id="chips" role="tablist" aria-label="Category filter"></div>
      </div>

      <div class="toolbar__group" role="group" aria-label="Add or remove">
        <button class="btn" data-act="add" type="button">+ Add card</button>
        <button class="btn btn--ghost" data-act="remove" type="button">− Remove last</button>
      </div>
    </section>

    <p class="status" id="status" role="status" aria-live="polite">Ready.</p>

    <ul class="grid" id="grid" aria-label="Animated card grid"></ul>

    <section class="note" aria-label="How FLIP works">
      <h2 class="note__title">How the FLIP technique works</h2>
      <ol class="flip-steps">
        <li class="flip-step">
          <span class="flip-step__k">F</span>
          <span class="flip-step__body">
            <strong>First</strong> — before touching the DOM, record every item's current
            bounding box with <code>getBoundingClientRect()</code>.
          </span>
        </li>
        <li class="flip-step">
          <span class="flip-step__k">L</span>
          <span class="flip-step__body">
            <strong>Last</strong> — mutate the DOM (reorder, filter, add) and let the browser
            reflow to the final layout. Measure the new boxes.
          </span>
        </li>
        <li class="flip-step">
          <span class="flip-step__k">I</span>
          <span class="flip-step__body">
            <strong>Invert</strong> — apply a <code>transform</code> that offsets each item back
            to where it started, so visually nothing moved yet.
          </span>
        </li>
        <li class="flip-step">
          <span class="flip-step__k">P</span>
          <span class="flip-step__body">
            <strong>Play</strong> — on the next frame, clear the transform with a transition.
            The GPU composites the slide — no layout thrash, no jank.
          </span>
        </li>
      </ol>
      <p class="note__foot">
        Because only <code>transform</code> / <code>opacity</code> animate, the work stays on the
        compositor thread. Animating <code>top</code>/<code>left</code>/<code>width</code> instead
        would trigger layout on every frame.
      </p>
    </section>
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