/* ===============================================
           Base Styles & Variables (共通設定)
        =============================================== */
      :root {
        --bg-color: #111; /* 背景色 */
        --text-color: #eee; /* テキスト色 */

        --accent-cyan: #00e5ff; /* アクセントカラー1: シアン */
        --accent-blue: #2979ff; /* アクセントカラー2: ブルー */
        --accent-pink: #ff0055; /* アクセントカラー3: ピンク */

        --font-jp: "Zen Kaku Gothic New", sans-serif;
        --font-en: "Manrope", sans-serif;

        --easing: cubic-bezier(0.25, 1, 0.5, 1); /* 共通イージング */
      }

      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }

      body {
        background-color: var(--bg-color);
        color: var(--text-color);
        font-family: var(--font-jp);
        line-height: 1.8;
        overflow-x: hidden; /* 横スクロール防止 */
      }

      img {
        max-width: 100%;
        height: auto;
        display: block;
      }

      /* セクション間のスペーサー */
      .spacer {
        height: 50vh;
        display: flex;
        align-items: center;
        justify-content: center;
        font-family: var(--font-en);
        font-size: 1.2rem;
        letter-spacing: 0.2em;
        opacity: 0.5;
        text-transform: uppercase;
      }

      .spacer.large {
        height: 80vh;
      }

      /* ===============================================
           SECTION 1: Vertical Flow (縦フロー)
        =============================================== */
      .flow-section {
        position: relative;
        padding: 10vh 0;
        max-width: 1400px;
        margin: 0 auto;
        width: 90%;
      }

      .flow-container {
        display: flex;
        justify-content: space-between;
        position: relative;
      }

      /* --- 左側：追従ナビゲーション --- */
      .flow-nav {
        width: 300px;
        height: 100vh;
        position: sticky; /* スクロール追従 */
        top: 0;
        display: flex;
        flex-direction: column;
        justify-content: center;
        padding-left: 20px;
        z-index: 10;
      }

      /* SVGプログレスサークル */
      .progress-circle {
        width: 120px;
        height: 120px;
        position: relative;
        margin-bottom: 2rem;
      }

      .progress-circle svg {
        width: 100%;
        height: 100%;
        transform: rotate(-90deg); /* 12時から開始 */
      }

      .progress-circle circle {
        fill: none;
        stroke-width: 8;
      }

      .circle-bg {
        stroke: rgba(255, 255, 255, 0.1);
      }
      .circle-fg {
        stroke: var(--accent-cyan);
        stroke-dasharray: 314; /* 円周の長さ */
        stroke-dashoffset: 314; /* 初期状態は隠す */
        stroke-linecap: round;
      }

      .current-step-display {
        position: absolute;
        top: 50%;
        left: 50%;
        transform: translate(-50%, -50%);
        font-family: var(--font-en);
        font-weight: 800;
        font-size: 2.5rem;
        color: var(--accent-cyan);
      }

      .flow-title {
        font-family: var(--font-en);
        font-size: 4rem;
        font-weight: 800;
        line-height: 1;
        margin-bottom: 1rem;
        background: linear-gradient(
          45deg,
          var(--text-color),
          rgba(255, 255, 255, 0.3)
        );
        -webkit-background-clip: text;
        -webkit-text-fill-color: transparent;
      }

      .flow-subtitle {
        font-size: 0.9rem;
        opacity: 0.6;
        letter-spacing: 0.1em;
      }

      /* --- 右側：コンテンツリスト --- */
      .flow-contents {
        width: 60%;
        padding-bottom: 20vh;
      }

      .flow-item {
        margin-bottom: 30vh;
        display: grid;
        grid-template-columns: 1fr;
        gap: 2rem;
        opacity: 0.2; /* アクティブ以外は薄く */
        transition: opacity 0.5s;
      }

      .flow-item.is-active {
        opacity: 1;
      }

      .item-text {
        order: 2;
      }

      .item-step-label {
        display: inline-block;
        font-family: var(--font-en);
        font-weight: 700;
        color: var(--accent-cyan);
        border: 1px solid var(--accent-cyan);
        padding: 4px 12px;
        border-radius: 50px;
        font-size: 0.8rem;
        margin-bottom: 1.5rem;
      }

      .item-heading {
        font-size: 2.5rem;
        margin-bottom: 1.5rem;
        line-height: 1.4;
        font-weight: 700;
      }

      .item-desc {
        font-size: 1rem;
        line-height: 2;
        color: #aaa;
        margin-bottom: 2rem;
      }

      .item-image {
        order: 1;
        position: relative;
        overflow: hidden;
        border-radius: 8px;
        aspect-ratio: 16 / 9;
      }

      /* 画像出現演出用マスク */
      .image-mask {
        width: 100%;
        height: 100%;
        overflow: hidden;
        clip-path: polygon(0 0, 0 0, 0 100%, 0 100%); /* 初期状態：隠す */
      }

      .image-mask img {
        width: 100%;
        height: 100%;
        object-fit: cover;
        transform: scale(1.3); /* 初期状態：拡大 */
      }

      /* ===============================================
           SECTION 2: Stacking Cards (積み重なるカード)
        =============================================== */
      .stack-section {
        padding: 10vh 0;
        background-color: #050505;
      }

      .stack-header {
        text-align: center;
        margin-bottom: 10vh;
      }

      .stack-title {
        font-family: var(--font-en);
        font-size: 3rem;
        font-weight: 800;
        background: linear-gradient(to right, #fff, #666);
        -webkit-background-clip: text;
        -webkit-text-fill-color: transparent;
      }

      .stack-container {
        max-width: 800px;
        margin: 0 auto;
        position: relative;
        padding-bottom: 10vh;
      }

      /* --- Sticky Card Logic --- */
      .stack-card {
        position: sticky; /* ここが重要：スクロールで吸着 */
        top: 15vh; /* 画面上部15vhの位置で止まる */

        height: 60vh;
        background: #1e1e1e;
        border: 1px solid rgba(255, 255, 255, 0.1);
        border-radius: 20px;
        margin-bottom: 10vh;
        display: flex;
        overflow: hidden;
        transform-origin: center top;
        box-shadow: 0 -10px 40px rgba(0, 0, 0, 0.5);
        /* will-change: transform, opacity, filter; 高負荷時に有効 */
      }

      /* カードのアクセントカラー */
      .stack-card:nth-child(1) {
        border-top: 4px solid var(--accent-cyan);
      }
      .stack-card:nth-child(2) {
        border-top: 4px solid var(--accent-blue);
      }
      .stack-card:nth-child(3) {
        border-top: 4px solid var(--accent-pink);
      }

      .stack-content {
        padding: 40px;
        flex: 1;
        display: flex;
        flex-direction: column;
        justify-content: center;
      }

      .stack-num {
        font-family: var(--font-en);
        font-size: 4rem;
        font-weight: 800;
        opacity: 0.1;
        line-height: 1;
        margin-bottom: 1rem;
      }

      .stack-h3 {
        font-size: 2rem;
        font-weight: 700;
        margin-bottom: 1.5rem;
      }

      .stack-p {
        color: #aaa;
        font-size: 1rem;
      }

      .stack-img {
        flex: 1;
        background-size: cover;
        background-position: center;
        opacity: 0.8;
        filter: grayscale(20%);
      }

      /* ===============================================
           Responsive Styles
        =============================================== */
      @media (max-width: 768px) {
        /* Section 1: Flow */
        .flow-container {
          flex-direction: column;
        }
        .flow-nav {
          width: 100%;
          height: auto;
          position: relative;
          padding: 40px 20px;
          flex-direction: row;
          align-items: center;
          justify-content: space-between;
          border-bottom: 1px solid #333;
          margin-bottom: 40px;
          background-color: rgba(17, 17, 17, 0.9);
        }
        .progress-circle {
          width: 60px;
          height: 60px;
          margin-bottom: 0;
        }
        .current-step-display {
          font-size: 1.2rem;
        }
        .flow-title {
          font-size: 2rem;
        }
        .flow-contents {
          width: 100%;
        }
        .flow-item {
          margin-bottom: 15vh;
        }

        /* Section 2: Stack */
        .stack-card {
          flex-direction: column;
          height: auto;
          min-height: 60vh;
          top: 10vh; /* スマホでは少し上に */
        }
        .stack-img {
          height: 200px;
          flex: none;
        }
      }