上スクロール時だけ現れるトップへ戻るボタン

<div class="d08-13-06-stage">
          <div class="d08-13-06-scroll">
            <div class="d08-13-06-body">
              <p>↑ 上スクロールでのみボタンが出現します</p>
              <div class="d08-13-06-block"></div>
              <div class="d08-13-06-block"></div>
              <div class="d08-13-06-block"></div>
              <div class="d08-13-06-block"></div>
              <div class="d08-13-06-block"></div>
            </div>
          </div>
          <button type="button" class="d08-13-06-top" aria-label="トップへ戻る">▲ TOP</button>
        </div>

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        .d08-13-06-body{padding:16px}
        .d08-13-06-body p{font-size:11.5px;color:#0f766e;margin:0 0 12px;text-align:center}
        .d08-13-06-block{height:70px;border-radius:8px;margin-bottom:12px;background:linear-gradient(135deg,#ccfbf1,#5eead4)}
        .d08-13-06-top{
          position:absolute;right:14px;bottom:14px;z-index:3;
          border:none;border-radius:999px;padding:10px 16px;font-size:12px;font-weight:700;cursor:pointer;
          background:#0f766e;color:#fff;box-shadow:0 8px 18px rgba(15,118,110,.35);
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          transition:opacity .25s ease,transform .25s ease,visibility .25s;
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        .d08-13-06-stage.is-up .d08-13-06-top{opacity:1;visibility:visible;transform:translateY(0)}

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        if(!stage)return;
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        var btn=stage.querySelector('.d08-13-06-top');
        var last=0;
        scroller.addEventListener('scroll',function(){
          var top=scroller.scrollTop;
          if(top<last-2&&top>0){stage.classList.add('is-up');}
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