コンパクト化する固定ヘッダー

<div class="d03-11-03-stage">
          <header class="d03-11-03-header">
            <span class="d03-11-03-logo">MINIMAL</span>
            <nav class="d03-11-03-nav">
              <a href="#" onclick="return false">サービス</a>
              <a href="#" onclick="return false">実績</a>
              <a href="#" onclick="return false">会社概要</a>
            </nav>
          </header>
          <div class="d03-11-03-scroll">
            <div class="d03-11-03-body">
              <p>↓ この領域をスクロールするとヘッダーが薄くなります</p>
              <div class="d03-11-03-block"></div>
              <div class="d03-11-03-block"></div>
              <div class="d03-11-03-block"></div>
              <div class="d03-11-03-block"></div>
            </div>
          </div>
        </div>

.prev.d03-11-03{padding:0;display:block}
        .d03-11-03-stage{position:relative;width:100%;height:100%;min-height:260px;overflow:hidden;background:#f8fafc}
        .d03-11-03-header{
          position:absolute;top:0;left:0;right:0;z-index:2;
          display:flex;align-items:center;justify-content:space-between;
          height:70px;padding:0 20px;background:#fff;
          box-shadow:0 2px 8px rgba(0,0,0,.08);
          transition:height .3s ease,padding .3s ease;
        }
        .d03-11-03-stage.compact .d03-11-03-header{height:42px;padding:0 16px}
        .d03-11-03-logo{font-weight:800;letter-spacing:.12em;color:#0f172a;font-size:17px;transition:font-size .3s ease}
        .d03-11-03-stage.compact .d03-11-03-logo{font-size:12px}
        .d03-11-03-nav{display:flex;gap:16px}
        .d03-11-03-nav a{color:#334155;text-decoration:none;font-size:12px;font-weight:600;transition:font-size .3s ease}
        .d03-11-03-stage.compact .d03-11-03-nav a{font-size:10.5px}
        .d03-11-03-nav a:hover{color:#0d9488}
        .d03-11-03-scroll{position:absolute;inset:0;overflow-y:auto;padding-top:70px}
        .d03-11-03-body{padding:16px}
        .d03-11-03-body p{font-size:11.5px;color:#64748b;margin:0 0 12px;text-align:center}
        .d03-11-03-block{
          height:80px;border-radius:8px;margin-bottom:12px;
          background:linear-gradient(135deg,#ccfbf1,#5eead4);
        }

(function(){
        var stage=document.querySelector('.d03-11-03-stage');
        if(!stage)return;
        var scroller=stage.querySelector('.d03-11-03-scroll');
        scroller.addEventListener('scroll',function(){
          stage.classList.toggle('compact',scroller.scrollTop>30);
        });
      })();