/* ==========================================================================
   Global base shared by Start & Portfolio
   - 背景層與堆疊（原 start.css §1）
   - 偏好減少動效：停用背景光暈動畫，並完全移除圖層以省 GPU
   ========================================================================== */
body {
  background-image:
    linear-gradient(rgba(185, 188, 201, 0.85), rgba(255,255,255,0)),
    url('/assets/img/background.jpeg');
  background-size: cover;
  background-position: center;
  background-attachment: fixed;
  position: relative;
  isolation: isolate;
}

body::before {
  content: "";
  position: fixed;
  inset: -20vmax;
  pointer-events: none;
  background:
    radial-gradient(40vmax 40vmax at 20% 10%, rgba(0,0,0,0.1), transparent 60%),
    radial-gradient(50vmax 50vmax at 80% 0%,  rgba(0,0,0,0.24), transparent 60%),
    radial-gradient(60vmax 60vmax at 50% 100%, rgba(0,0,0,0.35), transparent 60%);
  filter: blur(20px);
  animation: drift 5s ease-in-out infinite alternate;
  will-change: transform; /* 提示 compositor，避免 layout 抖動 */
}

body::after {
  content: "";
  position: fixed;
  inset: 0;
  pointer-events: none;
  background: linear-gradient(to bottom, rgba(255,255,255,0.20), rgba(255,255,255,0));
}

@keyframes drift {
  from { transform: translateY(-20px) }
  to   { transform: translateY(20px) }
}

/* iOS 小裝置如出現抖動，改回 scroll（可依需要調整臨界值） */
@media (max-width: 640px) {
  body { background-attachment: scroll; }
}

/* 偏好減少動效（共用背景動畫層）：不只停動畫，直接拿掉圖層以省電 */
@media (prefers-reduced-motion: reduce) {
  body::before { animation: none !important; display: none !important; }
}