/* 中線對折翻頁（fold.js）要的樣式：flip.html 與 book.html 共用 */
/* 對折：兩份複製各露出一半；折頁保留 3D，正反兩面都藏背面 */
.fold-layer { position: absolute; inset: 0; z-index: 5; perspective: 1600px; }
.half { position: absolute; overflow: hidden; }
.half.blank { background: #c4a27a; }
.half-shade { position: absolute; inset: 0; z-index: 50; pointer-events: none; background: #000; opacity: 0; }
.flap { position: absolute; z-index: 3; transform-style: preserve-3d; }
.flap > .half { backface-visibility: hidden; -webkit-backface-visibility: hidden; }
.crease { position: absolute; z-index: 4; pointer-events: none; background: #0000001f; }
.crease.h { left: 0; right: 0; top: 50%; height: 1px; }
.crease.v { top: 0; bottom: 0; left: 50%; width: 1px; }