/* ==========================================================================
   NUT "VE DAU TRANG"                                          02/09/2026
   ==========================================================================
   Thay cho <div id="back-to-top"> cu. Ban cu co ba diem yeu:
     - CHI hien tu 768px tro len (dieu kien "h > 767" trong js/main-script.js
       va js/library.js): tren dien thoai - noi trang dai nhat va nguoi ta can
       nhat - khong he co nut nay;
     - dung $.animate({scrollTop}) cua jQuery, phai tu tinh tung khung hinh
       tren main thread nen giat tren trang nhieu anh;
     - hien/an bang fadeIn/fadeOut goi trong su kien scroll, khong tiet che.
   Ban nay hoc theo giao dien 3 (.gd3-back-to-top): CSS lo phan hien/an qua
   mot lop .hien, JavaScript thuan lo phan con lai (xem js/len-dau.js).

   Vi tri: goc phai duoi. Khoi lien he noi (#button-contact-vr) nam o
   right:0 va cach day 80px (may tinh) / 62px (dien thoai), nen khoang duoi
   cung ben phai con trong - dat nut o do thi khong khoi nao de len khoi nao.
   ========================================================================== */
.len-dau {
	position: fixed;
	right: 20px;
	bottom: 20px;
	width: 44px;
	height: 44px;
	padding: 0;
	border: 0;
	border-radius: 50%;
	background: #000;
	color: #ffc600;
	font-size: 18px;
	line-height: 1;
	display: flex;
	align-items: center;
	justify-content: center;
	box-shadow: 0 2px 10px rgba(0, 0, 0, .25);
	cursor: pointer;
	/* Mac dinh an. Dung opacity + visibility (khong dung display:none) de con
	   chuyen dong mo dan; visibility de nut da an thi khong an chuot duoc. */
	opacity: 0;
	visibility: hidden;
	transform: translateY(8px);
	transition: opacity .15s ease, transform .15s ease, visibility .15s;
	z-index: 99998;   /* ngay duoi #button-contact-vr (99999) */
}

.len-dau.hien {
	opacity: 1;
	visibility: visible;
	transform: translateY(0);
}

/* Re chuot: dao mau - nen vang chu den. */
.len-dau:hover,
.len-dau:focus {
	background: #ffc600;
	color: #000;
	outline: none;
}

/* Duoi 768px khoi lien he noi duoc thu nho va ha xuong bottom:62px, nen nut
   nay cung thu lai mot chut va nam thap hon de hai khoi khong cham nhau. */
@media (max-width: 767px) {
	.len-dau {
		right: 12px;
		bottom: 14px;
		width: 40px;
		height: 40px;
		font-size: 16px;
	}
}

/* Nguoi dung dat he dieu hanh o che do "giam chuyen dong" thi bo hieu ung
   truot, chi bat/tat. */
@media (prefers-reduced-motion: reduce) {
	.len-dau { transition: none; transform: none; }
	.len-dau.hien { transform: none; }
}