/* Header sticky, mega-menu 3 cấp, off-canvas, ô tìm kiếm — port từ bản mẫu (.site-header/.category-menu/.mega-menu) */
.b4t-header{position:sticky;top:0;z-index:20;background:var(--b4t-color-paper);border-bottom:1px solid var(--b4t-color-line);transition:box-shadow .2s ease-out}
.b4t-header.is-scrolled{box-shadow:var(--b4t-shadow-rest)}
.b4t-header__inner{height:72px;display:flex;align-items:center;gap:var(--b4t-space-lg)}
.b4t-brand{display:flex;align-items:center;text-decoration:none;flex:none}
.b4t-brand__logo{display:block;height:44px;width:auto}
@media(max-width:480px){.b4t-brand__logo{height:38px}}
.b4t-nav{display:flex;align-items:center;gap:var(--b4t-space-lg);flex:1;min-width:0;height:100%}
.b4t-nav>a{color:var(--b4t-color-ink);text-decoration:none;font-weight:600;font-size:14px;padding:12px 0;white-space:nowrap}
.b4t-nav>a:hover{color:var(--b4t-color-primary)}
.b4t-menu-btn{display:none;flex:none;width:44px;height:44px;margin-left:auto;background:none;border:0;padding:8px;cursor:pointer;border-radius:var(--b4t-radius-sm)}
.b4t-menu-btn span:not(.screen-reader-text){display:block;width:24px;height:2px;background:var(--b4t-color-ink);margin:5px auto;transition:transform .2s ease-out,opacity .2s ease-out}
body.b4t-nav-open .b4t-menu-btn span:nth-child(1){transform:translateY(7px) rotate(45deg)}
body.b4t-nav-open .b4t-menu-btn span:nth-child(2){opacity:0}
body.b4t-nav-open .b4t-menu-btn span:nth-child(3){transform:translateY(-7px) rotate(-45deg)}
/* Dropdown 8 mục (yc6): mỗi cấp học → L2 (nested details) → L3 */
.b4t-drop{position:relative;height:100%;display:flex;align-items:center}
.b4t-drop>summary{list-style:none;cursor:pointer;color:var(--b4t-color-ink);font-weight:600;font-size:14px;padding:12px 0;white-space:nowrap;display:flex;align-items:center;gap:5px}
.b4t-drop>summary::-webkit-details-marker{display:none}
.b4t-drop[open]>summary{color:var(--b4t-color-primary)}
.b4t-drop__caret{transition:transform .18s}
.b4t-drop[open] .b4t-drop__caret{transform:rotate(180deg)}
.b4t-drop__panel{position:absolute;z-index:50;top:100%;left:0;width:max-content;max-width:min(680px,calc(100vw - 40px));padding:16px;background:var(--b4t-color-paper);border:1px solid var(--b4t-color-line);border-radius:0 0 var(--b4t-radius-lg) var(--b4t-radius-lg);box-shadow:var(--b4t-shadow-pop)}
.b4t-drop:not([open]) .b4t-drop__panel{display:none}
.b4t-drop__all{display:inline-block;margin-bottom:10px;color:var(--b4t-color-primary);font-weight:700;font-size:13px;text-decoration:none}
.b4t-drop__all:hover{text-decoration:underline}
.b4t-drop__cols{display:grid;grid-template-columns:repeat(3,minmax(150px,1fr));gap:2px 18px}
.b4t-drop__leaf{display:flex;align-items:center;min-height:38px;padding:5px 8px;color:var(--b4t-color-muted);text-decoration:none;font-size:13px;font-weight:600;border-radius:var(--b4t-radius-xs)}
.b4t-drop__leaf:hover{color:var(--b4t-color-primary);background:var(--b4t-color-primary-soft)}
.b4t-drop__sub{margin:0}
.b4t-drop__sub>summary{position:relative;list-style:none;cursor:pointer;display:flex;align-items:center;justify-content:space-between;gap:6px;min-height:38px;padding:5px 8px;color:var(--b4t-color-ink);font-size:13px;font-weight:700;border-radius:var(--b4t-radius-xs)}
.b4t-drop__sub>summary::-webkit-details-marker{display:none}
.b4t-drop__sub>summary svg{transition:transform .18s;color:var(--b4t-color-accent);flex:none}
.b4t-drop__sub[open]>summary{background:var(--b4t-color-primary-soft);color:var(--b4t-color-primary)}
.b4t-drop__sub[open]>summary svg{transform:rotate(180deg)}
.b4t-drop__subitems{display:flex;flex-direction:column;margin:1px 0 6px 12px;padding-left:11px;border-left:2px solid var(--b4t-color-line)}
.b4t-drop__subitems a{min-height:34px;display:flex;align-items:center;padding:3px 8px;color:var(--b4t-color-muted);text-decoration:none;font-size:12.5px;font-weight:500;border-radius:var(--b4t-radius-xs)}
.b4t-drop__subitems a:hover{color:var(--b4t-color-primary);background:var(--b4t-color-primary-soft)}
.b4t-drop__subhead{font-weight:700!important;color:var(--b4t-color-primary)!important}
/* Ô tìm kiếm */
.b4t-search{position:relative;display:flex;align-items:center;margin-left:auto;width:min(360px,100%);height:44px;background:var(--b4t-color-bg);border:1px solid var(--b4t-color-line);border-radius:var(--b4t-radius-full);padding:0 6px 0 16px}
.b4t-search:focus-within{border-color:var(--b4t-color-primary);box-shadow:0 0 0 3px var(--b4t-color-primary-soft)}
.b4t-search input{flex:1;min-width:0;border:0;outline:0;background:transparent;color:var(--b4t-color-ink);font:500 15px var(--b4t-font-sans)}
.b4t-search input::placeholder{color:var(--b4t-color-muted)}
.b4t-search button{flex:none;width:34px;height:34px;display:grid;place-items:center;border:0;border-radius:var(--b4t-radius-full);background:var(--b4t-color-primary);color:var(--b4t-color-on-primary);cursor:pointer}
.b4t-search button:hover{background:var(--b4t-color-primary-strong)}
.b4t-search__results{position:absolute;top:calc(100% + 6px);left:0;right:0;z-index:40;background:var(--b4t-color-paper);border:1px solid var(--b4t-color-line);border-radius:var(--b4t-radius-md);box-shadow:var(--b4t-shadow-pop);max-height:60vh;overflow:auto;padding:6px}
.b4t-search__results[hidden]{display:none}
.b4t-search__results [role=option]{display:block;padding:10px 12px;min-height:44px;border-radius:var(--b4t-radius-xs);color:var(--b4t-color-ink);text-decoration:none}
.b4t-search__results [role=option][aria-selected=true],.b4t-search__results [role=option]:hover{background:var(--b4t-color-primary-soft)}
.b4t-search__title{display:block;font-weight:600;font-size:14px;line-height:1.35}
.b4t-search__sub{display:block;font-size:12px;color:var(--b4t-color-muted)}
.b4t-search__empty{margin:0;padding:12px;color:var(--b4t-color-muted);font-size:14px}
@media (max-width:1199px){.b4t-drop__cols{grid-template-columns:repeat(2,minmax(150px,1fr))}}
@media (max-width:850px){
  .b4t-header__inner{height:60px;gap:var(--b4t-space-md)}
  .b4t-menu-btn{display:block}
  .b4t-nav{position:fixed;top:0;right:0;bottom:0;z-index:70;width:min(360px,88vw);height:auto;flex-direction:column;align-items:stretch;gap:var(--b4t-space-sm);padding:72px var(--b4t-space-md) var(--b4t-space-lg);background:var(--b4t-color-paper);box-shadow:var(--b4t-shadow-pop);overflow:auto;transform:translateX(100%);visibility:hidden;transition:transform .25s ease-out,visibility 0s linear .25s}
  body.b4t-nav-open .b4t-nav{transform:none;visibility:visible;transition:transform .25s ease-out,visibility 0s}
  body.b4t-nav-open{overflow:hidden}
  body.b4t-nav-open .b4t-header{z-index:75}  /* nâng header (chứa nav z70) lên trên backdrop, tránh bị lớp phủ nhốt trong stacking context sticky z20 */
  body.b4t-nav-open::before{content:"";position:fixed;inset:0;z-index:65;background:rgba(18,66,72,.45)}
  body.b4t-nav-open .b4t-menu-btn{position:fixed;top:8px;right:12px;z-index:80}
  .b4t-nav>a{padding:12px 8px;min-height:44px;display:flex;align-items:center}
  .b4t-drop{display:block;height:auto}
  .b4t-drop>summary{padding:12px 8px;min-height:44px;justify-content:space-between}
  .b4t-drop__panel{position:static;width:100%;max-width:none;margin-top:2px;padding:6px 4px 6px 12px;border:0;box-shadow:none;border-radius:0}
  .b4t-drop__cols{grid-template-columns:1fr}
  .b4t-search{order:-1;margin:0;width:100%}
}
@media (prefers-reduced-motion:reduce){.b4t-nav,.b4t-menu-btn span{transition:none}}

/* Mega "Sáng kiến kinh nghiệm" — gộp cả 6 cấp/nhóm thành cột, panel canh giữa */
.b4t-drop--mega .b4t-mega{position:fixed;left:50%;transform:translateX(-50%);top:72px;width:min(1240px,calc(100vw - 32px));max-width:none;max-height:calc(100vh - 96px);overflow:auto}
.b4t-mega__cols{display:grid;grid-template-columns:repeat(6,minmax(0,1fr));gap:6px 22px}
.b4t-mega__col{display:flex;flex-direction:column;min-width:0}
.b4t-mega__head{font:700 14px/1.3 var(--b4t-font-sans);text-transform:uppercase;color:var(--b4t-color-ink);text-decoration:none;padding:2px 6px 10px;margin-bottom:6px;border-bottom:1px solid var(--b4t-color-line)}
.b4t-mega__head:hover{color:var(--b4t-color-primary)}
@media (max-width:1199px){.b4t-mega__cols{grid-template-columns:repeat(3,minmax(0,1fr))}}
@media (max-width:850px){.b4t-drop--mega .b4t-mega{position:static;transform:none;top:auto;width:100%;max-height:none}.b4t-mega__cols{grid-template-columns:1fr;gap:0}.b4t-mega__head{margin-bottom:2px}}

/* Cấp con: nhãn là link (điều hướng), mũi tên/summary toggle danh mục con */
.b4t-drop__sub>summary>a{flex:1;min-width:0;color:inherit;text-decoration:none;padding:2px 0}
.b4t-drop__sub>summary>a:hover{color:var(--b4t-color-primary);text-decoration:underline;text-underline-offset:3px}
.b4t-drop__sub>summary>svg{padding:4px;box-sizing:content-box;margin:-4px -4px -4px 0;border-radius:var(--b4t-radius-xs)}
.b4t-drop__sub>summary>svg:hover{background:var(--b4t-color-primary-soft)}