:root {
  --bg-main: #f4fbf9;
  --bg-card: #ffffff;
  --bg-muted: #e6f4f1;
  --text-main: #0f172a;
  --text-secondary: #475569;
  --border-color: #cce3de;
  --accent: #0d9488;
  --accent-secondary: #0284c7;
  --accent-light: #ccfbf1;
  --shadow-sm: 0 1px 3px rgba(13, 148, 136, 0.08);
  --shadow-md: 0 8px 24px rgba(13, 148, 136, 0.12);
  --radius: 14px;
}

[data-theme="dark"] {
  --bg-main: #07111e;
  --bg-card: #112236;
  --bg-muted: #0b1829;
  --text-main: #f1f5f9;
  --text-secondary: #94a3b8;
  --border-color: #1e3a5f;
  --accent: #2dd4bf;
  --accent-secondary: #38bdf8;
  --accent-light: rgba(45, 212, 191, 0.16);
  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.35);
  --shadow-md: 0 8px 24px rgba(0, 0, 0, 0.45);
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: 'Noto Sans Sinhala', 'Iskoola Pota', system-ui, -apple-system, sans-serif;
  background-color: var(--bg-main);
  color: var(--text-main);
  line-height: 1.75;
  transition: background-color 0.25s ease, color 0.25s ease;
}

/* Header */
.top-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: var(--bg-card);
  border-bottom: 1px solid var(--border-color);
  padding: 0.65rem 1.25rem;
  display: flex;
  flex-direction: column;
  gap: 0.65rem;
  box-shadow: var(--shadow-sm);
  backdrop-filter: blur(8px);
}

.header-main-row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  width: 100%;
}

.brand {
  display: flex;
  align-items: center;
  gap: 0.65rem;
  flex-shrink: 0;
}

.mobile-menu-btn {
  display: none !important;
  font-size: 1.3rem;
  line-height: 1;
}

.brand-logo {
  width: 40px;
  height: 40px;
  border-radius: 10px;
  background: linear-gradient(135deg, #0d9488, #0284c7);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.35rem;
  color: #fff;
  box-shadow: 0 4px 12px rgba(13, 148, 136, 0.32);
  flex-shrink: 0;
}

.brand-title h1 {
  font-size: 1.15rem;
  font-weight: 800;
  line-height: 1.2;
  letter-spacing: -0.01em;
}

.brand-title p {
  font-size: 0.75rem;
  color: var(--text-secondary);
  line-height: 1.3;
}

.header-right-tools {
  display: flex;
  align-items: center;
  gap: 0.65rem;
  flex-shrink: 0;
}

/* Modern Search Box */
.search-box {
  position: relative;
  display: flex;
  align-items: center;
  width: 100%;
}

.desktop-search {
  flex: 1;
  max-width: 430px;
}

.search-box input {
  width: 100%;
  height: 42px;
  padding: 0 2.2rem 0 2.5rem;
  border-radius: 999px;
  border: 1.5px solid var(--border-color);
  background: var(--bg-muted);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.9rem;
  outline: none;
  transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
  box-shadow: inset 0 1px 2px rgba(0, 0, 0, 0.03);
}

.search-box input::placeholder {
  color: var(--text-secondary);
  opacity: 0.85;
}

.search-box input:focus {
  border-color: var(--accent);
  background: var(--bg-card);
  box-shadow: 0 0 0 3px var(--accent-light), 0 2px 8px rgba(0, 0, 0, 0.05);
}

.search-icon {
  position: absolute;
  left: 0.95rem;
  top: 50%;
  transform: translateY(-50%);
  font-size: 1rem;
  color: var(--text-secondary);
  pointer-events: none;
  display: flex;
  align-items: center;
  justify-content: center;
}

.search-clear-btn {
  position: absolute;
  right: 0.65rem;
  top: 50%;
  transform: translateY(-50%);
  width: 24px;
  height: 24px;
  border-radius: 50%;
  border: none;
  background: var(--border-color);
  color: var(--text-secondary);
  font-size: 0.75rem;
  font-weight: 700;
  display: none;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: all 0.15s;
}

.search-clear-btn:hover {
  background: var(--accent);
  color: #fff;
}

.search-box.has-text .search-clear-btn {
  display: flex;
}

.mobile-search-row {
  display: none;
  width: 100%;
}

.mobile-tabs-row {
  display: none;
  width: 100%;
}

/* Mode Tabs */
.mode-tabs {
  display: flex;
  background: var(--bg-muted);
  padding: 0.25rem;
  border-radius: 12px;
  border: 1px solid var(--border-color);
  gap: 0.2rem;
}

.mode-btn {
  border: none;
  background: transparent;
  color: var(--text-secondary);
  padding: 0.42rem 0.78rem;
  border-radius: 8px;
  font-family: inherit;
  font-size: 0.82rem;
  font-weight: 700;
  cursor: pointer;
  transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
  white-space: nowrap;
}

.mode-btn:hover {
  color: var(--text-main);
}

.mode-btn.active {
  background: var(--bg-card);
  color: var(--accent);
  box-shadow: var(--shadow-sm);
}

.icon-btn {
  border: 1px solid var(--border-color);
  background: var(--bg-muted);
  color: var(--text-main);
  width: 40px;
  height: 40px;
  border-radius: 10px;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.1rem;
  transition: all 0.2s;
  flex-shrink: 0;
}

.icon-btn:hover {
  border-color: var(--accent);
  background: var(--accent-light);
}

/* Sidebar overlay for mobile drawer */
.sidebar-overlay {
  position: fixed;
  inset: 0;
  background: rgba(15, 23, 42, 0.65);
  backdrop-filter: blur(4px);
  z-index: 190;
  opacity: 0;
  visibility: hidden;
  transition: all 0.25s ease;
}

.sidebar-overlay.open {
  opacity: 1;
  visibility: visible;
}

.sidebar-top-mobile {
  display: none;
  align-items: center;
  justify-content: space-between;
  padding-bottom: 0.75rem;
  margin-bottom: 0.75rem;
  border-bottom: 1px solid var(--border-color);
}

.sidebar-title {
  font-weight: 700;
  font-size: 0.95rem;
}

.sidebar-close-btn {
  background: var(--bg-muted);
  border: 1px solid var(--border-color);
  color: var(--text-main);
  width: 32px;
  height: 32px;
  border-radius: 8px;
  font-size: 0.9rem;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
}

/* Layout */
.app-layout {
  display: grid;
  grid-template-columns: 325px 1fr;
  max-width: 1500px;
  margin: 0 auto;
  min-height: calc(100vh - 68px);
}

/* Sidebar */
.sidebar {
  background: var(--bg-card);
  border-right: 1px solid var(--border-color);
  padding: 1.05rem;
  position: sticky;
  top: 68px;
  height: calc(100vh - 68px);
  overflow-y: auto;
  display: flex;
  flex-direction: column;
  gap: 0.85rem;
}

.progress-card {
  background: var(--bg-muted);
  border: 1px solid var(--border-color);
  border-radius: 12px;
  padding: 0.8rem 0.95rem;
}

.progress-header {
  display: flex;
  justify-content: space-between;
  font-size: 0.82rem;
  font-weight: 600;
  margin-bottom: 0.45rem;
}

.progress-bar-bg {
  width: 100%;
  height: 8px;
  background: var(--border-color);
  border-radius: 999px;
  overflow: hidden;
}

.progress-bar-fill {
  height: 100%;
  background: linear-gradient(90deg, #0d9488, #0284c7, #6366f1);
  width: 0%;
  transition: width 0.35s ease;
}

/* Sidebar Filter Chips */
.sidebar-filter-group {
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
}

.filter-chip {
  border: 1px solid var(--border-color);
  background: var(--bg-muted);
  color: var(--text-secondary);
  padding: 0.28rem 0.6rem;
  border-radius: 999px;
  font-family: inherit;
  font-size: 0.74rem;
  font-weight: 700;
  cursor: pointer;
  transition: all 0.18s ease;
}

.filter-chip:hover {
  border-color: var(--accent);
  color: var(--text-main);
}

.filter-chip.active {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
}

.lesson-nav-list {
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
}

.lesson-nav-item {
  display: flex;
  align-items: center;
  gap: 0.65rem;
  padding: 0.58rem 0.7rem;
  border-radius: 11px;
  border: 1px solid transparent;
  background: transparent;
  color: var(--text-main);
  text-align: left;
  font-family: inherit;
  cursor: pointer;
  transition: all 0.18s ease;
}

.lesson-nav-item:hover {
  background: var(--bg-muted);
}

.lesson-nav-item.active {
  background: var(--accent-light);
  border-color: var(--accent);
}

.lesson-num {
  width: 32px;
  height: 32px;
  border-radius: 8px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.78rem;
  font-weight: 700;
  color: #fff;
  flex-shrink: 0;
}

.lesson-nav-text {
  flex: 1;
  min-width: 0;
}

.lesson-nav-title {
  font-size: 0.85rem;
  font-weight: 600;
  line-height: 1.32;
  display: block;
}

.lesson-nav-meta {
  font-size: 0.7rem;
  color: var(--text-secondary);
  display: block;
}

.lesson-check {
  font-size: 0.9rem;
  color: #10b981;
}

/* Main Content */
.main-content {
  padding: 1.75rem 2.25rem 4rem;
  max-width: 1140px;
}

.lesson-hero {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 1.65rem 1.85rem;
  margin-bottom: 1.5rem;
  box-shadow: var(--shadow-sm);
  position: relative;
  overflow: hidden;
}

.lesson-hero-top {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 1rem;
  flex-wrap: wrap;
}

.lesson-tag {
  display: inline-flex;
  align-items: center;
  gap: 0.45rem;
  padding: 0.28rem 0.75rem;
  border-radius: 999px;
  font-size: 0.8rem;
  font-weight: 700;
  color: #fff;
  margin-bottom: 0.65rem;
}

.lesson-part-badge {
  display: inline-flex;
  align-items: center;
  padding: 0.25rem 0.65rem;
  border-radius: 999px;
  font-size: 0.75rem;
  font-weight: 700;
  background: var(--bg-muted);
  color: var(--text-secondary);
  border: 1px solid var(--border-color);
  margin-left: 0.4rem;
}

.lesson-hero h2 {
  font-size: 1.65rem;
  font-weight: 800;
  line-height: 1.3;
  margin-bottom: 0.25rem;
}

.lesson-subtitle {
  font-size: 0.9rem;
  color: var(--text-secondary);
  margin-bottom: 0.9rem;
}

.lesson-summary {
  font-size: 0.98rem;
  background: var(--bg-muted);
  padding: 0.95rem 1.15rem;
  border-radius: 10px;
  border-left: 4px solid var(--accent);
}

/* Subtopic Quick Jump Bar */
.subtopic-jump-bar {
  display: flex;
  flex-wrap: wrap;
  gap: 0.45rem;
  margin-top: 0.95rem;
  padding-top: 0.85rem;
  border-top: 1px dashed var(--border-color);
}

.subtopic-jump-pill {
  padding: 0.3rem 0.7rem;
  border-radius: 999px;
  background: var(--bg-muted);
  border: 1px solid var(--border-color);
  color: var(--text-main);
  font-size: 0.78rem;
  font-weight: 700;
  text-decoration: none;
  cursor: pointer;
  transition: all 0.15s;
}

.subtopic-jump-pill:hover {
  background: var(--accent-light);
  border-color: var(--accent);
  color: var(--accent);
}

.complete-toggle-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.45rem;
  padding: 0.55rem 1rem;
  border-radius: 10px;
  border: 1px solid var(--border-color);
  background: var(--bg-muted);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.2s;
}

.complete-toggle-btn.done {
  background: rgba(16, 185, 129, 0.14);
  border-color: #10b981;
  color: #059669;
}

[data-theme="dark"] .complete-toggle-btn.done {
  color: #34d399;
}

/* Subtopic Card */
.subtopic-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 1.5rem 1.75rem;
  margin-bottom: 1.35rem;
  box-shadow: var(--shadow-sm);
}

.subtopic-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
  flex-wrap: wrap;
  padding-bottom: 0.85rem;
  margin-bottom: 1rem;
  border-bottom: 1px solid var(--border-color);
}

.subtopic-header h3 {
  font-size: 1.22rem;
  font-weight: 700;
}

.subtopic-badge {
  font-size: 0.76rem;
  font-weight: 700;
  padding: 0.25rem 0.7rem;
  border-radius: 999px;
  background: var(--accent-light);
  color: var(--accent);
}

.points-list {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 0.65rem;
}

.points-list li {
  position: relative;
  padding-left: 1.5rem;
  font-size: 0.97rem;
}

.points-list li::before {
  content: "▸";
  position: absolute;
  left: 0.2rem;
  color: var(--accent);
  font-weight: bold;
}

/* Textbook Activity Box inside Subtopic */
.activity-box {
  margin-top: 1.2rem;
  background: linear-gradient(135deg, rgba(13, 148, 136, 0.08), rgba(2, 132, 199, 0.06));
  border: 1.5px solid var(--accent);
  border-radius: 12px;
  padding: 1.05rem 1.25rem;
}

.activity-box-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
  flex-wrap: wrap;
  margin-bottom: 0.65rem;
}

.activity-box-title {
  font-size: 1.02rem;
  font-weight: 800;
  color: var(--accent);
  display: flex;
  align-items: center;
  gap: 0.45rem;
}

.activity-sim-jump-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  background: linear-gradient(135deg, #0d9488, #0284c7);
  color: #fff;
  border: none;
  padding: 0.45rem 0.95rem;
  border-radius: 999px;
  font-family: inherit;
  font-size: 0.82rem;
  font-weight: 700;
  cursor: pointer;
  box-shadow: 0 3px 10px rgba(13, 148, 136, 0.28);
  transition: all 0.2s ease;
}

.activity-sim-jump-btn:hover {
  transform: translateY(-2px);
  filter: brightness(1.08);
  box-shadow: 0 5px 14px rgba(13, 148, 136, 0.4);
}

/* Subsections */
.subsection-box {
  margin-top: 1.15rem;
  background: var(--bg-muted);
  border: 1px solid var(--border-color);
  border-radius: 11px;
  padding: 1rem 1.2rem;
}

.subsection-box h4 {
  font-size: 1.02rem;
  font-weight: 700;
  margin-bottom: 0.6rem;
  color: var(--text-main);
}

/* Tables */
.table-wrapper {
  margin-top: 1.25rem;
  overflow-x: auto;
  border: 1px solid var(--border-color);
  border-radius: 11px;
}

.table-title {
  background: var(--bg-muted);
  padding: 0.65rem 1rem;
  font-size: 0.9rem;
  font-weight: 700;
  border-bottom: 1px solid var(--border-color);
}

.note-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.9rem;
}

.note-table th,
.note-table td {
  padding: 0.7rem 0.95rem;
  border-bottom: 1px solid var(--border-color);
  text-align: left;
  vertical-align: top;
}

.note-table th {
  background: var(--accent-light);
  color: var(--text-main);
  font-weight: 700;
}

.note-table tr:last-child td {
  border-bottom: none;
}

/* Images Grid */
.images-grid {
  margin-top: 1.35rem;
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(265px, 1fr));
  gap: 1rem;
}

.figure-card {
  background: var(--bg-muted);
  border: 1px solid var(--border-color);
  border-radius: 11px;
  overflow: hidden;
  display: flex;
  flex-direction: column;
  cursor: zoom-in;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.figure-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-md);
  border-color: var(--accent);
}

.figure-card img {
  width: 100%;
  height: 225px;
  object-fit: contain;
  background: #fff;
  padding: 0.45rem;
  border-bottom: 1px solid var(--border-color);
}

.figure-caption {
  padding: 0.65rem 0.85rem;
  font-size: 0.82rem;
  color: var(--text-secondary);
  line-height: 1.5;
}

/* Interactive Activity Simulation Section */
.sim-card {
  background: var(--bg-card);
  border: 2px solid var(--accent);
  border-radius: var(--radius);
  padding: 1.45rem 1.65rem;
  margin-top: 1.25rem;
  margin-bottom: 1.45rem;
  box-shadow: var(--shadow-md);
  transition: box-shadow 0.3s ease, transform 0.3s ease;
}

.sim-card.pulse-highlight {
  box-shadow: 0 0 0 4px var(--accent), 0 12px 32px rgba(13, 148, 136, 0.35);
}

.sim-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
  flex-wrap: wrap;
  padding-bottom: 0.8rem;
  margin-bottom: 1rem;
  border-bottom: 1px solid var(--border-color);
}

.sim-header h3 {
  font-size: 1.15rem;
  font-weight: 800;
  color: var(--accent);
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.sim-badge {
  background: linear-gradient(135deg, #0d9488, #0284c7);
  color: #fff;
  font-size: 0.74rem;
  font-weight: 700;
  padding: 0.25rem 0.75rem;
  border-radius: 999px;
}

.sim-desc {
  font-size: 0.92rem;
  color: var(--text-secondary);
  margin-bottom: 1rem;
}

.sim-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  align-items: center;
  background: var(--bg-muted);
  padding: 0.9rem 1.1rem;
  border-radius: 11px;
  border: 1px solid var(--border-color);
  margin-bottom: 1rem;
  width: 100%;
  box-sizing: border-box;
}

.sim-control-group {
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
  min-width: 145px;
  flex: 1 1 auto;
  box-sizing: border-box;
}

.sim-control-group label {
  font-size: 0.82rem;
  font-weight: 700;
  color: var(--text-main);
  display: flex;
  justify-content: space-between;
  gap: 0.5rem;
  flex-wrap: wrap;
}

.sim-control-group input[type="range"] {
  width: 100%;
  accent-color: var(--accent);
  cursor: pointer;
  min-height: 28px;
}

.sim-select, .sim-input {
  padding: 0.55rem 0.75rem;
  border-radius: 8px;
  border: 1px solid var(--border-color);
  background: var(--bg-card);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.88rem;
  font-weight: 600;
  min-height: 40px;
  box-sizing: border-box;
  max-width: 100%;
}

.sim-btn {
  padding: 0.55rem 1rem;
  border-radius: 9px;
  border: 1px solid var(--accent);
  background: var(--accent);
  color: #fff;
  font-family: inherit;
  font-size: 0.86rem;
  font-weight: 700;
  cursor: pointer;
  min-height: 40px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.4rem;
  transition: all 0.18s;
  box-sizing: border-box;
}

.sim-btn:hover {
  filter: brightness(1.08);
  transform: translateY(-1px);
}

.sim-btn.secondary {
  background: var(--bg-card);
  color: var(--text-main);
  border-color: var(--border-color);
}

.sim-btn.secondary:hover {
  border-color: var(--accent);
  background: var(--accent-light);
}

.sim-stage {
  background: var(--bg-muted);
  border: 1px solid var(--border-color);
  border-radius: 12px;
  padding: 1.1rem;
  min-height: 240px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 0.9rem;
  position: relative;
  overflow: hidden;
}

.sim-canvas-wrap {
  width: 100%;
  max-width: 100%;
  box-sizing: border-box;
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  align-items: center;
  justify-content: center;
  overflow: hidden;
}

.sim-canvas {
  background: #0f172a;
  border-radius: 12px;
  border: 2px solid #334155;
  width: 100% !important;
  max-width: 560px;
  height: auto !important;
  aspect-ratio: auto;
  box-sizing: border-box;
  display: block;
  margin: 0 auto;
  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.25);
  cursor: grab;
  touch-action: pan-y;
}

.sim-canvas:active {
  cursor: grabbing;
}

.sim-readout {
  width: 100%;
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-left: 4px solid var(--accent);
  border-radius: 9px;
  padding: 0.75rem 1rem;
  font-size: 0.9rem;
}

.sim-metrics-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(135px, 1fr));
  gap: 0.65rem;
  width: 100%;
}

.sim-metric-box {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: 9px;
  padding: 0.6rem 0.8rem;
  text-align: center;
}

.sim-metric-label {
  font-size: 0.75rem;
  color: var(--text-secondary);
  display: block;
}

.sim-metric-val {
  font-size: 1.05rem;
  font-weight: 800;
  color: var(--accent);
}

/* Lesson Quiz Section */
.quiz-section {
  background: var(--bg-card);
  border: 2px solid var(--border-color);
  border-radius: var(--radius);
  padding: 1.5rem 1.75rem;
  margin-top: 1.75rem;
}

.quiz-question-card {
  background: var(--bg-muted);
  border-radius: 11px;
  padding: 1rem 1.2rem;
  margin-top: 1rem;
}

.quiz-q-text {
  font-weight: 700;
  margin-bottom: 0.7rem;
  font-size: 0.96rem;
}

.quiz-options {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(210px, 1fr));
  gap: 0.55rem;
}

.quiz-opt-btn {
  padding: 0.55rem 0.85rem;
  border-radius: 8px;
  border: 1px solid var(--border-color);
  background: var(--bg-card);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.88rem;
  text-align: left;
  cursor: pointer;
  transition: all 0.15s;
}

.quiz-opt-btn:hover:not(:disabled) {
  border-color: var(--accent);
}

.quiz-opt-btn.correct {
  background: rgba(16, 185, 129, 0.18);
  border-color: #10b981;
  font-weight: 700;
}

.quiz-opt-btn.wrong {
  background: rgba(239, 68, 68, 0.16);
  border-color: #ef4444;
}

.quiz-explanation {
  margin-top: 0.65rem;
  font-size: 0.84rem;
  padding: 0.55rem 0.8rem;
  border-radius: 8px;
  background: var(--accent-light);
  display: none;
}

/* Flashcards Mode */
.flashcards-container {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(290px, 1fr));
  gap: 1.1rem;
}

.flashcard {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 1.35rem;
  min-height: 175px;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  cursor: pointer;
  box-shadow: var(--shadow-sm);
  transition: transform 0.2s, border-color 0.2s;
}

.flashcard:hover {
  transform: translateY(-2px);
  border-color: var(--accent);
}

.flashcard-term {
  font-size: 1.08rem;
  font-weight: 700;
  color: var(--accent);
  margin-bottom: 0.55rem;
}

.flashcard-def {
  font-size: 0.92rem;
  color: var(--text-main);
}

/* Lesson Footer Nav */
.lesson-footer-nav {
  display: flex;
  justify-content: space-between;
  gap: 1rem;
  margin-top: 2rem;
}

.nav-step-btn {
  padding: 0.75rem 1.25rem;
  border-radius: 11px;
  border: 1px solid var(--border-color);
  background: var(--bg-card);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.2s;
}

.nav-step-btn:hover:not(:disabled) {
  border-color: var(--accent);
  background: var(--accent-light);
}

.nav-step-btn:disabled {
  opacity: 0.4;
  cursor: not-allowed;
}

/* Lightbox Modal */
.lightbox {
  position: fixed;
  inset: 0;
  background: rgba(15, 23, 42, 0.88);
  z-index: 1000;
  display: none;
  align-items: center;
  justify-content: center;
  padding: 1.5rem;
}

.lightbox.open {
  display: flex;
}

.lightbox-inner {
  max-width: 940px;
  width: 100%;
  background: var(--bg-card);
  border-radius: var(--radius);
  overflow: hidden;
  box-shadow: var(--shadow-md);
}

.lightbox-inner img {
  width: 100%;
  max-height: 75vh;
  object-fit: contain;
  background: #fff;
  display: block;
}

.lightbox-bar {
  padding: 0.9rem 1.25rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
}

/* Mobile Quick Floating Pill & Back-to-Top Button (Mobile only) */
.mobile-quick-actions {
  position: fixed;
  bottom: 1.25rem;
  right: 1.25rem;
  display: none !important;
  align-items: center;
  gap: 0.6rem;
  z-index: 95;
  transition: transform 0.25s ease, opacity 0.25s ease;
}

.quick-pill-btn {
  background: var(--text-main);
  color: var(--bg-card);
  border: none;
  padding: 0.65rem 1.15rem;
  border-radius: 999px;
  font-family: inherit;
  font-size: 0.85rem;
  font-weight: 700;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  cursor: pointer;
  box-shadow: 0 6px 20px rgba(0, 0, 0, 0.25);
  transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

.quick-pill-btn:hover {
  transform: translateY(-2px);
  background: var(--accent);
  color: #fff;
}

.quick-top-btn {
  background: var(--bg-card);
  color: var(--text-main);
  border: 1px solid var(--border-color);
  width: 44px;
  height: 44px;
  border-radius: 50%;
  font-size: 0.95rem;
  font-weight: bold;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  box-shadow: 0 4px 14px rgba(0, 0, 0, 0.15);
  transition: all 0.2s ease;
}

.quick-top-btn:hover {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
  transform: translateY(-2px);
}

/* Responsive Media Queries */
@media (max-width: 960px) {
  .top-header {
    padding: 0.65rem 0.85rem;
    gap: 0.55rem;
  }

  .mobile-menu-btn {
    display: flex !important;
  }

  .mobile-quick-actions {
    display: flex !important;
  }

  .brand-title p {
    display: none;
  }

  .brand-title h1 {
    font-size: 1.05rem;
  }

  .desktop-search {
    display: none !important;
  }

  .desktop-mode-tabs {
    display: none !important;
  }

  .mobile-search-row {
    display: flex;
    width: 100%;
  }

  .mobile-tabs-row {
    display: flex;
    width: 100%;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
    scrollbar-width: none;
    padding-bottom: 0.15rem;
  }

  .mobile-tabs-row::-webkit-scrollbar {
    display: none;
  }

  .mobile-mode-tabs {
    width: 100%;
    display: flex;
    justify-content: space-between;
    padding: 0.2rem;
    background: var(--bg-muted);
    border-radius: 11px;
    gap: 0.15rem;
  }

  .mobile-mode-tabs .mode-btn {
    flex: 1;
    text-align: center;
    padding: 0.45rem 0.35rem;
    font-size: 0.78rem;
    border-radius: 7px;
  }

  .app-layout {
    grid-template-columns: 1fr;
  }

  .sidebar {
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    width: 325px;
    max-width: 86vw;
    height: 100vh;
    z-index: 200;
    background: var(--bg-card);
    transform: translateX(-100%);
    transition: transform 0.28s cubic-bezier(0.4, 0, 0.2, 1);
    box-shadow: 4px 0 24px rgba(0, 0, 0, 0.28);
    display: flex;
    flex-direction: column;
    padding: 1.15rem;
  }

  .sidebar.drawer-open {
    transform: translateX(0);
  }

  .sidebar-top-mobile {
    display: flex;
  }

  .main-content {
    padding: 1rem 0.9rem 5rem;
  }

  .lesson-hero {
    padding: 1.25rem 1.15rem;
    border-radius: 12px;
  }

  .lesson-hero h2 {
    font-size: 1.35rem;
  }

  .subtopic-card, .sim-card {
    padding: 1.15rem 1rem;
    border-radius: 12px;
  }

  .subtopic-header h3, .sim-header h3 {
    font-size: 1.1rem;
  }

  .table-wrapper {
    margin-left: -0.3rem;
    margin-right: -0.3rem;
    border-radius: 8px;
  }

  .note-table {
    font-size: 0.82rem;
  }

  .note-table th, .note-table td {
    padding: 0.55rem 0.7rem;
  }

  .images-grid {
    grid-template-columns: 1fr;
    gap: 0.85rem;
  }

  .figure-card img {
    height: 200px;
  }

  .quiz-options {
    grid-template-columns: 1fr;
  }

  .flashcards-container {
    grid-template-columns: 1fr;
  }

  .lesson-footer-nav {
    flex-direction: column;
    gap: 0.65rem;
  }

  .nav-step-btn {
    width: 100%;
    text-align: center;
    padding: 0.75rem 1rem;
  }
}

@media (max-width: 480px) {
  .top-header {
    padding: 0.5rem 0.65rem;
    gap: 0.45rem;
  }

  .brand {
    gap: 0.45rem;
  }

  .brand-logo {
    width: 32px;
    height: 32px;
    font-size: 1.1rem;
    border-radius: 8px;
  }

  .brand-title h1 {
    font-size: 0.96rem;
  }

  .icon-btn {
    width: 34px;
    height: 34px;
    font-size: 0.95rem;
    border-radius: 8px;
  }

  .mobile-mode-tabs .mode-btn {
    padding: 0.38rem 0.2rem;
    font-size: 0.72rem;
  }

  .search-box input {
    height: 38px;
    font-size: 0.85rem;
    padding: 0 2rem 0 2.25rem;
  }

  .search-icon {
    left: 0.8rem;
    font-size: 0.9rem;
  }

  .main-content {
    padding: 0.8rem 0.65rem 4.5rem;
  }

  .lesson-hero {
    padding: 1rem 0.85rem;
  }

  .subtopic-card, .sim-card {
    padding: 1rem 0.85rem;
  }

  .subtopic-header h3, .sim-header h3 {
    font-size: 1.02rem;
  }

  .quick-pill-btn {
    padding: 0.55rem 0.9rem;
    font-size: 0.78rem;
  }

  .quick-top-btn {
    width: 38px;
    height: 38px;
    font-size: 0.85rem;
  }
}

@media print {
  .top-header, .sidebar, .quiz-section, .lesson-footer-nav, .complete-toggle-btn, .mobile-quick-actions, .sidebar-overlay {
    display: none !important;
  }
  .app-layout {
    display: block;
  }
  .main-content {
    max-width: 100%;
    padding: 0;
  }
}

/* =========================================================================
   REALISTIC VIRTUAL LAB & ANIMATED SIMULATION UI (SciLearn Ultimate Style)
   ========================================================================= */
.vlab-container {
  background: var(--bg-card);
  background-image: radial-gradient(var(--border-color) 1px, transparent 1px);
  background-size: 20px 20px;
  border: 1px solid var(--border-color);
  border-radius: 20px;
  padding: 1.25rem;
  box-shadow: inset 0 0 40px rgba(148, 163, 184, 0.12);
}

.vlab-subtabs {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-bottom: 1.1rem;
}

.vlab-tab-btn {
  padding: 0.5rem 1rem;
  border-radius: 12px;
  border: 1.5px solid var(--border-color);
  background: var(--bg-card);
  color: var(--text-secondary);
  font-family: inherit;
  font-size: 0.85rem;
  font-weight: 700;
  cursor: pointer;
  transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

.vlab-tab-btn:hover {
  border-color: var(--accent);
  color: var(--accent);
  transform: translateY(-1px);
}

.vlab-tab-btn.active {
  background: linear-gradient(135deg, #2563eb, #0d9488);
  color: #ffffff;
  border-color: transparent;
  box-shadow: 0 6px 16px rgba(37, 99, 235, 0.28);
}

.vlab-grid {
  display: grid;
  grid-template-columns: 1.05fr 0.95fr;
  gap: 1.35rem;
  align-items: stretch;
}

@media (max-width: 820px) {
  .vlab-grid {
    grid-template-columns: 1fr;
  }
}

.vlab-panel {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: 18px;
  padding: 1.2rem;
  box-shadow: var(--shadow-sm);
  display: flex;
  flex-direction: column;
  justify-content: space-between;
}

.vlab-step-title {
  font-size: 0.95rem;
  font-weight: 800;
  color: var(--text-main);
  padding-bottom: 0.5rem;
  margin-bottom: 0.7rem;
  border-bottom: 1px solid var(--border-color);
  display: flex;
  align-items: center;
  gap: 0.55rem;
}

.vlab-step-num {
  width: 26px;
  height: 26px;
  border-radius: 8px;
  background: linear-gradient(135deg, #2563eb, #0d9488);
  color: #fff;
  font-size: 0.8rem;
  font-weight: 900;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  box-shadow: 0 3px 8px rgba(37, 99, 235, 0.25);
}

.vlab-btn-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 0.55rem;
  margin-bottom: 1rem;
}

.vlab-choice-btn {
  padding: 0.65rem 0.75rem;
  border-radius: 12px;
  border: 2px solid var(--border-color);
  background: var(--bg-muted);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.84rem;
  font-weight: 700;
  cursor: pointer;
  transition: all 0.2s ease;
  text-align: center;
}

.vlab-choice-btn:hover:not(:disabled) {
  border-color: #38bdf8;
  transform: translateY(-2px);
  box-shadow: 0 4px 12px rgba(56, 189, 248, 0.18);
}

.vlab-choice-btn.active {
  background: rgba(37, 99, 235, 0.12);
  border-color: #2563eb;
  color: #1d4ed8;
  box-shadow: 0 4px 12px rgba(37, 99, 235, 0.2);
}

[data-theme="dark"] .vlab-choice-btn.active {
  color: #60a5fa;
  border-color: #3b82f6;
}

.vlab-choice-btn:disabled {
  opacity: 0.45;
  cursor: not-allowed;
}

.vlab-action-row {
  display: flex;
  gap: 0.6rem;
  margin-top: 0.4rem;
}

.vlab-heat-btn {
  flex: 1;
  background: linear-gradient(135deg, #f97316, #dc2626);
  color: #fff;
  border: none;
  border-radius: 14px;
  padding: 0.75rem 1rem;
  font-family: inherit;
  font-size: 0.92rem;
  font-weight: 800;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 0.45rem;
  box-shadow: 0 6px 16px rgba(234, 88, 12, 0.32);
  transition: all 0.2s ease;
}

.vlab-heat-btn:hover:not(:disabled) {
  transform: translateY(-2px) scale(1.01);
  box-shadow: 0 8px 20px rgba(234, 88, 12, 0.45);
}

.vlab-heat-btn:disabled {
  opacity: 0.38;
  cursor: not-allowed;
  box-shadow: none;
}

.vlab-reset-btn {
  background: var(--bg-muted);
  color: var(--text-main);
  border: 1.5px solid var(--border-color);
  border-radius: 14px;
  padding: 0.75rem 1rem;
  font-family: inherit;
  font-size: 0.9rem;
  font-weight: 800;
  cursor: pointer;
  transition: all 0.2s ease;
}

.vlab-reset-btn:hover {
  border-color: var(--accent);
  color: var(--accent);
  transform: translateY(-2px);
}

.vlab-status-banner {
  margin-top: 0.85rem;
  padding: 0.65rem 0.9rem;
  border-radius: 12px;
  background: rgba(37, 99, 235, 0.08);
  border: 1px solid rgba(37, 99, 235, 0.25);
  color: var(--text-main);
  font-size: 0.85rem;
  font-weight: 700;
  text-align: center;
}

/* Realistic Glass Test Tube & Burner Bench */
.vlab-bench {
  background: linear-gradient(180deg, rgba(241, 245, 249, 0.6) 0%, rgba(226, 232, 240, 0.8) 100%);
  border: 1px solid var(--border-color);
  border-radius: 18px;
  padding: 1.5rem 1rem 1.2rem;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  position: relative;
  min-height: 340px;
  overflow: hidden;
}

[data-theme="dark"] .vlab-bench {
  background: linear-gradient(180deg, rgba(15, 23, 42, 0.85) 0%, rgba(30, 41, 59, 0.95) 100%);
}

.vlab-apparatus-wrap {
  position: relative;
  display: flex;
  flex-direction: column;
  align-items: center;
  margin-top: 1.2rem;
  margin-bottom: 1rem;
}

/* Animated Pipette Dropper */
.vlab-dropper {
  width: 14px;
  height: 44px;
  background: linear-gradient(to right, #cbd5e1, #f8fafc, #94a3b8);
  border-radius: 4px 4px 7px 7px;
  position: absolute;
  top: -54px;
  left: 50%;
  transform: translateX(-50%);
  opacity: 0;
  transition: opacity 0.3s ease, top 0.3s ease;
  z-index: 25;
}

.vlab-dropper::before {
  content: "";
  position: absolute;
  top: -16px;
  left: -3px;
  width: 20px;
  height: 18px;
  background: #ef4444;
  border-radius: 10px 10px 3px 3px;
}

.vlab-dropper.dropping {
  opacity: 1;
  top: -42px;
}

.vlab-drop {
  width: 10px;
  height: 13px;
  border-radius: 50% 50% 50% 50% / 60% 60% 40% 40%;
  position: absolute;
  top: -8px;
  left: 50%;
  transform: translateX(-50%);
  opacity: 0;
  z-index: 24;
}

.vlab-drop.animate-drop {
  animation: dropFall 0.65s ease-in forwards;
}

@keyframes dropFall {
  0% { transform: translate(-50%, 0) scale(0.7); opacity: 1; }
  85% { opacity: 1; }
  100% { transform: translate(-50%, 125px) scale(1.1); opacity: 0; }
}

.glass-tube {
  width: 68px;
  height: 205px;
  border: 3.5px solid rgba(148, 163, 184, 0.78);
  border-top: none;
  border-radius: 0 0 34px 34px;
  position: relative;
  overflow: hidden;
  background: rgba(255, 255, 255, 0.22);
  box-shadow:
    inset -8px -8px 20px rgba(0, 0, 0, 0.08),
    inset 8px 8px 20px rgba(255, 255, 255, 0.85),
    0 18px 28px rgba(0, 0, 0, 0.16);
  z-index: 10;
}

.glass-tube::before {
  content: "";
  position: absolute;
  top: 10px;
  left: 9px;
  width: 6px;
  height: 82%;
  background: linear-gradient(to bottom, rgba(255, 255, 255, 0.92), rgba(255, 255, 255, 0.08));
  border-radius: 10px;
  z-index: 20;
}

.liquid-level {
  position: absolute;
  bottom: 0;
  width: 100%;
  height: 5%;
  background-color: transparent;
  transition: height 0.95s cubic-bezier(0.4, 0, 0.2, 1), background-color 1.2s ease;
  border-radius: 0 0 30px 30px;
  box-shadow: inset 0 -10px 20px rgba(0, 0, 0, 0.22);
}

.liquid-level::after {
  content: "";
  position: absolute;
  top: -6px;
  left: 0;
  width: 100%;
  height: 12px;
  background: inherit;
  filter: brightness(1.2) contrast(1.1);
  border-radius: 50%;
  box-shadow: inset 0 2px 5px rgba(255, 255, 255, 0.75), 0 2px 5px rgba(0, 0, 0, 0.2);
  opacity: 0;
  transition: opacity 0.4s;
}

.liquid-level.filled::after {
  opacity: 1;
}

.oil-layer {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 0%;
  background: transparent;
  transition: height 0.8s ease, background-color 0.8s ease;
  z-index: 14;
}

/* Bunsen Burner & Flame */
.burner {
  width: 50px;
  height: 32px;
  background: linear-gradient(to right, #475569, #1e293b, #475569);
  margin: 12px auto 0;
  border-radius: 7px 7px 0 0;
  position: relative;
  box-shadow: 0 5px 12px rgba(0, 0, 0, 0.25);
}

.burner::after {
  content: "";
  position: absolute;
  bottom: 0;
  left: -14px;
  width: 78px;
  height: 7px;
  background: #334155;
  border-radius: 4px;
}

.burner-flame {
  width: 32px;
  height: 48px;
  background: radial-gradient(ellipse at bottom, #38bdf8 0%, #2563eb 45%, #f97316 80%, transparent 95%);
  position: absolute;
  bottom: 29px;
  left: 50%;
  transform: translateX(-50%);
  border-radius: 50% 50% 20% 20% / 60% 60% 40% 40%;
  animation: vlabFlicker 0.1s infinite alternate;
  display: none;
  filter: drop-shadow(0 0 10px #38bdf8) drop-shadow(0 0 22px #2563eb);
}

.burner-flame::after {
  content: "";
  position: absolute;
  bottom: 5px;
  left: 50%;
  transform: translateX(-50%);
  width: 13px;
  height: 22px;
  background: radial-gradient(ellipse at bottom, #ffffff 0%, #7dd3fc 100%);
  border-radius: 50% 50% 20% 20% / 60% 60% 40% 40%;
}

@keyframes vlabFlicker {
  0% { transform: translateX(-50%) scale(0.94) rotate(-2deg); opacity: 0.9; }
  100% { transform: translateX(-50%) scale(1.06) rotate(2deg); opacity: 1; }
}

.bubbles-container {
  position: absolute;
  bottom: 5px;
  width: 100%;
  height: 100%;
  display: none;
  z-index: 16;
}

.bubble {
  position: absolute;
  background: radial-gradient(circle at 30% 30%, rgba(255, 255, 255, 0.95), rgba(255, 255, 255, 0.25));
  border: 1px solid rgba(255, 255, 255, 0.6);
  border-radius: 50%;
  bottom: 0;
  animation: vlabRise 1.1s infinite ease-in;
}

@keyframes vlabRise {
  0% { transform: translateY(0) scale(0.5); opacity: 0; }
  25% { opacity: 1; }
  100% { transform: translateY(-145px) scale(1.45); opacity: 0; }
}

.vlab-shake {
  animation: vlabShake 0.45s cubic-bezier(0.36, 0.07, 0.19, 0.97) infinite;
}

@keyframes vlabShake {
  10%, 90% { transform: translate3d(-1px, 0, 0); }
  20%, 80% { transform: translate3d(2px, 0, 0); }
  30%, 50%, 70% { transform: translate3d(-3px, 0, 0); }
  40%, 60% { transform: translate3d(3px, 0, 0); }
}

.vlab-result-card {
  background: var(--bg-card);
  border: 2px solid var(--border-color);
  border-radius: 16px;
  padding: 0.85rem 1.15rem;
  width: 92%;
  max-width: 350px;
  text-align: center;
  box-shadow: 0 12px 28px rgba(0, 0, 0, 0.14);
  transition: all 0.35s cubic-bezier(0.175, 0.885, 0.32, 1.275);
  z-index: 20;
}

.vlab-result-card.success {
  border-color: #10b981;
  background: linear-gradient(135deg, var(--bg-card), rgba(16, 185, 129, 0.1));
}

/* =========================================================================
   SIMULATION MODE HEADER, TOOLBAR & MOBILE QUICK SELECTOR
   ========================================================================= */
.sim-mode-hero {
  border-left: 5px solid #0d9488 !important;
  box-shadow: var(--shadow-sm);
}

.sim-jump-bar {
  margin-top: 1.15rem;
  padding-top: 1rem;
  border-top: 1px dashed var(--border-color);
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.sim-jump-label {
  font-size: 0.88rem;
  font-weight: 700;
  color: var(--text-main);
  display: flex;
  align-items: center;
  gap: 0.4rem;
}

.sim-select-wrap {
  position: relative;
  width: 100%;
}

.sim-jump-select {
  width: 100%;
  padding: 0.72rem 1rem;
  border-radius: 12px;
  border: 2px solid var(--accent);
  background: var(--bg-card);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.9rem;
  font-weight: 700;
  cursor: pointer;
  box-shadow: 0 4px 14px rgba(13, 148, 136, 0.12);
  transition: all 0.2s ease;
  appearance: none;
  -webkit-appearance: none;
  background-image: url("data:image/svg+xml;charset=UTF-8,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%230d9488' stroke-width='2.5' stroke-linecap='round' stroke-linejoin='round'%3e%3cpolyline points='6 9 12 15 18 9'%3e%3c/polyline%3e%3c/svg%3e");
  background-repeat: no-repeat;
  background-position: right 0.85rem center;
  background-size: 1.15rem;
  padding-right: 2.5rem;
  box-sizing: border-box;
}

.sim-jump-select:focus {
  outline: none;
  border-color: #2563eb;
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.22);
}

.sim-toolbar {
  margin-top: 1rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.85rem;
  flex-wrap: wrap;
}

.sim-filter-chips {
  display: flex;
  flex-wrap: wrap;
  gap: 0.45rem;
}

.sim-search-box {
  position: relative;
  flex: 1;
  min-width: 240px;
}

.sim-search-box input {
  width: 100%;
  height: 40px;
  padding: 0 2.2rem 0 2.25rem;
  border-radius: 10px;
  border: 1.5px solid var(--border-color);
  background: var(--bg-card);
  color: var(--text-main);
  font-family: inherit;
  font-size: 0.88rem;
  transition: border-color 0.2s;
  box-sizing: border-box;
}

.sim-search-box input:focus {
  outline: none;
  border-color: var(--accent);
}

.sim-count-badge {
  font-size: 0.82rem;
  font-weight: 700;
  color: var(--text-secondary);
  margin-top: 0.65rem;
  display: inline-block;
}

.sim-empty-state {
  text-align: center;
  padding: 3rem 1.5rem;
  background: var(--bg-card);
  border: 1.5px dashed var(--border-color);
  border-radius: var(--radius);
  color: var(--text-secondary);
  font-size: 0.95rem;
  font-weight: 600;
}

/* =========================================================================
   COMPREHENSIVE MOBILE RESPONSIVENESS (<820px, <640px, <480px)
   ========================================================================= */
@media (max-width: 820px) {
  .vlab-grid {
    grid-template-columns: 1fr !important;
    gap: 1.15rem;
  }

  .vlab-panel {
    order: 1;
  }

  .vlab-bench {
    order: 2;
    min-height: 300px;
  }
}

@media (max-width: 640px) {
  /* Layout and Container Protection */
  html, body {
    overflow-x: hidden;
  }

  .main-content {
    padding: 0.85rem 0.65rem 4.75rem !important;
    width: 100%;
    max-width: 100vw;
    box-sizing: border-box;
    overflow-x: hidden;
  }

  /* Simulation Cards & Stage */
  .sim-card {
    padding: 1.1rem 0.85rem !important;
    border-radius: 14px !important;
    margin-bottom: 1.15rem;
    width: 100%;
    box-sizing: border-box;
  }

  .sim-header {
    flex-direction: column;
    align-items: flex-start;
    gap: 0.5rem;
  }

  .sim-header h3 {
    font-size: 1.05rem;
    line-height: 1.35;
  }

  .sim-controls {
    padding: 0.75rem 0.65rem !important;
    gap: 0.65rem !important;
  }

  .sim-control-group {
    min-width: 100% !important;
    width: 100% !important;
    flex: 1 1 100% !important;
  }

  .sim-control-group label {
    font-size: 0.8rem;
  }

  .sim-select, .sim-input {
    width: 100% !important;
    font-size: 0.9rem !important;
    min-height: 42px;
  }

  .sim-btn {
    width: 100% !important;
    justify-content: center;
    min-height: 42px;
    font-size: 0.9rem;
  }

  .sim-stage {
    padding: 0.85rem 0.45rem !important;
    border-radius: 10px;
    min-height: 210px;
    width: 100%;
    box-sizing: border-box;
  }

  .sim-canvas {
    width: 100% !important;
    height: auto !important;
    border-radius: 10px;
  }

  .sim-metrics-grid {
    grid-template-columns: repeat(2, 1fr) !important;
    gap: 0.45rem !important;
  }

  .sim-metric-box {
    padding: 0.5rem 0.35rem;
  }

  .sim-metric-val {
    font-size: 0.94rem;
  }

  .sim-readout {
    padding: 0.65rem 0.75rem !important;
    font-size: 0.84rem;
    line-height: 1.45;
  }

  /* Virtual Lab Apparatus Mobile */
  .vlab-container {
    padding: 0.85rem 0.55rem !important;
    border-radius: 14px !important;
  }

  .vlab-subtabs {
    display: flex !important;
    flex-wrap: nowrap !important;
    overflow-x: auto !important;
    -webkit-overflow-scrolling: touch;
    scrollbar-width: none;
    gap: 0.4rem !important;
    padding-bottom: 0.35rem;
    margin-bottom: 0.85rem;
  }

  .vlab-subtabs::-webkit-scrollbar {
    display: none;
  }

  .vlab-tab-btn {
    flex-shrink: 0 !important;
    white-space: nowrap !important;
    padding: 0.45rem 0.8rem !important;
    font-size: 0.78rem !important;
    border-radius: 10px;
  }

  .vlab-panel {
    padding: 0.95rem 0.75rem !important;
    border-radius: 14px;
  }

  .vlab-btn-grid {
    grid-template-columns: repeat(2, 1fr) !important;
    gap: 0.45rem !important;
  }

  .vlab-choice-btn {
    padding: 0.6rem 0.45rem !important;
    font-size: 0.8rem !important;
    min-height: 42px;
    display: flex;
    align-items: center;
    justify-content: center;
  }

  .vlab-action-row {
    flex-direction: column !important;
    gap: 0.5rem;
  }

  .vlab-heat-btn, .vlab-reset-btn {
    width: 100% !important;
    min-height: 44px;
    padding: 0.7rem 1rem !important;
    font-size: 0.88rem !important;
  }

  .vlab-bench {
    min-height: 275px !important;
    padding: 1.1rem 0.5rem 0.9rem !important;
    border-radius: 14px;
  }

  .glass-tube {
    width: 60px;
    height: 185px;
  }

  .vlab-result-card {
    width: 98% !important;
    max-width: 100% !important;
    padding: 0.75rem 0.75rem !important;
    font-size: 0.82rem !important;
  }

  .sim-toolbar {
    flex-direction: column;
    align-items: stretch;
    gap: 0.65rem;
  }

  .sim-search-box {
    width: 100% !important;
    min-width: 100% !important;
  }

  .activity-box-header {
    flex-direction: column;
    align-items: flex-start;
    gap: 0.5rem;
  }

  .activity-sim-jump-btn {
    width: 100% !important;
    justify-content: center;
    min-height: 40px;
    font-size: 0.84rem;
  }

  /* Table horizontal responsiveness */
  .table-wrapper {
    width: 100%;
    max-width: 100%;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
    border-radius: 10px;
  }
}

@media (max-width: 380px) {
  .vlab-btn-grid {
    grid-template-columns: 1fr !important;
  }

  .sim-metrics-grid {
    grid-template-columns: 1fr !important;
  }

  .sim-jump-select {
    font-size: 0.82rem;
    padding: 0.65rem 0.75rem;
  }
}