/* ===== RESET & BASE ===== */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --bg: #faf9f6;
  --bg-alt: #f0eeeb;
  --text: #2c2c2c;
  --text-muted: #666;
  --accent: #1a5276;
  --accent-light: #2980b9;
  --accent-bg: #eaf2f8;
  --border: #d5d5d5;
  --sidebar-w: 300px;
  --derivation-bg: #f7f5f0;
  --tooltip-bg: #2c3e50;
  --success: #27ae60;
  --warning: #e67e22;
  --eq-highlight: #fff8e1;
  --shadow: 0 2px 8px rgba(0,0,0,0.08);
  --radius: 6px;
  --transition: 0.25s ease;
  --font-serif: 'Merriweather', Georgia, serif;
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', monospace;
}

html { font-size: 17px; scroll-behavior: smooth; }
body {
  font-family: var(--font-serif);
  color: var(--text);
  background: var(--bg);
  line-height: 1.75;
  overflow-x: hidden;
}

/* ===== LAYOUT ===== */
.app { display: flex; min-height: 100vh; }

/* ===== SIDEBAR ===== */
.sidebar {
  position: fixed;
  top: 0; left: 0;
  width: var(--sidebar-w);
  height: 100vh;
  background: #1b2631;
  color: #ecf0f1;
  overflow-y: auto;
  z-index: 100;
  transition: transform var(--transition), width var(--transition);
  display: flex;
  flex-direction: column;
}
.sidebar-header {
  padding: 24px 20px 16px;
  border-bottom: 1px solid rgba(255,255,255,0.1);
  position: relative;
}
.sidebar-collapse {
  position: absolute;
  right: 10px;
  top: 50%;
  transform: translateY(-50%);
  background: none;
  border: 1px solid rgba(255,255,255,0.15);
  color: #95a5a6;
  width: 24px;
  height: 24px;
  border-radius: 4px;
  cursor: pointer;
  font-size: 1rem;
  line-height: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: all 0.15s;
  padding: 0;
}
.sidebar-collapse:hover { color: #fff; border-color: rgba(255,255,255,0.35); }
.sidebar-header h1 {
  font-family: var(--font-sans);
  font-size: 1.1rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  color: #fff;
}
.sidebar-header .subtitle {
  font-size: 0.72rem;
  color: #95a5a6;
  margin-top: 4px;
  font-family: var(--font-sans);
}

.sidebar-nav { flex: 1; padding: 12px 0; }
.sidebar-nav .chapter-link {
  display: flex;
  align-items: baseline;
  padding: 8px 20px;
  color: #bdc3c7;
  text-decoration: none;
  font-family: var(--font-sans);
  font-size: 0.82rem;
  cursor: pointer;
  transition: all 0.15s;
  border-left: 3px solid transparent;
}
.sidebar-nav .chapter-link:hover {
  background: rgba(255,255,255,0.05);
  color: #fff;
}
.sidebar-nav .chapter-link.active {
  background: rgba(26,82,118,0.3);
  color: #5dade2;
  border-left-color: #5dade2;
}
.sidebar-nav .chapter-link .ch-num {
  min-width: 28px;
  font-weight: 600;
  color: #7f8c8d;
  font-size: 0.75rem;
}
.sidebar-nav .chapter-link.active .ch-num { color: #5dade2; }

.sidebar-nav .section-links {
  display: none;
  padding: 2px 0 6px 48px;
}
.sidebar-nav .chapter-link.active + .section-links { display: block; }
.sidebar-nav .section-links a {
  display: block;
  padding: 3px 0;
  color: #95a5a6;
  text-decoration: none;
  font-size: 0.72rem;
  font-family: var(--font-sans);
  transition: color 0.15s;
}
.sidebar-nav .section-links a:hover { color: #5dade2; }
.sidebar-nav .section-links a.active { color: #5dade2; font-weight: 600; }

/* Collapsed sidebar */
.sidebar.collapsed {
  width: 48px;
  overflow: hidden;
}
.sidebar.collapsed .sidebar-header h1,
.sidebar.collapsed .sidebar-header .subtitle { display: none; }
.sidebar.collapsed .sidebar-header {
  padding: 12px 0;
  display: flex;
  justify-content: center;
}
.sidebar.collapsed .sidebar-collapse {
  position: static;
  transform: none;
  rotate: 180deg;
}
.sidebar.collapsed .sidebar-nav .chapter-link {
  padding: 8px 0;
  justify-content: center;
}
.sidebar.collapsed .sidebar-nav .chapter-link .ch-num {
  min-width: auto;
  font-size: 0.82rem;
}
.sidebar.collapsed .sidebar-nav .chapter-link .ch-title { display: none; }
.sidebar.collapsed .sidebar-nav .section-links { display: none !important; }
.app:has(.sidebar.collapsed) .progress-bar { left: 48px; }
.app:has(.sidebar.collapsed) .main-content { margin-left: 48px; }

/* ===== MAIN CONTENT ===== */
.main-content {
  margin-left: var(--sidebar-w);
  flex: 1;
  max-width: 820px;
  padding: 48px 60px 120px;
  transition: margin-left var(--transition);
}

/* ===== CHAPTER CONTAINER ===== */
.chapter { display: none; }
.chapter.active { display: block; }

/* ===== TYPOGRAPHY ===== */
.chapter-number {
  font-family: var(--font-sans);
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--accent-light);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  margin-bottom: 8px;
}
.chapter-title {
  font-family: var(--font-sans);
  font-size: 2.2rem;
  font-weight: 800;
  color: var(--text);
  line-height: 1.2;
  margin-bottom: 8px;
  letter-spacing: -0.03em;
}
.chapter-intro {
  font-size: 1.05rem;
  color: var(--text-muted);
  margin-bottom: 36px;
  border-bottom: 1px solid var(--border);
  padding-bottom: 24px;
}

/* ===== CHAPTER TABLE OF CONTENTS ===== */
.chapter-toc {
  background: var(--bg-alt);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 4px 24px 4px;
  margin-bottom: 36px;
  max-width: 480px;
}
.chapter-toc[open] {
  padding: 4px 24px 12px;
}
.toc-title {
  font-family: var(--font-sans);
  font-weight: 700;
  font-size: 0.95rem;
  color: var(--text);
  padding: 10px 0;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  cursor: pointer;
  user-select: none;
}
.toc-title::-webkit-details-marker {
  margin-right: 8px;
}
.toc-list {
  margin: 8px 0 0;
  padding-left: 8px;
  list-style: none;
}
.toc-list > li {
  margin-bottom: 4px;
}
.toc-list a {
  color: var(--accent);
  text-decoration: none;
  font-size: 0.92rem;
  font-family: var(--font-sans);
  transition: color 0.15s;
}
.toc-list a:hover {
  color: var(--text);
  text-decoration: underline;
}

h2 {
  font-family: var(--font-sans);
  font-size: 1.45rem;
  font-weight: 700;
  color: var(--text);
  margin: 48px 0 16px;
  letter-spacing: -0.02em;
  padding-bottom: 8px;
  border-bottom: 2px solid var(--accent-bg);
}
h3 {
  font-family: var(--font-sans);
  font-size: 1.15rem;
  font-weight: 600;
  color: var(--accent);
  margin: 32px 0 12px;
}
h4 {
  font-family: var(--font-sans);
  font-size: 1rem;
  font-weight: 600;
  color: var(--text);
  margin: 24px 0 8px;
}
p { margin-bottom: 16px; }
ul, ol { margin: 0 0 16px 24px; }
li { margin-bottom: 6px; }

/* ===== EQUATIONS ===== */
.equation {
  display: flex;
  align-items: center;
  justify-content: center;
  margin: 20px 0;
  padding: 16px 20px;
  background: var(--eq-highlight);
  border-radius: var(--radius);
  position: relative;
  overflow-x: auto;
}
.equation .eq-number {
  position: absolute;
  right: 16px;
  font-family: var(--font-sans);
  font-size: 0.8rem;
  color: var(--text-muted);
}
.equation.key-result {
  background: var(--accent-bg);
  border-left: 4px solid var(--accent-light);
}

.inline-math { /* KaTeX handles styling */ }

/* ===== DERIVATION BLOCKS ===== */
.derivation {
  margin: 20px 0;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
}
.derivation-header {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 12px 16px;
  background: var(--derivation-bg);
  cursor: pointer;
  font-family: var(--font-sans);
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--accent);
  user-select: none;
  transition: background 0.15s;
}
.derivation-header:hover { background: #efe9df; }
.derivation-header .arrow {
  transition: transform var(--transition);
  font-size: 0.7rem;
  color: var(--text-muted);
}
.derivation.open .derivation-header .arrow { transform: rotate(90deg); }
.derivation-body {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.4s ease;
}
.derivation.open .derivation-body { max-height: 5000px; }
.derivation-body-inner {
  padding: 16px 20px;
  border-top: 1px solid var(--border);
  background: #fff;
  font-size: 0.95rem;
}
.derivation-body-inner .step {
  padding: 8px 0 8px 20px;
  border-left: 2px solid var(--accent-bg);
  margin-bottom: 12px;
}
.derivation-body-inner .step-label {
  font-family: var(--font-sans);
  font-size: 0.78rem;
  font-weight: 700;
  color: var(--accent-light);
  text-transform: uppercase;
  letter-spacing: 0.04em;
  margin-bottom: 4px;
}

/* ===== TOOLTIPS ===== */
.tooltip-trigger {
  border-bottom: 1px dashed var(--accent-light);
  color: var(--accent);
  cursor: help;
  position: relative;
}
.tooltip-content {
  display: none;
  position: absolute;
  bottom: calc(100% + 8px);
  left: 50%;
  transform: translateX(-50%);
  background: var(--tooltip-bg);
  color: #ecf0f1;
  padding: 10px 14px;
  border-radius: var(--radius);
  font-family: var(--font-sans);
  font-size: 0.8rem;
  line-height: 1.5;
  width: max-content;
  max-width: 320px;
  z-index: 200;
  box-shadow: 0 4px 16px rgba(0,0,0,0.2);
  pointer-events: none;
}
.tooltip-content::after {
  content: '';
  position: absolute;
  top: 100%;
  left: 50%;
  transform: translateX(-50%);
  border: 6px solid transparent;
  border-top-color: var(--tooltip-bg);
}
.tooltip-trigger:hover .tooltip-content { display: block; }

/* ===== INFO BOXES ===== */
.info-box {
  margin: 20px 0;
  padding: 16px 20px;
  border-radius: var(--radius);
  font-size: 0.92rem;
}
.info-box.concept {
  background: #e8f8f5;
  border-left: 4px solid #1abc9c;
}
.info-box.warning {
  background: #fef9e7;
  border-left: 4px solid var(--warning);
}
.info-box.example {
  background: #f4ecf7;
  border-left: 4px solid #8e44ad;
}
.info-box.physical {
  background: #eaf2f8;
  border-left: 4px solid var(--accent-light);
}
.info-box .box-title {
  font-family: var(--font-sans);
  font-weight: 700;
  font-size: 0.85rem;
  margin-bottom: 6px;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}
.info-box.concept .box-title { color: #1abc9c; }
.info-box.warning .box-title { color: var(--warning); }
.info-box.example .box-title { color: #8e44ad; }
.info-box.physical .box-title { color: var(--accent-light); }
.info-box.explore {
  background: #f0f4f8;
  border-left: 4px solid #5b7ea8;
}
.info-box.explore .box-title { color: #5b7ea8; }
.info-box.explore ul { margin: 6px 0 0; padding-left: 18px; }
.info-box.explore li { margin-bottom: 4px; }
.info-box.explore a { color: #2c5282; text-decoration: none; border-bottom: 1px solid rgba(44,82,130,0.3); }
.info-box.explore a:hover { border-bottom-color: #2c5282; }
.info-box.explore .link-desc { color: #666; font-size: 0.85em; }

/* ===== INTERACTIVE VISUALIZATIONS ===== */
.visualization {
  margin: 24px 0;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  background: #fff;
}
.vis-header {
  padding: 10px 16px;
  background: var(--bg-alt);
  font-family: var(--font-sans);
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--text);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  gap: 8px;
}
.vis-header .vis-icon { font-size: 1rem; }
.vis-canvas-wrap {
  padding: 16px;
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 200px;
}
.vis-canvas-wrap canvas {
  display: block;
  max-width: 100%;
}
.vis-controls {
  padding: 10px 16px;
  background: var(--bg-alt);
  border-top: 1px solid var(--border);
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
  font-family: var(--font-sans);
  font-size: 0.8rem;
}
.vis-controls label {
  display: flex;
  align-items: center;
  gap: 6px;
  color: var(--text-muted);
}
.vis-controls input[type="range"] {
  width: 120px;
  accent-color: var(--accent-light);
}
.vis-controls button {
  padding: 5px 14px;
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 4px;
  cursor: pointer;
  font-family: var(--font-sans);
  font-size: 0.78rem;
  font-weight: 600;
  transition: background 0.15s;
}
.vis-controls button:not(.control-btn):hover { background: var(--accent-light); }
.vis-controls .button-group {
  display: flex;
  gap: 6px;
  flex-wrap: wrap;
}
.vis-controls .button-group .control-btn {
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text-muted);
  font-weight: 500;
}
.vis-controls .button-group .control-btn:hover {
  background: rgba(41, 128, 185, 0.15);
  border-color: var(--accent-light);
  color: var(--accent);
}
.vis-controls .button-group .control-btn.active {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
  font-weight: 600;
}
.bb-obj-btn { background: var(--bg-alt, #1e293b) !important; color: var(--text-dim, #94a3b8) !important; border: 1px solid var(--border, #334155) !important; }
.bb-obj-btn:hover { background: var(--bg, #0f172a) !important; color: var(--text, #e2e8f0) !important; }
.bb-obj-btn.active { background: var(--accent) !important; color: #fff !important; border-color: var(--accent) !important; }
.vis-controls select {
  padding: 4px 8px;
  border: 1px solid var(--border);
  border-radius: 4px;
  font-family: var(--font-sans);
  font-size: 0.78rem;
  background: var(--bg);
  color: var(--text);
}
.vis-controls .value-display {
  font-weight: 600;
  color: var(--accent);
  min-width: 36px;
  text-align: center;
}

/* ===== INTERACTIVE CONTAINERS (ch1-3 style) ===== */
.interactive-container {
  margin: 24px 0;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  background: #fff;
  padding: 16px;
}
.interactive-container h3 {
  font-family: var(--font-sans);
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--accent);
  margin: 0 0 8px;
}
.interactive-container canvas {
  display: block;
  width: 100%;
  max-width: 600px;
  margin: 12px auto;
  border-radius: var(--radius);
}
.interactive-container .vis-controls {
  border-top: none;
  padding: 8px 0;
  background: transparent;
}
.vis-caption {
  font-family: var(--font-sans);
  font-size: 0.82rem;
  color: var(--text-muted);
  line-height: 1.5;
  margin: 4px 0 8px;
  padding: 0 16px;
}
.figure-label + .vis-caption {
  margin-top: 4px;
  margin-bottom: 12px;
}
figure .vis-caption {
  text-align: center;
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
}

/* Interactive wrappers (ch5+ style) */
.interactive-wrapper {
  margin: 24px 0;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  background: #fff;
  padding: 16px;
}
.interactive-wrapper h4 {
  font-family: var(--font-sans);
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--accent);
  margin: 0 0 10px;
}
.interactive-wrapper canvas {
  display: block;
  width: 100%;
  max-width: 700px;
  margin: 0 auto 12px;
  border-radius: var(--radius);
}
.interactive-wrapper .controls {
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
  font-family: var(--font-sans);
  font-size: 0.8rem;
  padding: 8px 0 0;
  border-top: 1px solid var(--border);
}
.interactive-wrapper .controls label {
  display: flex;
  align-items: center;
  gap: 6px;
  color: var(--text-muted);
}
.interactive-wrapper .controls input[type="range"] {
  width: 120px;
  accent-color: var(--accent-light);
}
.interactive-wrapper .controls button {
  padding: 5px 14px;
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 4px;
  cursor: pointer;
  font-family: var(--font-sans);
  font-size: 0.78rem;
  font-weight: 600;
  transition: background 0.15s;
}
.interactive-wrapper .controls button:not(.control-btn):hover { background: var(--accent-light); }
.interactive-wrapper .controls .button-group {
  display: flex;
  gap: 6px;
  flex-wrap: wrap;
}
.interactive-wrapper .controls .button-group .control-btn {
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text-muted);
  font-weight: 500;
}
.interactive-wrapper .controls .button-group .control-btn:hover {
  background: rgba(41, 128, 185, 0.25);
  border-color: var(--accent-light);
  color: #fff;
}
.interactive-wrapper .controls .button-group .control-btn.active {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
  font-weight: 600;
}
.interactive-wrapper .controls select {
  padding: 4px 8px;
  border: 1px solid var(--border);
  border-radius: 4px;
  font-family: var(--font-sans);
  font-size: 0.78rem;
}
.interactive-wrapper .controls .value-display {
  font-weight: 600;
  color: var(--accent);
  min-width: 36px;
  text-align: center;
}

/* Slider containers (ch7+ style) */
.slider-container {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 8px 0;
  font-family: var(--font-sans);
  font-size: 0.82rem;
  color: var(--text-muted);
}
.slider-container input[type="range"] {
  width: 140px;
  accent-color: var(--accent-light);
}
.slider-value {
  font-weight: 600;
  color: var(--accent);
  min-width: 40px;
  display: inline-block;
  text-align: center;
  font-family: var(--font-mono);
  font-size: 0.8rem;
}

/* ===== SUMMARY BOXES ===== */
.summary {
  margin: 32px 0;
  padding: 20px 24px;
  background: linear-gradient(135deg, #eaf2f8 0%, #e8f8f5 100%);
  border-radius: var(--radius);
  border: 1px solid #d4e6f1;
}
.summary h3 {
  font-family: var(--font-sans);
  font-size: 1rem;
  color: var(--accent);
  margin: 0 0 12px;
}
.summary ul { margin-left: 16px; }
.summary li { margin-bottom: 8px; font-size: 0.95rem; }

/* ===== MOBILE HAMBURGER ===== */
.hamburger {
  display: none;
  position: fixed;
  top: 12px;
  left: 12px;
  z-index: 200;
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 6px;
  width: 40px;
  height: 40px;
  cursor: pointer;
  font-size: 1.2rem;
}
.overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.4);
  z-index: 90;
}

/* ===== PROGRESS BAR ===== */
.progress-bar {
  position: fixed;
  top: 0;
  left: var(--sidebar-w);
  right: 0;
  height: 3px;
  background: var(--bg-alt);
  z-index: 50;
  transition: left var(--transition);
}
.progress-bar .fill {
  height: 100%;
  background: var(--accent-light);
  width: 0%;
  transition: width 0.3s;
}

/* ===== LANDING PAGE ===== */
.landing {
  display: none;
  text-align: center;
  padding: 80px 20px;
}
.landing.active {
  display: block;
}
.landing h1 {
  font-family: var(--font-sans);
  font-size: 2.8rem;
  font-weight: 800;
  letter-spacing: -0.03em;
  margin-bottom: 12px;
}
.landing .author {
  font-size: 1.1rem;
  color: var(--text-muted);
  margin-bottom: 40px;
}

/* ===== RESPONSIVE ===== */
@media (max-width: 900px) {
  .sidebar { transform: translateX(-100%); }
  .sidebar.open { transform: translateX(0); }
  .main-content { margin-left: 0; padding: 48px 24px 120px; }
  .hamburger { display: block; }
  .overlay.show { display: block; }
  .progress-bar { left: 0; }
}

/* ===== SCROLLBAR ===== */
.sidebar::-webkit-scrollbar { width: 6px; }
.sidebar::-webkit-scrollbar-track { background: transparent; }
.sidebar::-webkit-scrollbar-thumb { background: rgba(255,255,255,0.15); border-radius: 3px; }

/* ===== KATEX OVERRIDES ===== */
.katex-display { margin: 0; }
.katex { font-size: 1.05em; }

/* ===== ANIMATION CANVAS ===== */
canvas.sim { border-radius: 4px; background: #1b2631; }

/* ===== TABLE ===== */
table {
  width: 100%;
  border-collapse: collapse;
  margin: 16px 0;
  font-size: 0.92rem;
}
th, td {
  padding: 8px 12px;
  border: 1px solid var(--border);
  text-align: left;
}
th {
  background: var(--bg-alt);
  font-family: var(--font-sans);
  font-weight: 600;
  font-size: 0.85rem;
}

/* ===== FIGURE LABEL (shared by Interactives & Figures) ===== */
.figure-label {
  padding: 10px 16px;
  background: var(--bg-alt);
  font-family: var(--font-sans);
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--text);
  border-bottom: 1px solid var(--border);
  display: flex;
  cursor: pointer;
  align-items: center;
  gap: 8px;
}
.interactive-container .figure-label,
.interactive-wrapper .figure-label {
  margin: -16px -16px 8px;
}
figure .figure-label {
  margin: 0 0 0;
  border-bottom: none;
}

/* ===== FIGURES ===== */
figure {
  margin: 24px 0;
  text-align: center;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  background: #fff;
}
figure img {
  max-width: 100%;
  height: auto;
}
figure svg {
  max-width: 100%;
  height: auto;
}
figcaption {
  margin-top: 0;
  padding: 4px 16px 8px;
  font-family: var(--font-sans);
  font-size: 0.82rem;
  color: var(--text-muted);
  line-height: 1.4;
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
}

/* ===== FOOTNOTES ===== */
.footnote {
  font-size: 0.85rem;
  color: var(--text-muted);
  border-top: 1px solid var(--border);
  margin-top: 12px;
  padding-top: 8px;
}

/* ===== BLOCKQUOTE ===== */
blockquote {
  margin: 20px 0;
  padding: 12px 20px;
  border-left: 3px solid var(--accent-light);
  background: var(--accent-bg);
  font-style: italic;
  color: var(--text);
}
blockquote cite {
  display: block;
  margin-top: 8px;
  font-style: normal;
  font-size: 0.85rem;
  color: var(--text-muted);
}

/* ===== PROBLEMS SECTIONS ===== */
.problems-section {
  margin: 16px 0;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
}
.problems-section summary {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 14px 20px;
  background: var(--bg-alt);
  cursor: pointer;
  font-family: var(--font-sans);
  font-size: 1rem;
  font-weight: 700;
  color: var(--accent);
  user-select: none;
  transition: background 0.15s;
  list-style: none;
}
.problems-section summary::-webkit-details-marker { display: none; }
.problems-section summary::before {
  content: '\25B6';
  font-size: 0.7rem;
  color: var(--text-muted);
  transition: transform var(--transition);
}
.problems-section[open] summary::before { transform: rotate(90deg); }
.problems-section summary:hover { background: #e8e6e3; }
.problems-section .problems-body {
  padding: 20px 24px;
  border-top: 1px solid var(--border);
  background: #fff;
}

.problem {
  margin-bottom: 28px;
  padding-bottom: 24px;
  border-bottom: 1px solid #eee;
}
.problem:last-child { border-bottom: none; margin-bottom: 0; padding-bottom: 0; }
.problem-label {
  font-family: var(--font-sans);
  font-weight: 700;
  font-size: 0.95rem;
  color: var(--accent);
  margin-bottom: 8px;
}
.problem p { margin-bottom: 10px; }

/* Multiple choice options */
.mc-options {
  display: flex;
  flex-direction: column;
  gap: 8px;
  margin: 12px 0 16px;
  padding-left: 8px;
}
.mc-options label {
  display: flex;
  align-items: flex-start;
  gap: 8px;
  cursor: pointer;
  font-size: 0.95rem;
  padding: 6px 10px;
  border-radius: 4px;
  transition: background 0.15s;
}
.mc-options label:hover { background: var(--accent-bg); }
.mc-options input[type="radio"] {
  margin-top: 4px;
  accent-color: var(--accent-light);
}

/* Short answer input */
.sa-input {
  display: flex;
  align-items: center;
  gap: 10px;
  margin: 12px 0 16px;
}
.sa-input input[type="text"] {
  padding: 8px 12px;
  border: 1px solid var(--border);
  border-radius: 4px;
  font-family: var(--font-mono);
  font-size: 0.9rem;
  width: 200px;
  transition: border-color 0.15s;
}
.sa-input input[type="text"]:focus {
  outline: none;
  border-color: var(--accent-light);
  box-shadow: 0 0 0 3px rgba(41,128,185,0.15);
}

/* Check answer button */
.check-answer-btn {
  padding: 8px 20px;
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 4px;
  cursor: pointer;
  font-family: var(--font-sans);
  font-size: 0.85rem;
  font-weight: 600;
  transition: background 0.15s;
}
.check-answer-btn:hover { background: var(--accent-light); }

/* Answer feedback */
.answer-feedback {
  margin-top: 10px;
  padding: 10px 14px;
  border-radius: 4px;
  font-family: var(--font-sans);
  font-size: 0.9rem;
  display: none;
}
.answer-feedback.correct {
  display: block;
  background: #e8f8f0;
  border-left: 4px solid var(--success);
  color: #1a7a42;
}
.answer-feedback.incorrect {
  display: block;
  background: #fdecea;
  border-left: 4px solid #e74c3c;
  color: #c0392b;
}

/* Long problem parts */
.problem-part {
  margin: 12px 0 12px 20px;
  padding-left: 16px;
  border-left: 2px solid var(--accent-bg);
}
.part-label {
  font-family: var(--font-sans);
  font-weight: 700;
  font-size: 0.85rem;
  color: var(--accent-light);
  margin-bottom: 4px;
}

/* Problem diagrams/figures */
.problem-figure {
  margin: 16px auto;
  text-align: center;
  max-width: 500px;
}
.problem-figure svg { max-width: 100%; height: auto; }
.problem-figure .caption {
  font-family: var(--font-sans);
  font-size: 0.8rem;
  color: var(--text-muted);
  margin-top: 6px;
}

/* ===== LOADING INDICATOR ===== */
.chapter-loading {
  text-align: center;
  padding: 60px;
  color: var(--text-muted);
  font-family: var(--font-sans);
}

/* ===== PRINT STYLES ===== */
@media print {
  .sidebar, .hamburger, .overlay, .progress-bar, .vis-controls { display: none !important; }
  .main-content { margin-left: 0; max-width: 100%; }
  .chapter { display: block !important; page-break-before: always; }
  .derivation-body { max-height: none !important; }
}