/* Hand-crafted modern courseware — §11-1 */
:root {
  --bg-0: #0a0e14;
  --bg-1: #121821;
  --bg-2: #1a212d;
  --bg-card: rgba(255,255,255,0.035);
  --bg-card-hover: rgba(255,255,255,0.06);
  --border-weak: rgba(255,255,255,0.07);
  --border: rgba(255,255,255,0.12);
  --border-strong: rgba(255,255,255,0.2);
  --accent: #ffb74d;
  --accent-2: #64b5f6;
  --accent-3: #81c784;
  --accent-4: #e57373;
  --text: #e8eef6;
  --text-dim: #a0aec0;
  --text-mute: #6b7a8f;
  --font-sans: "Inter", "Source Han Sans SC", "Microsoft YaHei", "PingFang SC", sans-serif;
  --font-mono: "JetBrains Mono", "Fira Code", "Consolas", monospace;
  --font-serif: "Source Han Serif SC", "Songti SC", "SimSun", serif;
}

* { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; scroll-padding-top: 72px; }
body {
  font-family: var(--font-sans);
  background: var(--bg-0);
  color: var(--text);
  line-height: 1.7;
  -webkit-font-smoothing: antialiased;
  background:
    radial-gradient(ellipse 1200px 600px at 0% 0%, rgba(100,181,246,0.08), transparent 60%),
    radial-gradient(ellipse 800px 400px at 100% 0%, rgba(255,183,77,0.06), transparent 60%),
    var(--bg-0);
  background-attachment: fixed;
}

/* Top nav */
.topnav {
  position: sticky; top: 0; z-index: 100;
  background: rgba(10,14,20,0.85);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border-weak);
  display: flex; align-items: center;
  padding: 12px 28px; gap: 20px;
}
.topnav .logo {
  font-weight: 700; font-size: 1em; color: var(--text);
  text-decoration: none; display: flex; align-items: center; gap: 10px;
}
.topnav .logo::before {
  content: ''; width: 18px; height: 18px;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  border-radius: 4px;
}
.topnav .crumbs { color: var(--text-dim); font-size: 0.9em; }
.topnav .crumbs a { color: var(--accent-2); text-decoration: none; }
.topnav .progress {
  margin-left: auto; font-size: 0.82em; color: var(--text-mute);
  font-family: var(--font-mono);
}

/* Language switch (中 / EN) */
.lang-switch {
  display: inline-flex; align-items: center; gap: 2px;
  font-size: 0.82em; color: var(--text-mute);
  font-family: var(--font-mono);
  padding: 4px 10px; border-radius: 6px;
  border: 1px solid var(--border-weak);
  background: rgba(255,255,255,0.03);
  text-decoration: none;
  transition: all 160ms;
  margin-left: 16px;
  white-space: nowrap;
}
.lang-switch:hover {
  color: var(--text);
  border-color: var(--border);
  background: var(--bg-card-hover);
}
.lang-switch .lang-active {
  color: var(--accent); font-weight: 600;
}
.lang-switch .lang-sep {
  color: var(--border); margin: 0 4px;
}
.topnav .progress-bar {
  display: inline-block; width: 120px; height: 5px;
  background: rgba(255,255,255,0.08); border-radius: 3px; vertical-align: middle;
  margin-left: 8px; overflow: hidden;
}
.topnav .progress-fill {
  height: 100%; background: linear-gradient(90deg, var(--accent), var(--accent-2));
  width: 0; transition: width 400ms ease;
}

/* Layout */
.layout {
  display: grid; grid-template-columns: 240px 1fr;
  max-width: 1280px; margin: 0 auto; position: relative;
}
.section-nav {
  position: sticky; top: 56px;
  height: calc(100vh - 56px);
  padding: 28px 16px;
  overflow-y: auto;
  border-right: 1px solid var(--border-weak);
}
.section-nav h3 {
  font-size: 0.7em; color: var(--text-mute);
  letter-spacing: 0.12em; text-transform: uppercase;
  margin-bottom: 10px; font-weight: 600; padding-left: 12px;
}
.section-nav a {
  display: block; padding: 6px 12px;
  color: var(--text-dim); text-decoration: none;
  border-left: 2px solid transparent;
  font-size: 0.88em; line-height: 1.4;
  transition: all 180ms;
}
.section-nav a:hover { color: var(--text); background: rgba(255,255,255,0.03); }
.section-nav a.active {
  color: var(--accent); border-left-color: var(--accent);
  background: linear-gradient(90deg, rgba(255,183,77,0.08), transparent);
}
.section-nav a .idnum {
  font-family: var(--font-mono);
  font-size: 0.85em; color: var(--text-mute);
  margin-right: 8px;
}
.section-nav a.active .idnum { color: var(--accent); opacity: 0.8; }

/* Content column */
.content {
  padding: 44px 52px 120px;
  max-width: 900px; width: 100%;
  min-height: calc(100vh - 56px);
}

/* Hero */
.hero {
  margin-bottom: 48px; padding-bottom: 28px;
  border-bottom: 1px solid var(--border-weak);
}
.hero .kicker {
  color: var(--accent); font-size: 0.78em;
  letter-spacing: 0.18em; text-transform: uppercase;
  font-weight: 600; font-family: var(--font-mono);
  margin-bottom: 12px;
}
.hero h1 {
  font-size: 2.6em; font-weight: 700;
  letter-spacing: -0.02em; line-height: 1.15;
  margin-bottom: 16px;
  background: linear-gradient(135deg, #fff 0%, #a0aec0 100%);
  -webkit-background-clip: text; background-clip: text;
  color: transparent;
}
.hero .lede {
  font-size: 1.12em; color: var(--text-dim);
  line-height: 1.65; max-width: 740px;
}
.hero .lede strong { color: var(--accent); font-weight: 600; }

/* Sections */
.subsect {
  margin: 64px 0;
  scroll-margin-top: 72px;
  min-height: 60vh;  /* uniform visual weight */
}
.subsect-header {
  display: flex; align-items: baseline; gap: 14px;
  margin-bottom: 22px;
  padding-bottom: 14px; border-bottom: 1px solid var(--border-weak);
}
.subsect-num {
  font-family: var(--font-mono); font-size: 0.92em;
  color: var(--accent); font-weight: 600;
  padding: 3px 10px; border-radius: 4px;
  background: rgba(255,183,77,0.1);
  border: 1px solid rgba(255,183,77,0.25);
}
.subsect h2 {
  font-size: 1.65em; font-weight: 600; color: var(--text);
  letter-spacing: -0.01em;
}
.subsect-tag {
  font-size: 0.78em; color: var(--text-mute);
  padding: 3px 10px; background: rgba(255,255,255,0.04);
  border-radius: 12px; margin-left: auto;
}

.subsect p { margin-bottom: 14px; color: var(--text); font-size: 1em; }
.subsect p:last-child { margin-bottom: 0; }
.subsect p strong { color: var(--accent); font-weight: 600; }
.subsect p em { color: var(--accent-2); font-style: normal; font-weight: 500; }
.subsect ul, .subsect ol { margin: 14px 0 14px 28px; color: var(--text); }
.subsect li { margin-bottom: 6px; }

/* ─── Progressive reveal ─── */
.reveal { opacity: 0; transform: translateY(12px); transition: opacity 240ms ease-out, transform 240ms ease-out; }
.reveal.shown { opacity: 1; transform: translateY(0); }

.sect-nav-btn {
  display: inline-flex; align-items: center; gap: 6px;
  padding: 10px 22px;
  margin: 20px 8px 0 0;
  background: linear-gradient(135deg, var(--accent), #ff9800);
  color: #1a1a1a; border: none; border-radius: 8px;
  font-size: 0.95em; font-weight: 600; font-family: inherit;
  cursor: pointer; transition: all 180ms;
  box-shadow: 0 4px 12px rgba(255,152,0,0.25);
}
.sect-nav-btn:hover { transform: translateY(-1px); box-shadow: 0 6px 18px rgba(255,152,0,0.4); }
.sect-nav-btn:disabled { opacity: 0.3; cursor: not-allowed; }
.sect-nav-btn.secondary {
  background: rgba(255,255,255,0.1); color: var(--text); border: 1px solid var(--border);
  box-shadow: none;
}
.sect-nav-btn.secondary:hover { background: rgba(255,255,255,0.15); }
/* btn-next: even more prominent (green accent) to stand out as "next section" */
.sect-nav-btn.btn-next {
  background: linear-gradient(135deg, #66bb6a, #43a047);
  color: #1a1a1a;
  box-shadow: 0 4px 14px rgba(67,160,71,0.35);
}
.sect-nav-btn.btn-next:hover { box-shadow: 0 6px 20px rgba(67,160,71,0.55); }

/* Callout */
.callout {
  background: linear-gradient(90deg, rgba(100,181,246,0.08), transparent 70%);
  border-left: 3px solid var(--accent-2);
  padding: 16px 22px; margin: 22px 0;
  border-radius: 0 8px 8px 0; color: var(--text-dim);
}
.callout.question {
  background: linear-gradient(90deg, rgba(255,183,77,0.1), transparent 70%);
  border-left-color: var(--accent); color: var(--text);
}
.callout .label {
  display: inline-block;
  font-size: 0.7em; font-family: var(--font-mono);
  letter-spacing: 0.1em; text-transform: uppercase;
  color: var(--accent-2); font-weight: 700; margin-bottom: 4px;
}
.callout.question .label { color: var(--accent); }

/* Triptych for three states */
.triptych {
  display: grid; grid-template-columns: repeat(3, 1fr);
  gap: 16px; margin: 24px 0;
}
.eq-card {
  background: var(--bg-card);
  border: 1px solid var(--border-weak);
  border-radius: 12px; padding: 18px 18px 0;
  display: flex; flex-direction: column;
  transition: all 220ms; overflow: hidden;
}
.eq-card:hover { border-color: var(--border); background: var(--bg-card-hover); transform: translateY(-2px); }
.eq-card .eq-badge {
  display: inline-flex; align-items: center; gap: 6px;
  padding: 3px 10px; border-radius: 20px;
  font-size: 0.8em; font-weight: 600;
  margin-bottom: 10px; align-self: flex-start;
}
.eq-badge.stable   { background: rgba(129,199,132,0.15); color: var(--accent-3); border: 1px solid rgba(129,199,132,0.3); }
.eq-badge.unstable { background: rgba(229,115,115,0.15); color: var(--accent-4); border: 1px solid rgba(229,115,115,0.3); }
.eq-badge.critical { background: rgba(255,183,77,0.15);  color: var(--accent);   border: 1px solid rgba(255,183,77,0.3); }
.eq-card h3 { font-size: 1.05em; font-weight: 600; margin-bottom: 5px; }
.eq-card .desc { color: var(--text-dim); font-size: 0.9em; line-height: 1.5; margin-bottom: 14px; flex: 1; }

/* SWF frame wrapper — transparent, click-to-advance */
.swf-frame {
  background: transparent;
  border-top: 1px solid var(--border-weak);
  margin: 0 -18px;
  position: relative;
  overflow: hidden;
  cursor: pointer;
}
.swf-frame.contained {
  margin: 0; border-radius: 8px;
  border: 1px solid var(--border-weak);
  background: transparent;
}
.swf-frame .swf-stage { width: 100%; height: 100%; display: block; position: relative; }
.swf-frame ruffle-player, .swf-frame ruffle-embed {
  width: 100% !important; height: 100% !important;
  display: block !important; background: transparent !important;
}
.swf-replay {
  position: absolute; top: 8px; right: 8px; z-index: 5;
  background: rgba(0,0,0,0.6); color: var(--text);
  border: 1px solid rgba(255,255,255,0.2);
  padding: 5px 12px; border-radius: 20px; cursor: pointer;
  font-family: inherit; font-size: 0.78em;
  backdrop-filter: blur(6px);
  opacity: 0.4; transition: opacity 180ms;
}
.swf-frame:hover .swf-replay { opacity: 1; }
.swf-replay:hover { background: var(--accent); color: #000; border-color: var(--accent); }

/* SWF captions globally hidden per user preference (2026-04-19) */
.swf-caption { display: none !important; }

/* Small play button at bottom-right, icon only */
.swf-play-btn {
  position: absolute; bottom: 6px; right: 6px;
  width: 28px; height: 28px;
  border-radius: 50%;
  background: rgba(255,183,77,0.85);
  color: #1a1a1a;
  border: none;
  cursor: pointer;
  z-index: 6;
  display: flex; align-items: center; justify-content: center;
  font-size: 12px;
  box-shadow: 0 2px 6px rgba(0,0,0,0.4);
  opacity: 0.7;
  transition: opacity 180ms, transform 180ms, background 180ms;
  font-family: inherit;
  line-height: 1;
  padding: 0;
}
.swf-play-btn::before { content: '▶'; margin-left: 1px; }
.swf-play-btn.done::before { content: '↻'; margin-left: 0; }
.swf-frame:hover .swf-play-btn { opacity: 1; }
.swf-play-btn:hover { background: var(--accent); transform: scale(1.1); }
.swf-play-btn:disabled { opacity: 0.3; cursor: default; }

.swf-stage-dots {
  position: absolute; top: 8px; left: 10px;
  display: flex; gap: 4px; z-index: 5;
  pointer-events: none;
  opacity: 0.6;
  transition: opacity 180ms;
}
.swf-frame:hover .swf-stage-dots { opacity: 1; }
.swf-stage-dots .dot {
  width: 5px; height: 5px;
  border-radius: 50%;
  background: rgba(255,255,255,0.25);
  transition: background 200ms;
}
.swf-stage-dots .dot.done { background: var(--accent); }
.swf-stage-dots .dot.current { background: #fff; box-shadow: 0 0 6px rgba(255,255,255,0.7); }

/* SVG force-arrow overlays on balls */
.swf-overlay {
  position: absolute; inset: 0;
  pointer-events: none; z-index: 4;
}
.swf-overlay .arrow {
  stroke: #ff5722; stroke-width: 3;
  fill: none;
  stroke-linecap: round;
  filter: drop-shadow(0 0 6px rgba(255,87,34,0.7));
}
.swf-overlay .arrow-head { fill: #ff5722; stroke: none; }
.swf-overlay .force-label {
  font-family: var(--font-math); fill: #ff5722;
  font-size: 18px; font-weight: bold; font-style: italic;
}
.swf-overlay .pulse { animation: pulseFade 2.5s infinite; }
@keyframes pulseFade {
  0%, 100% { opacity: 0.2; transform-origin: var(--tx, 50%) var(--ty, 50%); transform: translateX(-5px); }
  30% { opacity: 1; transform: translateX(0); }
  60% { opacity: 1; transform: translateX(0); }
}
.swf-overlay .trajectory {
  stroke: #4caf50; stroke-width: 2; fill: none;
  stroke-dasharray: 4 4;
  opacity: 0.8;
  animation: trajPulse 3s ease-in-out infinite;
}
@keyframes trajPulse {
  0%, 100% { opacity: 0.2; }
  50% { opacity: 0.9; }
}

/* F-Δ curve overlays (animated draw-in via stroke-dashoffset) */
.swf-overlay .curve-stable {
  stroke: #4caf50; stroke-width: 2.5; fill: none;
  stroke-dasharray: 1000; stroke-dashoffset: 1000;
  animation: drawCurve 3s ease-out 0.5s forwards;
  filter: drop-shadow(0 0 4px rgba(76,175,80,0.6));
}
.swf-overlay .curve-unstable {
  stroke: #f44336; stroke-width: 2.5; fill: none;
  stroke-dasharray: 6 4;
  stroke-dashoffset: 1000;
  animation: drawCurve 3s ease-out 1.5s forwards;
  filter: drop-shadow(0 0 4px rgba(244,67,54,0.5));
}
.swf-overlay .curve-critical-pt {
  fill: #ffb74d; stroke: #fff; stroke-width: 1.5;
  opacity: 0; animation: fadePt 0.6s ease-out 2.5s forwards;
}
.swf-overlay .curve-label {
  fill: var(--accent); font-family: var(--font-math);
  font-size: 11px; font-style: italic;
  opacity: 0; animation: fadePt 0.5s ease-out 3s forwards;
}
.swf-overlay .curve-label-2 {
  fill: #4caf50; font-family: var(--font-math);
  font-size: 10px; font-style: italic;
  opacity: 0; animation: fadePt 0.5s ease-out 3.5s forwards;
}
.swf-overlay .curve-label-3 {
  fill: #f44336; font-family: var(--font-math);
  font-size: 10px; font-style: italic;
  opacity: 0; animation: fadePt 0.5s ease-out 4s forwards;
}
@keyframes drawCurve {
  to { stroke-dashoffset: 0; }
}
@keyframes fadePt {
  to { opacity: 1; }
}

/* Replay overlays should reset animations */
.swf-overlay.reset .curve-stable,
.swf-overlay.reset .curve-unstable { stroke-dashoffset: 1000; }
.swf-overlay.reset .curve-critical-pt,
.swf-overlay.reset .curve-label,
.swf-overlay.reset .curve-label-2,
.swf-overlay.reset .curve-label-3 { opacity: 0; }

/* Old replay button is no longer used — replaced by unified .swf-play-btn */
.swf-replay { display: none; }

/* Fixed aspect ratios based on SWF original dimensions */
.eq-card .swf-frame { aspect-ratio: 213 / 176; }

/* Stage feature — formula + animation side by side */
.stage-feature {
  margin: 28px 0;
  background: linear-gradient(180deg, var(--bg-1), var(--bg-2));
  border: 1px solid var(--border-weak);
  border-radius: 14px; padding: 22px;
  display: grid; grid-template-columns: 1.1fr 1fr;
  gap: 24px; align-items: center;
}
.stage-feature .swf-frame {
  aspect-ratio: 4 / 3; margin: 0; border-radius: 10px;
  border: 1px solid var(--border-weak);
}
.stage-feature h3 { font-size: 1.2em; margin-bottom: 8px; color: var(--text); }
.stage-feature .tag {
  display: inline-block;
  font-size: 0.72em; font-family: var(--font-mono);
  padding: 3px 10px; background: rgba(100,181,246,0.12);
  border: 1px solid rgba(100,181,246,0.3);
  color: var(--accent-2); border-radius: 4px; margin-bottom: 8px;
}

/* Compare grid */
.compare-grid {
  display: grid; grid-template-columns: 1fr 1fr;
  gap: 20px; margin: 24px 0;
}
.compare-card {
  background: var(--bg-card);
  border: 1px solid var(--border-weak);
  border-radius: 12px; padding: 20px;
}
.compare-card h3 {
  font-size: 1.12em; font-weight: 600; margin-bottom: 6px;
  display: flex; align-items: center; gap: 10px;
}
.compare-card h3::before {
  content: ''; width: 7px; height: 7px; border-radius: 50%;
}
.compare-card.type1 h3::before { background: var(--accent-2); box-shadow: 0 0 8px var(--accent-2); }
.compare-card.type2 h3::before { background: var(--accent-4); box-shadow: 0 0 8px var(--accent-4); }
.compare-card h3 .subtitle {
  font-size: 0.7em; font-weight: 400; color: var(--text-mute);
  margin-left: auto; font-family: var(--font-mono);
}
.compare-card .swf-frame {
  aspect-ratio: 4/3; margin: 12px 0; border-radius: 8px;
  border: 1px solid var(--border-weak);
}
.compare-card ul {
  margin: 12px 0 0; padding-left: 20px;
  color: var(--text-dim); font-size: 0.92em;
}
.compare-card li { margin-bottom: 4px; }
.compare-card li strong { color: var(--text); }

/* Examples grid */
.examples-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 18px; margin: 22px 0;
}
.ex-card {
  background: var(--bg-card);
  border: 1px solid var(--border-weak);
  border-radius: 10px; padding: 14px 16px 0;
  display: flex; flex-direction: column;
  transition: all 220ms; overflow: hidden;
}
.ex-card:hover {
  border-color: var(--border); transform: translateY(-2px);
  box-shadow: 0 6px 18px rgba(0,0,0,0.3);
}
.ex-card .caption {
  font-weight: 500; margin-bottom: 3px; color: var(--text); font-size: 0.98em;
}
.ex-card .desc {
  font-size: 0.84em; color: var(--text-mute);
  margin-bottom: 12px; line-height: 1.5; flex: 1;
}
.ex-card .swf-frame { aspect-ratio: 4/3; margin: 0 -16px; }

/* Formula (KaTeX display) — CSS Grid: eq centered, number flush right */
.formula-block {
  margin: 18px 0;
  padding: 18px 22px;
  background: var(--bg-card);
  border: 1px solid var(--border-weak);
  border-left: 3px solid var(--accent);
  border-radius: 8px;
  display: grid;
  grid-template-columns: 1fr auto 1fr;
  align-items: center;
  gap: 8px;
  overflow-x: auto;
}
.formula-block .eq {
  grid-column: 2;
  text-align: center;
  min-width: 0;
}
.formula-block .eq-num {
  grid-column: 3;
  justify-self: end;
  padding-right: 4px;
  color: var(--text-mute);
  font-family: var(--font-mono);
  font-size: 0.92em;
  white-space: nowrap;
  font-feature-settings: "tnum" 1;
}
.formula-block .caption {
  grid-column: 1 / -1;
  display: block; margin-top: 10px;
  text-align: center;
  font-size: 0.82em; color: var(--text-mute);
  letter-spacing: 0.05em;
}
/* SWF label — top-right corner of swf-frame, avoids .swf-replay */
.swf-label {
  position: absolute; top: 8px; right: 8px; z-index: 5;
  font-family: var(--font-mono); font-size: 0.72em;
  color: var(--text-dim);
  background: rgba(0,0,0,0.55);
  border: 1px solid rgba(255,255,255,0.12);
  padding: 3px 9px; border-radius: 12px;
  backdrop-filter: blur(6px);
  pointer-events: none;
  opacity: 0.72;
  letter-spacing: 0.04em;
  font-feature-settings: "tnum" 1;
  transition: opacity 180ms;
}
.swf-frame:hover .swf-label { opacity: 1; }

/* Fix KaTeX color for dark theme */
.katex { color: var(--text); }
.katex .frac-line { border-color: var(--text); }

/* Inline markup */
.tag-inline {
  display: inline-block; padding: 1px 7px; margin: 0 2px;
  background: rgba(255,255,255,0.06); border-radius: 4px;
  font-family: var(--font-mono); font-size: 0.88em;
  color: var(--accent-2);
}

/* Table */
table.data {
  width: 100%; border-collapse: collapse; margin: 20px 0;
  background: var(--bg-card); border-radius: 10px; overflow: hidden;
  font-size: 0.94em;
}
table.data th, table.data td {
  padding: 11px 14px; text-align: left;
  border-bottom: 1px solid var(--border-weak);
}
table.data thead th {
  background: rgba(255,255,255,0.04);
  color: var(--accent); font-weight: 600;
  font-size: 0.82em; letter-spacing: 0.05em;
}
table.data tbody tr:last-child td { border-bottom: none; }
table.data tbody tr:hover { background: rgba(255,255,255,0.02); }
table.data td strong { color: var(--text); }

/* Footer */
.page-footer {
  padding: 30px 60px; text-align: center;
  color: var(--text-mute); font-size: 0.88em;
  border-top: 1px solid var(--border-weak); background: var(--bg-1);
}
.page-footer a { color: var(--accent-2); text-decoration: none; }

/* Responsive */
@media (max-width: 900px) {
  .layout { grid-template-columns: 1fr; }
  .section-nav { display: none; }
  .content { padding: 28px 20px 100px; }
  .triptych, .compare-grid, .examples-grid { grid-template-columns: 1fr; }
  .stage-feature { grid-template-columns: 1fr; }
  .hero h1 { font-size: 2em; }
  .topnav { padding: 10px 16px; }
  .topnav .progress { display: none; }
}