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

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  background-color: #f5f5f7;
  color: #1d1d1f;
  min-height: 100vh;
}

header {
  text-align: center;
  padding: 32px 24px 16px;
}

header h1 {
  font-size: 1.75rem;
  font-weight: 700;
  letter-spacing: -0.02em;
}

header p {
  color: #6e6e73;
  margin-top: 4px;
  font-size: 0.95rem;
}

.hero-equation {
  margin-top: 28px;
  font-size: 2.8rem;
  overflow-x: auto;
}

.hero-equation--compact {
  font-size: 1.6rem;
}

.app {
  display: flex;
  flex-direction: column;
  gap: 32px;
  max-width: 1200px;
  margin: 0 auto;
  padding: 16px 32px 48px;
}

/* ---------- Top row: canvas + controls ---------- */

.top-row {
  display: flex;
  gap: 32px;
  align-items: flex-start;
}

.canvas-col {
  flex: 1 1 0;
  display: flex;
  align-items: center;
  justify-content: center;
  min-width: 0;
}

.canvas-wrap {
  position: relative;
}

#plot {
  width: 100%;
  aspect-ratio: 1;
  max-width: 600px;
  background: #fff;
  border: 1px solid #d2d2d7;
  border-radius: 12px;
  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.06);
}

.tooltip {
  position: absolute;
  pointer-events: none;
  background: rgba(29, 29, 31, 0.92);
  color: #fff;
  padding: 6px 10px;
  border-radius: 6px;
  font-size: 0.78rem;
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
  line-height: 1.4;
  white-space: nowrap;
  transform: translate(-50%, calc(-100% - 10px));
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
}

canvas {
  background: #fff;
  border: 1px solid #d2d2d7;
  border-radius: 12px;
  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.06);
  display: block;
  max-width: 100%;
}

.controls-card {
  flex: 0 0 360px;
  align-self: stretch;
}

.card {
  background: #fff;
  border: 1px solid #d2d2d7;
  border-radius: 12px;
  padding: 24px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.04);
}

.card h2 {
  font-size: 1rem;
  font-weight: 600;
  margin-bottom: 16px;
  color: #1d1d1f;
}

/* Sliders */

.slider-group {
  margin-bottom: 18px;
}

.slider-group:last-child {
  margin-bottom: 0;
}

.slider-group label {
  display: flex;
  justify-content: space-between;
  font-size: 0.9rem;
  font-weight: 500;
  margin-bottom: 6px;
}

input[type="range"] {
  -webkit-appearance: none;
  appearance: none;
  width: 100%;
  height: 6px;
  border-radius: 3px;
  background: #e5e5ea;
  outline: none;
}

input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none;
  appearance: none;
  width: 18px;
  height: 18px;
  border-radius: 50%;
  background: #2563eb;
  cursor: pointer;
  border: 2px solid #fff;
  box-shadow: 0 1px 4px rgba(0, 0, 0, 0.2);
}

.toggle-group {
  margin-top: 18px;
}

.trace-btn {
  margin-top: 18px;
  width: 100%;
  padding: 10px 14px;
  font-size: 0.95rem;
  font-weight: 600;
  color: #fff;
  background: #2563eb;
  border: none;
  border-radius: 8px;
  cursor: pointer;
}

.trace-btn:hover {
  background: #1d4ed8;
}

.toggle-group label {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 0.9rem;
  font-weight: 500;
  cursor: pointer;
}

.toggle-group input[type="checkbox"] {
  width: 16px;
  height: 16px;
  accent-color: #2563eb;
  cursor: pointer;
}

/* Geometry derivation (<details>) */

.geometry-details {
  background: #fff;
  border: 1px solid #d2d2d7;
  border-radius: 12px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.04);
  overflow: hidden;
}

.geometry-details > summary {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 16px 24px;
  font-size: 1rem;
  font-weight: 600;
  color: #1d1d1f;
  cursor: pointer;
  user-select: none;
  list-style: none;
}

.geometry-details > summary::-webkit-details-marker {
  display: none;
}

.geometry-details > summary::after {
  content: "▾";
  font-size: 3rem;
  line-height: 1;
  color: #6e6e73;
  transition: transform 0.15s ease;
}

.geometry-details[open] > summary::after {
  transform: rotate(180deg);
}

.geometry-details > summary:hover {
  background: #f5f5f7;
}

.geometry-panel {
  border-top: 1px solid #e5e5ea;
  padding: 24px 32px;
}

/* Explanation panel (zero-md) */

.explanation-panel {
  background: #fff;
  border: 1px solid #d2d2d7;
  border-radius: 12px;
  padding: 24px 32px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.04);
}

.credit {
  text-align: center;
  margin-top: -8px;
}

.credit span {
  display: inline-block;
  background: #fff;
  border: 1px solid #d2d2d7;
  padding: 8px 18px;
  color: #6e6e73;
  font-size: 1rem;
}

.credit a {
  color: #2563eb;
  text-decoration: none;
  font-weight: 500;
}

.credit a:hover {
  text-decoration: underline;
}

/* ---------- Responsive ---------- */

@media (max-width: 800px) {
  header {
    padding: 24px 16px 8px;
  }

  header h1 {
    font-size: 1.4rem;
  }

  header p {
    font-size: 0.88rem;
  }

  .hero-equation {
    margin-top: 18px;
    font-size: 1.3rem;
  }

  .top-row {
    flex-direction: column;
    gap: 20px;
    align-items: stretch;
  }

  .canvas-col {
    width: 100%;
  }

  .controls-card {
    flex: none;
    width: 100%;
    align-self: auto;
  }

  .app {
    padding: 16px;
    gap: 20px;
  }

  .explanation-panel {
    padding: 20px;
  }

  .geometry-panel {
    padding: 16px;
  }
}