:root {
  --paper: #f7f8fa;
  --surface: #ffffff;
  --ink: #17202a;
  --muted: #66717e;
  --faint: #98a2ad;
  --line: #dce1e6;
  --line-strong: #bcc5cf;
  --blue: #2563eb;
  --blue-soft: #eaf0ff;
  --green: #13795b;
  --green-soft: #e6f5ef;
  --red: #c2413b;
  --red-soft: #fff0ee;
  --yellow: #f2c94c;
  --yellow-soft: #fff9df;
  --font: 'Noto Sans SC', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
}

*, *::before, *::after { box-sizing: border-box; }

html { font-size: 15px; background: var(--paper); }

body {
  margin: 0;
  min-width: 320px;
  background:
    linear-gradient(rgba(23, 32, 42, .024) 1px, transparent 1px),
    linear-gradient(90deg, rgba(23, 32, 42, .024) 1px, transparent 1px),
    var(--paper);
  background-size: 24px 24px;
  color: var(--ink);
  font-family: var(--font);
  line-height: 1.65;
  letter-spacing: 0;
}

button, input { font: inherit; letter-spacing: 0; }
button { -webkit-tap-highlight-color: transparent; }

.lab-shell {
  width: min(1440px, 100%);
  margin: 0 auto;
  padding: 28px 32px 56px;
}

.lab-header {
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  gap: 32px;
  padding: 4px 0 22px;
  border-bottom: 1px solid var(--line-strong);
}

.lab-kicker {
  margin: 0 0 6px;
  color: var(--green);
  font-size: 12px;
  font-weight: 700;
}

.lab-header h1 {
  margin: 0;
  font-size: 28px;
  line-height: 1.25;
  font-weight: 700;
}

.lab-header-note {
  margin: 0 0 3px;
  color: var(--muted);
  font-size: 13px;
  text-align: right;
}

.question-switcher {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  border-bottom: 1px solid var(--line-strong);
}

.question-tab {
  display: grid;
  grid-template-columns: 36px minmax(0, 1fr);
  gap: 12px;
  min-height: 78px;
  padding: 15px 18px;
  border: 0;
  border-right: 1px solid var(--line);
  border-bottom: 3px solid transparent;
  background: rgba(255, 255, 255, .55);
  color: var(--muted);
  text-align: left;
  cursor: pointer;
}

.question-tab:last-child { border-right: 0; }
.question-tab:hover { background: var(--surface); color: var(--ink); }
.question-tab:focus-visible, .phase-step:focus-visible, .choice-button:focus-visible,
.primary-action:focus-visible, .secondary-action:focus-visible, .icon-button:focus-visible,
.point-probe:focus-visible, .transfer-choice:focus-visible {
  outline: 3px solid rgba(37, 99, 235, .28);
  outline-offset: 2px;
}

.question-tab.is-active {
  border-bottom-color: var(--green);
  background: var(--surface);
  color: var(--ink);
}

.question-tab-index {
  color: var(--faint);
  font: 600 12px/1 var(--font);
  padding-top: 4px;
}

.question-tab b, .question-tab small { display: block; }
.question-tab b { font-size: 14px; font-weight: 700; }
.question-tab small { margin-top: 2px; font-size: 11.5px; color: var(--faint); }

.learning-workspace {
  display: grid;
  grid-template-columns: minmax(380px, .82fr) minmax(520px, 1.18fr);
  min-height: 650px;
  background: var(--surface);
  border-bottom: 1px solid var(--line-strong);
}

.lesson-pane {
  min-width: 0;
  padding: 24px 30px 32px;
  border-right: 1px solid var(--line-strong);
}

.visual-pane {
  min-width: 0;
  padding: 24px 28px 28px;
  background: #fbfcfd;
}

.phase-rail {
  display: flex;
  align-items: center;
  margin-bottom: 28px;
}

.phase-rail i {
  flex: 1;
  height: 1px;
  min-width: 8px;
  background: var(--line);
}

.phase-step {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 3px;
  border: 0;
  background: transparent;
  color: var(--faint);
  cursor: pointer;
  white-space: nowrap;
}

.phase-step:disabled { cursor: default; opacity: .55; }
.phase-step span {
  display: inline-grid;
  place-items: center;
  width: 24px;
  height: 24px;
  border: 1px solid var(--line-strong);
  border-radius: 50%;
  background: var(--surface);
  font-size: 11px;
  font-weight: 700;
}

.phase-step b { font-size: 11px; font-weight: 600; }
.phase-step.is-unlocked { color: var(--ink); }
.phase-step.is-unlocked span { border-color: var(--green); color: var(--green); }
.phase-step.is-current { color: var(--ink); }
.phase-step.is-current span { border-color: var(--green); background: var(--green); color: #fff; }

.source-line {
  display: flex;
  align-items: center;
  gap: 10px;
  margin-bottom: 15px;
  color: var(--muted);
  font-size: 12px;
}

.source-line span {
  padding: 2px 7px;
  border: 1px solid var(--line);
  border-radius: 4px;
  background: var(--paper);
  color: var(--ink);
  font-weight: 700;
}

.question-stem {
  min-height: 78px;
  margin: 0 0 20px;
  font-size: 17px;
  line-height: 1.85;
  font-weight: 500;
}

.question-stem .katex { font-size: 1.02em; }

.phase-heading {
  margin: 0 0 4px;
  font-size: 20px;
  line-height: 1.35;
}

.phase-prompt {
  margin: 0 0 16px;
  color: var(--muted);
  font-size: 13px;
}

.choice-list, .transfer-list {
  display: grid;
  gap: 8px;
}

.choice-button, .transfer-choice {
  display: grid;
  grid-template-columns: 28px minmax(0, 1fr);
  align-items: center;
  gap: 10px;
  width: 100%;
  min-height: 48px;
  padding: 9px 13px;
  border: 1px solid var(--line);
  border-radius: 6px;
  background: var(--surface);
  color: var(--ink);
  text-align: left;
  cursor: pointer;
}

.choice-button:hover, .transfer-choice:hover { border-color: var(--blue); background: #f8faff; }
.choice-button.is-selected, .transfer-choice.is-selected { border-color: var(--blue); background: var(--blue-soft); }
.choice-button.is-correct, .transfer-choice.is-correct { border-color: var(--green); background: var(--green-soft); }
.choice-button.is-wrong, .transfer-choice.is-wrong { border-color: var(--red); background: var(--red-soft); }

.choice-key {
  display: grid;
  place-items: center;
  width: 26px;
  height: 26px;
  border: 1px solid var(--line-strong);
  border-radius: 50%;
  font-size: 12px;
  font-weight: 700;
}

.action-row {
  display: flex;
  align-items: center;
  gap: 10px;
  min-height: 44px;
  margin-top: 16px;
}

.primary-action, .secondary-action {
  min-height: 40px;
  padding: 9px 16px;
  border-radius: 6px;
  font-size: 13px;
  font-weight: 700;
  cursor: pointer;
}

.primary-action { border: 1px solid var(--green); background: var(--green); color: #fff; }
.primary-action:hover { background: #0f664c; }
.primary-action:disabled { border-color: var(--line); background: var(--line); color: var(--faint); cursor: default; }
.secondary-action { border: 1px solid var(--line-strong); background: var(--surface); color: var(--ink); }
.secondary-action:hover { border-color: var(--blue); color: var(--blue); }

.answer-feedback {
  margin-top: 14px;
  padding: 11px 13px;
  border-left: 3px solid var(--red);
  background: var(--red-soft);
  color: #8e322e;
  font-size: 13px;
}

.answer-feedback.is-correct { border-left-color: var(--green); background: var(--green-soft); color: #0f654b; }

.phase-note {
  margin: 14px 0 0;
  padding: 12px 14px;
  border-left: 3px solid var(--yellow);
  background: var(--yellow-soft);
  color: #665219;
  font-size: 13px;
}

.formula-chain {
  display: grid;
  margin-top: 18px;
  border-top: 1px solid var(--line);
}

.formula-step {
  display: grid;
  grid-template-columns: 32px minmax(0, 1fr);
  gap: 12px;
  padding: 13px 2px;
  border-bottom: 1px solid var(--line);
}

.formula-step > span {
  display: grid;
  place-items: center;
  width: 26px;
  height: 26px;
  border-radius: 50%;
  background: var(--blue-soft);
  color: var(--blue);
  font-size: 11px;
  font-weight: 700;
}

.formula-step b, .formula-step small { display: block; }
.formula-step b { font-size: 14px; font-weight: 600; }
.formula-step small { margin-top: 3px; color: var(--muted); font-size: 12px; }

.transfer-box {
  margin-top: 18px;
  padding-top: 18px;
  border-top: 2px solid var(--ink);
}

.transfer-box h3 { margin: 0 0 5px; font-size: 16px; }
.transfer-box p { margin: 0 0 13px; color: var(--muted); font-size: 13px; line-height: 1.75; }

.visual-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  min-height: 44px;
  margin-bottom: 12px;
}

.visual-toolbar > div { min-width: 0; }
.visual-label {
  display: block;
  margin-bottom: 2px;
  color: var(--green);
  font-size: 11px;
  font-weight: 700;
}
.visual-toolbar b { display: block; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; font-size: 15px; }

.icon-button {
  display: grid;
  place-items: center;
  flex: 0 0 auto;
  width: 36px;
  height: 36px;
  border: 1px solid var(--line);
  border-radius: 6px;
  background: var(--surface);
  color: var(--muted);
  cursor: pointer;
  font-size: 20px;
}
.icon-button:hover { border-color: var(--ink); color: var(--ink); }

.visual-stage {
  position: relative;
  width: 100%;
  aspect-ratio: 16 / 10;
  min-height: 360px;
  overflow: hidden;
  border: 1px solid var(--line-strong);
  border-radius: 6px;
  background:
    linear-gradient(rgba(37, 99, 235, .06) 1px, transparent 1px),
    linear-gradient(90deg, rgba(37, 99, 235, .06) 1px, transparent 1px),
    #fff;
  background-size: 28px 28px;
}

.visual-stage svg { display: block; width: 100%; height: 100%; }
.visual-stage text { font-family: var(--font); letter-spacing: 0; }

.visual-caption {
  min-height: 58px;
  padding: 12px 2px 0;
  color: var(--muted);
  font-size: 13px;
}
.visual-caption b { color: var(--ink); }

.visual-empty {
  display: grid;
  place-items: center;
  height: 100%;
  padding: 32px;
  text-align: center;
  color: var(--muted);
}
.visual-empty span { display: block; margin: 0 auto 16px; width: 68px; height: 68px; border: 1px solid var(--line-strong); border-radius: 50%; line-height: 66px; color: var(--green); font-size: 25px; }
.visual-empty b { display: block; margin-bottom: 5px; color: var(--ink); font-size: 16px; }
.visual-empty p { max-width: 360px; margin: 0; font-size: 13px; }

.visual-controls {
  position: absolute;
  left: 22px;
  right: 22px;
  bottom: 18px;
  display: grid;
  grid-template-columns: 150px minmax(0, 1fr) 58px;
  align-items: center;
  gap: 12px;
  padding: 10px 12px;
  border: 1px solid var(--line);
  border-radius: 6px;
  background: rgba(255, 255, 255, .94);
  box-shadow: 0 6px 22px rgba(23, 32, 42, .08);
}
.visual-controls label { font-size: 11.5px; font-weight: 700; }
.visual-controls output { color: var(--blue); font-size: 13px; font-weight: 700; text-align: right; }
input[type='range'] { width: 100%; accent-color: var(--blue); cursor: pointer; }

.point-probe { cursor: pointer; outline: none; }
.point-probe circle { transition: transform .18s ease, fill .18s ease; transform-box: fill-box; transform-origin: center; }
.point-probe:hover circle, .point-probe:focus-visible circle { transform: scale(1.18); }

.legend-row {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
  color: var(--muted);
  font-size: 11px;
}
.legend-row span { display: inline-flex; align-items: center; gap: 5px; }
.legend-row i { display: inline-block; width: 18px; height: 3px; border-radius: 2px; background: var(--blue); }
.legend-row i.green { background: var(--green); }
.legend-row i.dashed { height: 0; border-top: 2px dashed var(--red); background: none; }

@media (max-width: 980px) {
  .lab-shell { padding: 20px 18px 44px; }
  .learning-workspace { grid-template-columns: 1fr; }
  .lesson-pane { border-right: 0; border-bottom: 1px solid var(--line-strong); }
  .visual-stage { min-height: 0; }
}

@media (max-width: 680px) {
  .lab-shell { padding: 14px 0 32px; }
  .lab-header { align-items: flex-start; padding: 4px 16px 16px; }
  .lab-header h1 { font-size: 22px; }
  .lab-header-note { display: none; }
  .question-switcher { display: flex; overflow-x: auto; scroll-snap-type: x mandatory; scrollbar-width: thin; }
  .question-tab { flex: 0 0 76%; min-height: 62px; padding: 11px 16px; border-right: 1px solid var(--line); border-bottom: 3px solid transparent; scroll-snap-align: start; }
  .question-tab.is-active { border-bottom-color: var(--green); }
  .question-tab small { display: none; }
  .learning-workspace { min-height: 0; }
  .lesson-pane, .visual-pane { padding: 20px 16px 24px; }
  .phase-rail { margin-bottom: 22px; }
  .phase-step b { display: none; }
  .phase-step { min-width: 40px; min-height: 40px; justify-content: center; }
  .phase-step span { width: 30px; height: 30px; }
  .question-stem { min-height: 0; font-size: 16px; }
  .visual-toolbar b { font-size: 13px; }
  .choice-button, .transfer-choice, .primary-action, .secondary-action { min-height: 44px; }
  .icon-button { width: 44px; height: 44px; }
  .visual-stage { aspect-ratio: 4 / 3; }
  .visual-controls { left: 10px; right: 10px; bottom: 10px; grid-template-columns: 90px minmax(0, 1fr) 45px; gap: 7px; padding: 8px; }
  .visual-controls label { font-size: 10.5px; }
  .action-row { flex-wrap: wrap; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { scroll-behavior: auto !important; transition: none !important; }
}