/* ---------- lesson player ---------- */
.player {
  min-height: 100dvh;
  display: flex;
  flex-direction: column;
  background: var(--bg);
}
.player-top {
  position: sticky;
  top: 0;
  z-index: 30;
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 10px var(--gutter);
  background: color-mix(in srgb, var(--bg) 90%, transparent);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
}
.player-title {
  font-weight: 700;
  font-size: 14px;
  color: var(--ink-3);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  max-width: 30%;
}
.segbar {
  flex: 1;
  display: flex;
  gap: 4px;
  height: 10px;
}
.segbar span {
  flex: 1;
  border-radius: var(--r-pill);
  background: var(--surface-2);
  position: relative;
  overflow: hidden;
}
.segbar span::after {
  content: "";
  position: absolute;
  inset: 0;
  border-radius: inherit;
  background: var(--cc, var(--success-bright));
  transform: scaleX(0);
  transform-origin: left;
  transition: transform var(--t-slow) var(--ease-out);
}
.segbar span.done::after {
  transform: scaleX(1);
}
.segbar span.current::after {
  transform: scaleX(0.35);
  opacity: 0.45;
}
@media (max-width: 600px) {
  .player-title {
    display: none;
  }
}

.stage {
  flex: 1;
  width: 100%;
  max-width: calc(var(--prose) + 2 * var(--gutter));
  margin: 0 auto;
  padding: 20px var(--gutter) 180px;
}
@media (max-width: 520px) {
  /* the action bar wraps onto two rows on phones: keep the last card clear of it */
  .stage { padding-bottom: 240px; }
}
.step {
  animation: step-in 360ms var(--ease-out) both;
}
.step.leaving {
  animation: step-out 200ms var(--ease) both;
}
@keyframes step-in {
  from { opacity: 0; transform: translateX(28px); }
  to { opacity: 1; transform: none; }
}
@keyframes step-out {
  to { opacity: 0; transform: translateX(-28px); }
}

.step-kicker {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin-bottom: 14px;
  font-size: 12px;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--cc, var(--ink-3));
}
.step-kicker svg {
  width: 16px;
  height: 16px;
}
.step-title {
  font-family: var(--font-display);
  font-weight: 500;
  font-size: clamp(28px, 4.4vw, 36px);
  letter-spacing: -0.015em;
  line-height: 1.12;
  margin-bottom: 16px;
}
.prose {
  font-size: 18px;
  line-height: 1.6;
}
.prose p {
  margin-bottom: 0.85em;
}
.prose ul,
.prose ol {
  margin: 0 0 0.9em;
  padding-left: 1.3em;
}
.prose li {
  margin: 0.3em 0;
}
.prose strong {
  font-weight: 700;
}
.prose .callout {
  margin: 1em 0;
  padding: 14px 18px;
  border-radius: var(--r-md);
  background: var(--surface);
  border-left: 4px solid var(--cc, var(--blue));
}
.prose .key {
  margin: 1.1em 0;
  padding: 16px 20px;
  border-radius: var(--r-lg);
  background: var(--cc-soft, var(--blue-soft));
  font-size: 18px;
}
.prose .key::before {
  content: "Key idea";
  display: block;
  margin-bottom: 4px;
  font-size: 12px;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--cc, var(--blue));
}
.prompt {
  margin-top: 18px;
  font-size: 19px;
  font-weight: 600;
  line-height: 1.45;
}

.figure {
  margin: 20px 0;
  border-radius: var(--r-lg);
  background: var(--surface);
  overflow: hidden;
  position: relative;
}
.figure svg.fig {
  width: 100%;
  height: auto;
  /* a swipe on a figure still scrolls the page; draggable handles opt out themselves */
  touch-action: pan-y;
  user-select: none;
  -webkit-user-select: none;
}
.figure .caption {
  padding: 0 16px 12px;
  font-size: 14px;
  color: var(--ink-3);
  text-align: center;
}

/* ---------- choices ---------- */
.choices {
  display: grid;
  gap: 12px;
  margin-top: 20px;
}
.choices.cols-2 {
  grid-template-columns: repeat(2, minmax(0, 1fr));
}
@media (max-width: 480px) {
  .choices.cols-2 {
    grid-template-columns: 1fr;
  }
}
.choice {
  position: relative;
  display: flex;
  align-items: center;
  gap: 14px;
  width: 100%;
  min-height: 60px;
  padding: 14px 18px;
  text-align: left;
  font-size: 18px;
  line-height: 1.35;
  background: var(--card);
  border: 2px solid var(--line-2);
  border-radius: var(--r-lg);
  box-shadow: 0 4px 0 var(--edge);
  transition: transform var(--t-fast) var(--ease), box-shadow var(--t-fast) var(--ease),
    border-color var(--t-fast) var(--ease), background-color var(--t-fast) var(--ease);
}
.choice:hover:not([disabled]) {
  border-color: color-mix(in srgb, var(--select) 40%, var(--line-2));
}
.choice:active:not([disabled]) {
  transform: translateY(3px);
  box-shadow: 0 1px 0 var(--edge);
}
.choice .key-hint {
  flex: none;
  display: grid;
  place-items: center;
  width: 28px;
  height: 28px;
  border-radius: 8px;
  border: 2px solid var(--line-2);
  font-size: 13px;
  font-weight: 800;
  color: var(--ink-3);
  font-family: var(--font-mono);
}
.choice.multi .key-hint {
  border-radius: 7px;
}
.choice[aria-checked="true"] {
  border-color: var(--select);
  background: var(--select-tint);
  box-shadow: 0 4px 0 var(--select-edge);
}
.choice[aria-checked="true"] .key-hint {
  border-color: var(--select);
  background: var(--select);
  color: #fff;
}
.choice.is-correct {
  border-color: var(--success);
  background: var(--success-tint);
  box-shadow: 0 4px 0 var(--success-edge);
}
.choice.is-correct .key-hint {
  border-color: var(--success);
  background: var(--success);
  color: #fff;
}
.choice.is-wrong {
  border-color: var(--error);
  background: var(--error-tint);
  box-shadow: 0 4px 0 var(--error-edge);
}
.choice.is-wrong .key-hint {
  border-color: var(--error);
  background: var(--error);
  color: #fff;
}
.choice[disabled] {
  cursor: default;
}
.choice.is-dim {
  opacity: 0.55;
}
.choice .ctext {
  flex: 1;
  min-width: 0;
}
.choice .ctext .katex {
  font-size: 1.12em;
}

/* ---------- numeric ---------- */
.numeric {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-top: 20px;
  flex-wrap: wrap;
}
.numeric .lhs {
  font-size: 20px;
}
.num-input {
  width: 180px;
  max-width: 100%;
  height: 60px;
  padding: 0 18px;
  font-size: 22px;
  font-weight: 650;
  font-variant-numeric: tabular-nums;
  border-radius: var(--r-lg);
  background: var(--card);
  border: 2px solid var(--line-2);
  box-shadow: 0 4px 0 var(--edge);
  outline: none;
  transition: border-color var(--t-fast) var(--ease);
}
.num-input:focus {
  border-color: var(--select);
  box-shadow: 0 4px 0 var(--select-edge);
}
.num-input.is-correct {
  border-color: var(--success);
  background: var(--success-tint);
  box-shadow: 0 4px 0 var(--success-edge);
}
.num-input.is-wrong {
  border-color: var(--error);
  background: var(--error-tint);
  box-shadow: 0 4px 0 var(--error-edge);
}
.numeric .unit {
  font-size: 20px;
  color: var(--ink-2);
}

/* ---------- order ---------- */
.order-list {
  list-style: none;
  margin: 20px 0 0;
  padding: 0;
  display: grid;
  gap: 10px;
}
.order-item {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 12px 14px;
  background: var(--card);
  border: 2px solid var(--line-2);
  border-radius: var(--r-lg);
  box-shadow: 0 4px 0 var(--edge);
  font-size: 17px;
  touch-action: pan-y;
  user-select: none;
  transition: transform var(--t) var(--ease), border-color var(--t-fast) var(--ease);
}
.order-item.dragging {
  z-index: 5;
  border-color: var(--select);
  box-shadow: var(--shadow-pop);
  transition: none;
  cursor: grabbing;
}
.order-item .grip {
  color: var(--ink-3);
  cursor: grab;
  display: grid;
  place-items: center;
  width: 40px;
  height: 44px;
  margin: -4px -6px -4px -8px;
  touch-action: none;
}
.order-item .grip svg {
  width: 18px;
  height: 18px;
}
.order-item .otext {
  flex: 1;
}
.order-item .moves {
  display: flex;
  gap: 2px;
}
.order-item .moves button {
  width: 34px;
  height: 34px;
  border-radius: 8px;
  color: var(--ink-3);
  display: grid;
  place-items: center;
}
.order-item .moves button[disabled] {
  opacity: 0.25;
  pointer-events: none;
}
.order-item .moves button:hover {
  background: var(--surface);
  color: var(--ink);
}
.order-item .moves svg {
  width: 18px;
  height: 18px;
}
.order-item.is-correct {
  border-color: var(--success);
  background: var(--success-tint);
}
.order-item.is-wrong {
  border-color: var(--error);
  background: var(--error-tint);
}

/* ---------- action bar & feedback ---------- */
.actionbar {
  position: fixed;
  left: 0;
  right: 0;
  bottom: 0;
  z-index: 40;
  border-top: 2px solid var(--line);
  background: var(--bg);
  transition: background-color var(--t) var(--ease), border-color var(--t) var(--ease);
}
.actionbar .inner {
  max-width: calc(var(--prose) + 2 * var(--gutter));
  margin: 0 auto;
  padding: 16px var(--gutter) calc(16px + env(safe-area-inset-bottom));
  display: flex;
  align-items: center;
  gap: 12px;
}
.actionbar .fb {
  flex: 1;
  min-width: 0;
  display: none;
}
.actionbar .grow {
  flex: 1;
}
.actionbar .btn-main {
  min-width: 160px;
}
@media (max-width: 520px) {
  .actionbar .inner {
    flex-wrap: wrap;
  }
  .actionbar .btn-main {
    flex: 1;
  }
}
.actionbar.is-correct {
  background: var(--success-tint);
  border-color: var(--success-edge);
}
.actionbar.is-wrong {
  background: var(--error-tint);
  border-color: var(--error-edge);
}
.actionbar.is-correct .fb,
.actionbar.is-wrong .fb {
  display: flex;
  align-items: center;
  gap: 12px;
  animation: fb-in 380ms var(--spring) both;
}
.actionbar.is-correct .grow,
.actionbar.is-wrong .grow {
  display: none;
}
@media (max-width: 520px) {
  .actionbar .fb {
    flex-basis: 100%;
  }
}
@keyframes fb-in {
  from { opacity: 0; transform: translateY(12px) scale(0.98); }
  to { opacity: 1; transform: none; }
}
.fb-badge {
  flex: none;
  display: grid;
  place-items: center;
  width: 44px;
  height: 44px;
  border-radius: 50%;
  color: #fff;
}
.fb-badge svg {
  width: 24px;
  height: 24px;
}
.is-correct .fb-badge {
  background: var(--success);
  animation: badge-pop 520ms var(--spring) both;
}
.is-wrong .fb-badge {
  background: var(--error);
}
:root[data-theme="dark"] .fb-badge {
  color: #22272e;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) .fb-badge {
    color: #22272e;
  }
}
@keyframes badge-pop {
  0% { transform: scale(0.3); }
  100% { transform: scale(1); }
}
.fb-text {
  min-width: 0;
}
.fb-text strong {
  display: block;
  font-size: 19px;
  line-height: 1.2;
}
.is-correct .fb-text strong {
  color: var(--success);
}
.is-wrong .fb-text strong {
  color: var(--error);
}
.fb-text span {
  display: block;
  font-size: 15px;
  color: var(--ink-2);
  line-height: 1.4;
  margin-top: 2px;
}
.fb-text span .katex {
  font-size: 1em;
}
/* "+10 XP": a small pill that floats up under the top bar and fades */
.xp-pop {
  position: fixed;
  top: calc(58px + env(safe-area-inset-top));
  left: 50%;
  translate: -50% 0;
  z-index: 60;
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 7px 14px 7px 11px;
  border-radius: var(--r-pill);
  background: var(--card);
  border: 2px solid var(--select-edge);
  box-shadow: var(--shadow-soft);
  color: var(--select);
  font-weight: 800;
  font-size: 15px;
  white-space: nowrap;
  pointer-events: none;
  animation: xp-pop 1700ms var(--ease-out) both;
}
.xp-pop svg {
  width: 16px;
  height: 16px;
  color: var(--amber);
}
@keyframes xp-pop {
  0% { opacity: 0; transform: translateY(10px) scale(0.7); }
  14% { opacity: 1; transform: translateY(0) scale(1.06); }
  24% { transform: translateY(0) scale(1); }
  72% { opacity: 1; transform: translateY(-4px); }
  100% { opacity: 0; transform: translateY(-22px); }
}
@media (prefers-reduced-motion: reduce) {
  .xp-pop { animation: xp-fade 1400ms linear both; }
  @keyframes xp-fade { 0%, 100% { opacity: 0; } 15%, 75% { opacity: 1; } }
}

/* feedback card in the step body (why an answer is wrong or right, hints) */
.note {
  scroll-margin-bottom: 200px;
  margin-top: 20px;
  padding: 14px 18px;
  border-radius: var(--r-md);
  border: 2px solid;
  animation: page-in var(--t) var(--ease-out) both;
}
.note.is-wrong {
  background: var(--error-tint);
  border-color: var(--error-edge);
}
.note.is-correct {
  background: var(--success-tint);
  border-color: var(--success-edge);
}
.note-part + .note-part {
  margin-top: 12px;
  padding-top: 12px;
  border-top: 1px solid var(--line-2);
}
.note h4 {
  font-size: 13px;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 4px;
}
.note.is-wrong h4 { color: var(--error); }
.note.is-correct h4 { color: var(--success); }
.note .prose {
  font-size: 16px;
  color: var(--ink);
}
.note .prose p:last-child { margin-bottom: 0; }

/* explanation drawer */
.why {
  scroll-margin-bottom: 150px;
  margin-top: 24px;
  padding: 18px 20px;
  border-radius: var(--r-lg);
  background: var(--surface);
  border: 2px solid var(--line);
  animation: page-in var(--t-slow) var(--ease-out) both;
}
.why h4 {
  font-size: 13px;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-3);
  margin-bottom: 8px;
}
.why .prose {
  font-size: 17px;
}
.hint {
  scroll-margin-bottom: 150px;
  margin-top: 18px;
  padding: 14px 18px;
  border-radius: var(--r-md);
  background: var(--warn-tint);
  color: var(--ink);
  font-size: 16px;
  animation: page-in var(--t) var(--ease-out) both;
}
.hint b {
  color: var(--warn);
}

.shake {
  animation: shake 360ms var(--ease);
}
@keyframes shake {
  0%, 100% { transform: translateX(0); }
  20% { transform: translateX(-8px); }
  40% { transform: translateX(7px); }
  60% { transform: translateX(-5px); }
  80% { transform: translateX(3px); }
}

/* particle burst */
.burst {
  position: fixed;
  pointer-events: none;
  z-index: 100;
}
.burst i {
  position: absolute;
  width: 9px;
  height: 9px;
  border-radius: 2px;
  animation: burst 800ms var(--ease-out) forwards;
}
@keyframes burst {
  0% { transform: translate(0, 0) rotate(0) scale(1); opacity: 1; }
  100% { transform: translate(var(--dx), var(--dy)) rotate(var(--rot)) scale(0.4); opacity: 0; }
}
@media (prefers-reduced-motion: reduce) {
  .burst { display: none; }
}

/* ---------- completion ---------- */
.complete {
  text-align: center;
  padding-top: 32px;
}
.complete .medal {
  width: 168px;
  height: 168px;
  margin: 0 auto 20px;
}
.complete .medal svg {
  width: 100%;
  height: 100%;
}
.complete h1 {
  font-family: var(--font-display);
  font-weight: 500;
  font-size: clamp(34px, 5vw, 46px);
  letter-spacing: -0.02em;
}
.complete .sub {
  margin-top: 8px;
  color: var(--ink-2);
  font-size: 18px;
}
.stat-row {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 12px;
  margin: 28px 0;
}
.stat {
  padding: 14px 10px;
  border-radius: var(--r-lg);
  border: 2px solid var(--line);
  box-shadow: 0 4px 0 var(--edge);
}
.stat b {
  display: block;
  font-size: 26px;
  font-weight: 800;
  font-variant-numeric: tabular-nums;
}
.stat span {
  font-size: 12px;
  font-weight: 800;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--ink-3);
}
.stat.s-xp b { color: var(--select); }
.stat.s-acc b { color: var(--success); }
.stat.s-streak b { color: #e8590c; }
.recap {
  text-align: left;
  margin-top: 8px;
  padding: 18px 20px;
  border-radius: var(--r-lg);
  background: var(--cc-soft, var(--surface));
}
.recap h4 {
  font-size: 13px;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--cc, var(--ink-3));
  margin-bottom: 8px;
}
.recap ul {
  margin: 0;
  padding-left: 1.2em;
}
.recap li {
  margin: 6px 0;
}

.medal-draw {
  stroke-dasharray: 1;
  stroke-dashoffset: 1;
  animation: medal-draw 900ms 200ms var(--ease-out) forwards;
}
@keyframes medal-draw {
  to { stroke-dashoffset: 0; }
}
.medal-pop {
  transform-box: fill-box;
  transform-origin: center;
  animation: badge-pop 700ms var(--spring) both;
}

/* exit dialog */
.dialog-backdrop {
  position: fixed;
  inset: 0;
  z-index: 300;
  background: var(--overlay);
  display: grid;
  place-items: center;
  padding: 16px;
  animation: fade-in var(--t) var(--ease) both;
}
.dialog {
  width: 100%;
  max-width: 420px;
  padding: 26px;
  border-radius: var(--r-xl);
  background: var(--card);
  box-shadow: var(--shadow-pop);
  animation: fb-in var(--t-slow) var(--spring) both;
}
.dialog h3 {
  font-size: 22px;
  margin-bottom: 6px;
}
.dialog p {
  color: var(--ink-2);
}
.dialog .row {
  display: flex;
  gap: 10px;
  margin-top: 20px;
  justify-content: flex-end;
  flex-wrap: wrap;
}
@keyframes fade-in {
  from { opacity: 0; }
}

.md-table {
  border-collapse: collapse;
  margin: 0.6em 0 1em;
  font-variant-numeric: tabular-nums;
  font-size: 16px;
}
.md-table th,
.md-table td {
  padding: 7px 16px;
  text-align: left;
  border-bottom: 1px solid var(--line);
}
.md-table th {
  font-size: 12px;
  font-weight: 800;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--ink-3);
  border-bottom: 2px solid var(--line-2);
}

/* back to the previous step */
.actionbar .back-btn {
  flex: none;
  padding: 0 18px 0 14px;
}
.actionbar .back-btn svg {
  width: 20px;
  height: 20px;
}
@media (max-width: 520px) {
  .actionbar .fb {
    order: -1;
  }
  .actionbar .back-btn {
    padding: 0 14px;
  }
  .actionbar .back-btn span {
    display: none;
  }
}