/* ==========================================================================
   Tiếng Việt textbook: warm-minimal theme (Wealthsimple-inspired)
   ========================================================================== */

/* ---------- Design tokens ---------- */

:root {
  color-scheme: light;

  --bg: #fcfcfc;
  --surface: #f6f4f1;
  --surface-2: #efedeb;
  --surface-3: #e6e3e0;
  --text: #32302f;
  --text-strong: #09090a;
  --muted: #686664;
  --border: #e4e2e1;
  --border-strong: #cdc9c6;
  --btn-bg: #09090a;
  --btn-fg: #fcfcfc;
  --btn-bg-hover: #32302f;
  --south: #f3ece1;
  --south-ink: #6b5536;
  --good: #2e7350;
  --good-bg: #e5f1e9;
  --bad: #b03f2c;
  --bad-bg: #f8e8e4;
  --focus: #09090a;

  --font-serif: "Source Serif 4", Georgia, "Times New Roman", serif;
  --font-sans: Inter, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --font-mono: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;

  --r-pill: 999px;
  --r-card: 24px;
  --r-md: 16px;
  --r-sm: 12px;

  --header-h: 64px;
  --sidebar-w: 288px;
  --measure: 720px;
}

/* Dark: used when chosen with the toggle, or when the OS is dark and nothing was chosen. */
:root[data-theme="dark"] {
  color-scheme: dark;

  --bg: #141312;
  --surface: #1c1b1a;
  --surface-2: #262523;
  --surface-3: #312f2d;
  --text: #f3f1ee;
  --text-strong: #ffffff;
  --muted: #a3a09c;
  --border: #2f2d2b;
  --border-strong: #46433f;
  --btn-bg: #f3f1ee;
  --btn-fg: #141312;
  --btn-bg-hover: #ffffff;
  --south: #2a241c;
  --south-ink: #d9bf98;
  --good: #74c897;
  --good-bg: #1b2c22;
  --bad: #f08b77;
  --bad-bg: #35211c;
  --focus: #f3f1ee;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    color-scheme: dark;

    --bg: #141312;
    --surface: #1c1b1a;
    --surface-2: #262523;
    --surface-3: #312f2d;
    --text: #f3f1ee;
    --text-strong: #ffffff;
    --muted: #a3a09c;
    --border: #2f2d2b;
    --border-strong: #46433f;
    --btn-bg: #f3f1ee;
    --btn-fg: #141312;
    --btn-bg-hover: #ffffff;
    --south: #2a241c;
    --south-ink: #d9bf98;
    --good: #74c897;
    --good-bg: #1b2c22;
    --bad: #f08b77;
    --bad-bg: #35211c;
    --focus: #f3f1ee;
  }
}

/* ---------- Base ---------- */

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

[hidden] { display: none !important; }

/* Set by ui.js for one frame while the theme switches, so colors change instantly. */
.theme-switching *, .theme-switching *::before, .theme-switching *::after { transition: none !important; }

html {
  -webkit-text-size-adjust: 100%;
  scroll-padding-top: calc(var(--header-h) + 24px);
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: var(--font-sans);
  font-size: 16px;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

:focus-visible {
  outline: 2px solid var(--focus);
  outline-offset: 2px;
}

::selection { background: var(--surface-3); }

/* ---------- Top bar ---------- */

.topbar {
  position: sticky;
  top: 0;
  z-index: 30;
  display: flex;
  align-items: center;
  gap: 12px;
  height: var(--header-h);
  padding: 0 24px;
  background: var(--bg);
  border-bottom: 1px solid var(--border);
}

.wordmark {
  font-family: var(--font-serif);
  font-size: 24px;
  letter-spacing: -0.01em;
  color: var(--text-strong);
  text-decoration: none;
  white-space: nowrap;
}

.topbar-tag {
  padding: 4px 10px;
  border-radius: var(--r-pill);
  background: var(--surface-2);
  color: var(--muted);
  font-size: 12px;
  font-weight: 500;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  white-space: nowrap;
}

.theme-toggle { margin-left: auto; }
.topbar .icon-btn { flex-shrink: 0; }

.icon-btn {
  display: inline-grid;
  place-items: center;
  width: 40px;
  height: 40px;
  padding: 0;
  border: 0;
  border-radius: 50%;
  background: transparent;
  color: var(--text);
  cursor: pointer;
  transition: background-color 0.15s ease;
}
.icon-btn:hover { background: var(--surface-2); }

.icon-sun { display: none; }
:root[data-theme="dark"] .icon-sun { display: block; }
:root[data-theme="dark"] .icon-moon { display: none; }
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) .icon-sun { display: block; }
  :root:not([data-theme="light"]) .icon-moon { display: none; }
}

.menu-btn { display: none; margin-left: -8px; }

/* ---------- Layout ---------- */

.layout {
  display: grid;
  grid-template-columns: var(--sidebar-w) minmax(0, 1fr);
  max-width: 1200px;
  margin: 0 auto;
}

.sidebar {
  position: sticky;
  top: var(--header-h);
  height: calc(100vh - var(--header-h));
  overflow-y: auto;
  padding: 32px 16px 32px 24px;
}

.main {
  min-width: 0;
  padding: 56px 48px 96px;
}

.page-head,
.prose,
.pager {
  max-width: var(--measure);
}

/* ---------- Sidebar navigation ---------- */

.nav-item {
  display: flex;
  align-items: baseline;
  gap: 12px;
  padding: 10px 16px;
  border-radius: var(--r-pill);
  color: var(--text);
  font-size: 15px;
  text-decoration: none;
  transition: background-color 0.15s ease;
}
.nav-item:hover { background: var(--surface-2); }
.nav-item.is-active {
  background: var(--surface-2);
  color: var(--text-strong);
  font-weight: 500;
}

.nav-num {
  font-size: 13px;
  font-variant-numeric: tabular-nums;
  color: var(--muted);
  font-weight: 400;
}

.nav-label {
  margin: 28px 16px 8px;
  font-size: 12px;
  font-weight: 500;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}

.nav-chapters,
.nav-sections {
  list-style: none;
  margin: 0;
  padding: 0;
}
.nav-chapters > li + li { margin-top: 2px; }

.nav-sections {
  margin: 6px 0 10px 27px;
  padding-left: 16px;
  border-left: 1px solid var(--border);
}
.nav-sections a {
  display: block;
  padding: 6px 8px;
  font-size: 14px;
  line-height: 1.35;
  color: var(--muted);
  text-decoration: none;
  border-radius: 8px;
}
.nav-sections a:hover { color: var(--text-strong); }

.nav-footer {
  margin-top: 28px;
  padding-top: 16px;
  border-top: 1px solid var(--border);
}

/* ---------- Page heading ---------- */

.eyebrow {
  display: block;
  margin: 0 0 12px;
  font-size: 13px;
  font-weight: 500;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}

.page-head { margin-bottom: 40px; }

h1, h2 {
  font-family: var(--font-serif);
  font-weight: 400;
  letter-spacing: -0.01em;
  color: var(--text-strong);
}

h1 {
  margin: 0;
  font-size: 56px;
  line-height: 1.12;
}

/* ---------- Prose ---------- */

.prose > :first-child { margin-top: 0; }

.prose h2 {
  margin: 72px 0 16px;
  font-size: 36px;
  line-height: 1.2;
}

.prose h3 {
  margin: 40px 0 8px;
  font-size: 20px;
  line-height: 1.3;
  font-weight: 500;
  color: var(--text-strong);
}

.prose p { margin: 0 0 16px; }

.prose .lead,
.lead {
  font-size: 20px;
  line-height: 1.5;
  color: var(--muted);
  margin-bottom: 24px;
}

.caption,
.prose small {
  font-size: 14px;
  line-height: 1.4;
  color: var(--muted);
}

.prose a:not(.btn):not(.card):not(.chip):not(.toc-link) {
  color: var(--text-strong);
  text-decoration: underline;
  text-decoration-thickness: 1px;
  text-decoration-color: var(--border-strong);
  text-underline-offset: 3px;
  transition: text-decoration-color 0.15s ease;
}
.prose a:not(.btn):not(.card):not(.chip):not(.toc-link):hover { text-decoration-color: currentColor; }

.prose strong { font-weight: 600; color: var(--text-strong); }

.prose ul,
.prose ol {
  margin: 0 0 16px;
  padding-left: 24px;
}
.prose li { margin: 6px 0; }
.prose li::marker { color: var(--muted); }

.prose blockquote {
  margin: 32px 0;
  padding: 4px 0 4px 24px;
  border-left: 2px solid var(--text-strong);
  font-family: var(--font-serif);
  font-size: 24px;
  line-height: 1.4;
  color: var(--text-strong);
}
.prose blockquote p:last-child { margin-bottom: 0; }

.prose code {
  padding: 2px 6px;
  border-radius: 6px;
  background: var(--surface-2);
  font-family: var(--font-mono);
  font-size: 0.875em;
}

.prose pre {
  margin: 24px 0;
  padding: 20px 24px;
  overflow-x: auto;
  border-radius: var(--r-md);
  background: var(--surface);
  line-height: 1.6;
}
.prose pre code { padding: 0; background: none; }

.prose hr {
  margin: 56px 0;
  border: 0;
  border-top: 1px solid var(--border);
}

/* ---------- Tables ---------- */

.table-wrap {
  margin: 24px 0;
  overflow-x: auto;
  border: 1px solid var(--border);
  border-radius: var(--r-md);
}

.prose table {
  width: 100%;
  border-collapse: collapse;
  font-size: 15px;
}

.prose th,
.prose td {
  padding: 14px 20px;
  text-align: left;
  vertical-align: middle;
  border-bottom: 1px solid var(--border);
}

.prose th {
  background: var(--surface);
  font-size: 13px;
  font-weight: 500;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: var(--muted);
}

.prose tr:last-child td { border-bottom: 0; }

/* ---------- Clickable words ---------- */

.say {
  display: inline;
  margin: 0;
  padding: 0;
  border: 0;
  background: none;
  font: inherit;
  color: var(--text-strong);
  text-align: inherit;
  cursor: pointer;
  border-radius: 6px;
}

.say-text {
  text-decoration: underline dotted var(--muted);
  text-decoration-thickness: 1.5px;
  text-underline-offset: 0.22em;
  transition: text-decoration-color 0.15s ease;
}

.say-icon {
  display: inline-grid;
  place-items: center;
  width: 1.3em;
  height: 1.3em;
  margin-left: 0.3em;
  border-radius: 50%;
  background: var(--surface-2);
  color: var(--text);
  /* Centre the chip on the text box. (Aligning by baseline would use the icon's
     bottom edge and push the chip down.) */
  vertical-align: middle;
  position: relative;
  top: var(--say-nudge, -0.1em);
  transition: background-color 0.15s ease, color 0.15s ease;
}
.say-icon svg { display: block; width: 62%; height: 62%; }

.say:hover .say-text,
.demo-hover .say .say-text {
  text-decoration-style: solid;
  text-decoration-color: var(--text-strong);
}
.say:hover .say-icon,
.demo-hover .say .say-icon { background: var(--surface-3); }

.say--playing .say-text,
.demo-playing .say .say-text {
  text-decoration-style: solid;
  text-decoration-color: var(--text-strong);
}
.say--playing .say-icon,
.demo-playing .say .say-icon {
  background: var(--btn-bg);
  color: var(--btn-fg);
}
.say--playing .wave,
.demo-playing .say .wave { animation: say-wave 1s ease-in-out infinite; }
.say--playing .wave-2,
.demo-playing .say .wave-2 { animation-delay: 0.2s; }

@keyframes say-wave {
  0%, 100% { opacity: 1; }
  50% { opacity: 0.15; }
}

.say--missing { color: var(--muted); cursor: help; }
.say--missing .say-text { text-decoration-style: dashed; }
.say--missing .say-icon { opacity: 0.55; }
.say--missing:hover .say-text { text-decoration-style: dashed; text-decoration-color: var(--muted); }
.say--missing:hover .say-icon { background: var(--surface-2); }

/* Style guide only: show the "has audio" look on words that have no audio yet. */
.demo-normal .say--missing,
.demo-hover .say--missing,
.demo-playing .say--missing { color: var(--text-strong); cursor: pointer; }
.demo-normal .say--missing .say-text,
.demo-hover .say--missing .say-text,
.demo-playing .say--missing .say-text { text-decoration-style: dotted; }
.demo-hover .say--missing .say-text,
.demo-playing .say--missing .say-text { text-decoration-style: solid; }
.demo-normal .say--missing .say-icon,
.demo-hover .say--missing .say-icon,
.demo-playing .say--missing .say-icon { opacity: 1; }

/* ---------- Buttons ---------- */

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  height: 48px;
  padding: 0 28px;
  border: 1px solid transparent;
  border-radius: var(--r-pill);
  font-family: var(--font-sans);
  font-size: 16px;
  font-weight: 500;
  letter-spacing: 0.025em;
  line-height: 1;
  text-decoration: none;
  white-space: nowrap;
  cursor: pointer;
  transition: background-color 0.15s ease, border-color 0.15s ease, color 0.15s ease;
}
.btn svg { width: 18px; height: 18px; }

.btn-primary { background: var(--btn-bg); color: var(--btn-fg); }
.btn-primary:hover { background: var(--btn-bg-hover); }

.btn-secondary { background: transparent; color: var(--text-strong); border-color: var(--text-strong); }
.btn-secondary:hover { background: var(--surface-2); }

.btn-ghost { background: var(--surface-2); color: var(--text-strong); }
.btn-ghost:hover { background: var(--surface-3); }

.btn-sm { height: 36px; padding: 0 16px; font-size: 14px; }

.btn:disabled,
.btn[aria-disabled="true"] { opacity: 0.45; cursor: not-allowed; }

.btn-row {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 12px;
  margin: 24px 0;
}

/* ---------- Cards ---------- */

.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
  gap: 16px;
  margin: 24px 0;
}

.card {
  display: flex;
  flex-direction: column;
  gap: 8px;
  padding: 28px;
  border: 1px solid transparent;
  border-radius: var(--r-card);
  background: var(--surface);
  color: var(--text);
  text-decoration: none;
}
.card .eyebrow { margin: 0; }
.card p { margin: 0; color: var(--muted); }

.card-link { transition: border-color 0.15s ease, background-color 0.15s ease; }
.card-link:hover { border-color: var(--border-strong); }

.card-title {
  font-family: var(--font-serif);
  font-size: 24px;
  line-height: 1.2;
  letter-spacing: -0.01em;
  color: var(--text-strong);
}

.card-meta {
  margin-top: auto;
  padding-top: 16px;
  font-size: 14px;
  color: var(--muted);
}

/* ---------- Table of contents (home page) ---------- */

.toc { margin: 8px 0 0; }
.prose .toc-part {
  margin: 36px 0 4px;
  font-size: 13px;
  font-weight: 500;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}
.prose .toc-list {
  margin: 0;
  padding: 0;
  list-style: none;
  border-top: 1px solid var(--border);
}
.prose .toc-chapter {
  margin: 0;
  padding: 18px 0 16px;
  border-bottom: 1px solid var(--border);
}
.toc-title {
  display: flex;
  align-items: baseline;
  gap: 16px;
  color: var(--text-strong);
  text-decoration: none;
}
.toc-num {
  min-width: 24px;
  font-size: 14px;
  font-variant-numeric: tabular-nums;
  color: var(--muted);
}
.toc-name {
  font-family: var(--font-serif);
  font-size: 24px;
  line-height: 1.25;
  letter-spacing: -0.01em;
}
.toc-title:hover .toc-name { text-decoration: underline; text-decoration-thickness: 1px; text-underline-offset: 4px; }
.prose .toc-sections {
  margin: 8px 0 0 40px;
  padding: 0;
  list-style: none;
}
.prose .toc-sections li { margin: 0; }
.toc-sections .toc-link {
  display: flex;
  gap: 12px;
  padding: 3px 0;
  font-size: 15px;
  line-height: 1.5;
  color: var(--muted);
  text-decoration: none;
  transition: color 0.15s ease;
}
.toc-sections .toc-link:hover { color: var(--text-strong); }
.toc-snum { min-width: 28px; font-variant-numeric: tabular-nums; }

/* ---------- Callouts ---------- */

.tip,
.south,
.warn {
  margin: 28px 0;
  padding: 24px 28px;
  border-radius: var(--r-card);
  background: var(--surface);
}
.tip > :last-child,
.south > :last-child,
.warn > :last-child { margin-bottom: 0; }

.tip::before,
.south::before,
.warn::before {
  display: block;
  margin-bottom: 8px;
  font-size: 12px;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}
.tip::before { content: "Tip"; }
.south::before { content: "Southern pronunciation"; color: var(--south-ink); }
.warn::before { content: "Watch out"; color: var(--bad); }

.south { background: var(--south); }
.warn { background: var(--bad-bg); }

/* ---------- Dialogue ---------- */

.dialogue { margin: 28px 0; }
.dialogue ul {
  display: flex;
  flex-direction: column;
  gap: 10px;
  margin: 0;
  padding: 0;
  list-style: none;
}
.dialogue li {
  max-width: 82%;
  margin: 0;
  padding: 12px 18px 14px;
  border-radius: 20px 20px 20px 6px;
  background: var(--surface-2);
}
.dialogue li:nth-child(even) {
  align-self: flex-end;
  border-radius: 20px 20px 6px 20px;
  background: transparent;
  border: 1px solid var(--border-strong);
}
.dialogue li > strong:first-child {
  display: block;
  margin-bottom: 2px;
  font-size: 12px;
  font-weight: 500;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--muted);
}
.dialogue li em {
  display: block;
  margin-top: 4px;
  font-size: 14px;
  font-style: normal;
  color: var(--muted);
}

/* ---------- Details / reveal ---------- */

.prose details {
  margin: 16px 0;
  border: 1px solid var(--border);
  border-radius: var(--r-md);
}
.prose details[open] { background: var(--surface); border-color: transparent; }
.prose summary {
  position: relative;
  display: block;
  padding: 16px 52px 16px 20px;
  font-weight: 500;
  color: var(--text-strong);
  cursor: pointer;
  list-style: none;
}
.prose summary::-webkit-details-marker { display: none; }
.prose summary::after {
  position: absolute;
  top: 50%;
  right: 20px;
  transform: translateY(-50%);
  content: "+";
  font-size: 22px;
  font-weight: 400;
  line-height: 1;
  color: var(--muted);
}
.prose details[open] summary::after { content: "−"; }
.prose details > :not(summary) { margin-left: 20px; margin-right: 20px; }
.prose details > :last-child { margin-bottom: 18px; }

/* ---------- Forms ---------- */

.input {
  width: 100%;
  max-width: 420px;
  height: 52px;
  padding: 0 22px;
  border: 1px solid var(--border);
  border-radius: var(--r-pill);
  background: var(--bg);
  color: var(--text-strong);
  font: inherit;
  transition: border-color 0.15s ease;
}
.input::placeholder { color: var(--muted); }
.input:hover { border-color: var(--border-strong); }
.input:focus { outline: none; border-color: var(--text-strong); }

.choices {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  margin: 16px 0;
}
.choice {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  height: 44px;
  padding: 0 20px;
  border: 1px solid var(--border-strong);
  border-radius: var(--r-pill);
  cursor: pointer;
  transition: background-color 0.15s ease, color 0.15s ease, border-color 0.15s ease;
}
.choice input { position: absolute; opacity: 0; pointer-events: none; }
.choice:hover { background: var(--surface-2); }
.choice:has(input:checked) { background: var(--btn-bg); border-color: var(--btn-bg); color: var(--btn-fg); }
.choice:has(input:focus-visible) { outline: 2px solid var(--focus); outline-offset: 2px; }

/* ---------- Drill components (<tone-drill>, <word-drill>, <minimal-pair>) ---------- */

tone-drill,
word-drill,
minimal-pair,
say-it {
  display: block;
  margin: 28px 0;
  padding: 24px;
  border-radius: var(--r-card);
  background: var(--surface);
}

.drill-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(84px, 1fr));
  gap: 10px;
}
minimal-pair .drill-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }

.drill-cell {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 4px;
  min-height: 100px;
  padding: 16px 8px;
  border: 1px solid transparent;
  border-radius: 18px;
  background: var(--bg);
  text-align: center;
  transition: border-color 0.15s ease, background-color 0.15s ease;
}
minimal-pair .drill-cell { min-height: 132px; }
/* In practice grids the chip always sits under the word, never wrapping unpredictably. */
.drill-cell .say {
  display: inline-flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  font-size: 28px;
  line-height: 1.2;
}
.drill-cell .say-icon { margin-left: 0; top: 0; width: 30px; height: 30px; }
minimal-pair .drill-cell .say { font-size: 36px; }
.drill-label { font-size: 13px; color: var(--muted); }

.is-quiz .drill-cell { cursor: pointer; }
.is-quiz .drill-cell:hover { border-color: var(--border-strong); }
.drill-cell.is-correct { border-color: var(--good); background: var(--good-bg); }
.drill-cell.is-wrong { border-color: var(--bad); background: var(--bad-bg); }

.drill-bar {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 10px;
  margin-top: 16px;
}
.drill-status { font-size: 14px; color: var(--muted); }
.drill-status.is-good { color: var(--good); }
.drill-status.is-bad { color: var(--bad); }

/* ---------- Tone checker (<say-it>) ---------- */

.sayit-head {
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  gap: 8px 16px;
  margin-bottom: 12px;
}
.sayit-head .say { font-size: 32px; line-height: 1.2; }
.sayit-tone { font-size: 15px; color: var(--muted); }
.sayit-tone strong { color: var(--text-strong); font-weight: 500; }

.sayit-chart {
  display: block;
  width: 100%;
  height: auto;
  border-radius: 18px;
  background: var(--bg);
}
.sayit-grid { stroke: var(--border); stroke-width: 1; }
.sayit-axis { fill: var(--muted); font-size: 12px; font-family: var(--font-sans); }
.sayit-target-band { fill: none; stroke: var(--surface-3); stroke-width: 22; stroke-linecap: round; stroke-linejoin: round; }
.sayit-target { fill: none; stroke: var(--muted); stroke-width: 2; stroke-dasharray: 5 6; stroke-linecap: round; }
.sayit-mine { fill: none; stroke: var(--text-strong); stroke-width: 3; stroke-linecap: round; stroke-linejoin: round; }

/* The Record button glows with your voice level while listening. */
say-it .btn-primary { box-shadow: 0 0 0 calc(var(--level, 0) * 8px) var(--surface-3); transition: box-shadow 0.08s linear, background-color 0.15s ease; }

say-it .sayit-foot { margin: 24px 0 0; font-size: 13px; color: var(--muted); }
.sayit-link {
  padding: 0;
  border: 0;
  background: none;
  font: inherit;
  color: var(--text-strong);
  text-decoration: underline;
  text-underline-offset: 3px;
  cursor: pointer;
}

/* ---------- Tone chart (<tone-chart>) ---------- */

tone-chart {
  display: block;
  margin: 28px 0;
  padding: 24px;
  border-radius: var(--r-card);
  background: var(--surface);
}
.tonechart-line { fill: none; stroke: var(--border-strong); stroke-width: 3; stroke-linecap: round; stroke-linejoin: round; transition: stroke 0.15s ease, stroke-width 0.15s ease; }
.tonechart-line.is-active { stroke: var(--text-strong); stroke-width: 4.5; }
.tonechart-label { fill: var(--muted); font-size: 13px; font-family: var(--font-sans); font-weight: 500; }
.tonechart-label.is-active { fill: var(--text-strong); }
.tonechart-words {
  display: flex;
  flex-wrap: wrap;
  gap: 8px 24px;
  margin-top: 16px;
  font-size: 22px;
}
.tonechart-word { display: flex; flex-direction: column; gap: 2px; padding: 6px 10px; border-radius: 14px; cursor: pointer; }
.tonechart-word.is-active { background: var(--surface-3); }
.tonechart-word .caption { font-size: 13px; }

/* ---------- Sentence builder (<sentence-builder>) ---------- */

sentence-builder {
  display: block;
  margin: 24px 0;
  padding: 24px;
  border-radius: var(--r-card);
  background: var(--surface);
}
.sb-top { display: flex; align-items: flex-start; gap: 16px; }
.sb-play .say-icon { width: 44px; height: 44px; margin: 4px 0 0; top: 0; }
.sb-words { min-width: 0; }
.prose .sb-sentence {
  margin: 0;
  font-family: var(--font-serif);
  font-size: 28px;
  line-height: 1.6;
  color: var(--text-strong);
}
.sb-chip {
  margin: 0 1px;
  padding: 0 10px;
  border: 1px solid var(--border-strong);
  border-radius: 12px;
  background: var(--bg);
  color: inherit;
  font: inherit;
  line-height: 1.35;
  cursor: pointer;
  transition: background-color 0.15s ease, border-color 0.15s ease;
}
.sb-chip:hover, .sb-chip.is-hot { border-color: var(--text-strong); background: var(--surface-3); }
.prose .sb-english { margin: 2px 0 0; font-size: 16px; color: var(--muted); }

.sb-rows { display: grid; gap: 10px; margin-top: 20px; padding-top: 18px; border-top: 1px solid var(--border); }
.sb-row { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; }
.sb-label {
  min-width: 76px;
  font-size: 12px;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}
.sb-option {
  display: inline-flex;
  align-items: baseline;
  gap: 6px;
  min-height: 36px;
  padding: 6px 14px;
  border: 1px solid var(--border-strong);
  border-radius: var(--r-pill);
  background: transparent;
  color: var(--text-strong);
  font: inherit;
  font-size: 15px;
  cursor: pointer;
  transition: background-color 0.15s ease, color 0.15s ease, border-color 0.15s ease;
}
.sb-option small { font-size: 12px; color: var(--muted); }
.sb-option:hover { background: var(--surface-2); }
.sb-option[aria-pressed="true"] { background: var(--btn-bg); border-color: var(--btn-bg); color: var(--btn-fg); }
.sb-option[aria-pressed="true"] small { color: inherit; opacity: 0.7; }
.sb-shuffle { justify-self: start; margin-top: 4px; }

/* ---------- Math (MathML, converted from LaTeX at build time) ---------- */

math {
  color: var(--text-strong);
  font-family: "STIX Two Math", "Cambria Math", "Latin Modern Math", math;
  font-size: 1.05em;
}
math mtext { font-family: var(--font-serif); }
.math-display {
  margin: 14px 0;
  padding: 2px 0;
  overflow-x: auto;
  overflow-y: hidden;
  scrollbar-width: thin; /* only long formulas on narrow screens should ever scroll */
}
.math-display math { font-size: 1.12em; }
/* Piecewise definitions (cases) read left to right. */
.math-display mtd { text-align: left; padding: 0.15em 0.5em; }

/* ---------- Cheat sheet ---------- */

.sheet-card {
  margin: 20px 0;
  padding: 24px 28px;
  border-radius: var(--r-card);
  background: var(--surface);
}
.prose .sheet-card h3 { margin: 0 0 8px; font-size: 20px; }
.sheet-card > :last-child { margin-bottom: 0; }
.sheet-card .table-wrap { background: var(--bg); }

.chips {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 18px 0 0;
}
.chip {
  display: inline-flex;
  align-items: center;
  height: 32px;
  padding: 0 14px;
  border-radius: var(--r-pill);
  background: var(--surface-3);
  color: var(--text-strong);
  font-size: 13px;
  font-weight: 500;
  text-decoration: none;
  transition: background-color 0.15s ease;
}
.chip:hover { background: var(--border-strong); }
.chip::after { content: " →"; white-space: pre; }

/* ---------- Pager ---------- */

.pager {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 16px;
  margin-top: 80px;
  padding-top: 32px;
  border-top: 1px solid var(--border);
}
.pager:empty { display: none; }

.pager-link {
  display: flex;
  align-items: center;
  gap: 16px;
  min-width: 0;
  padding: 20px 24px;
  border-radius: 20px;
  background: var(--surface);
  color: var(--text-strong);
  text-decoration: none;
  transition: background-color 0.15s ease;
}
.pager-link:hover { background: var(--surface-2); }
.pager-prev { grid-column: 1; }
.pager-next { grid-column: 2; justify-content: flex-end; text-align: right; }

.pager-text { display: flex; flex-direction: column; gap: 4px; min-width: 0; }
.pager-label {
  font-size: 12px;
  font-weight: 500;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}
.pager-title {
  overflow: hidden;
  font-family: var(--font-serif);
  font-size: 22px;
  line-height: 1.25;
  letter-spacing: -0.01em;
  text-overflow: ellipsis;
  white-space: nowrap;
}

.pager-arrow {
  display: grid;
  flex-shrink: 0;
  place-items: center;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  background: var(--surface-3);
  transition: background-color 0.15s ease, color 0.15s ease, transform 0.15s ease;
}
.pager-link:hover .pager-arrow { background: var(--btn-bg); color: var(--btn-fg); }
.pager-prev:hover .pager-arrow { transform: translateX(-3px); }
.pager-next:hover .pager-arrow { transform: translateX(3px); }

/* ---------- Style guide helpers ---------- */

.swatches {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
  gap: 12px;
  margin: 24px 0;
}
.swatch {
  overflow: hidden;
  border: 1px solid var(--border);
  border-radius: var(--r-md);
  font-size: 13px;
}
.swatch span { display: block; height: 64px; }
.swatch code { display: block; padding: 10px 12px; background: none; }

.state-row {
  display: flex;
  flex-wrap: wrap;
  gap: 32px;
  margin: 24px 0;
  font-size: 20px;
}
.state-row > div { display: flex; flex-direction: column; gap: 6px; }
.state-row .caption { font-size: 13px; }

/* ---------- Small screens ---------- */

@media (max-width: 900px) {
  .layout { display: block; }
  .menu-btn { display: inline-grid; }

  .sidebar {
    position: fixed;
    top: var(--header-h);
    left: 0;
    bottom: 0;
    z-index: 20;
    width: min(320px, 86vw);
    height: auto;
    background: var(--bg);
    border-right: 1px solid var(--border);
    transform: translateX(-100%);
    transition: transform 0.2s ease;
  }
  .nav-open .sidebar { transform: none; }

  .nav-backdrop {
    position: fixed;
    inset: var(--header-h) 0 0 0;
    z-index: 10;
    background: rgb(0 0 0 / 0.25);
  }

  .main { padding: 40px 24px 80px; }
}

@media (max-width: 600px) {
  .topbar { padding: 0 16px; }
  .prose .toc-sections { margin-left: 0; }
  .prose .toc-chapter { padding-bottom: 12px; }
  sentence-builder { padding: 16px; }
  .prose .sb-sentence { font-size: 22px; }
  .sb-label { min-width: 100%; }
  .toc-name { font-size: 21px; }
  .main { padding: 32px 16px 64px; }
  h1 { font-size: 40px; }
  .prose h2 { font-size: 30px; margin-top: 56px; }
  .prose th, .prose td { padding: 12px 14px; }
  .tip, .south, .warn, .sheet-card { padding: 20px; }
  tone-drill, word-drill, minimal-pair, say-it, tone-chart { padding: 16px; }
  .card { padding: 24px; }
  .pager { grid-template-columns: 1fr; }
  .pager-next { grid-column: 1; }
  .pager-link { padding: 16px 18px; }
  .pager-title { font-size: 19px; }
  .dialogue li { max-width: 92%; }
}

@media (max-width: 400px) {
  .topbar-tag { display: none; }
  .wordmark { font-size: 22px; }
}

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