.textbook-main { min-height: 620px; }
.textbook-view[hidden] { display: none; }
.textbook-context { display: flex; flex-wrap: wrap; align-items: center; gap: 7px; min-height: 34px; margin: 0 0 16px; }
.textbook-context a,
.textbook-context span { display: inline-flex; min-height: 32px; align-items: center; border: 1px solid var(--line); border-radius: 999px; background: var(--surface); color: var(--ink); padding: 5px 11px; font-size: .72rem; font-weight: 800; text-decoration: none; }
.textbook-context a:hover,
.textbook-context a:focus-visible { border-color: var(--mint-strong); background: var(--mint); outline: 0; }
.textbook-context i { color: var(--muted); font-style: normal; }
.textbook-tiles { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 16px; }
.textbook-book-tiles { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.textbook-tile {
  position: relative;
  display: flex;
  min-height: 210px;
  flex-direction: column;
  justify-content: flex-end;
  border: 1px solid var(--line);
  border-radius: 27px;
  background: linear-gradient(145deg, var(--surface-strong), var(--mint));
  box-shadow: var(--shadow);
  color: var(--ink);
  padding: clamp(22px, 4vw, 34px);
  text-decoration: none;
  transition: transform .18s ease, border-color .18s ease;
}
.textbook-tile.is-complete { border-color: var(--learning-correct); box-shadow: 0 16px 38px color-mix(in srgb, var(--learning-correct) 18%, transparent); }
.chapter-complete-mark {
  position: absolute;
  top: 18px;
  right: 18px;
  display: grid;
  width: 42px;
  height: 42px;
  place-items: center;
  border: 2px solid var(--learning-correct);
  border-radius: 50%;
  background: var(--learning-correct-bg);
  box-shadow: 0 8px 22px color-mix(in srgb, var(--learning-correct) 24%, transparent);
  color: var(--learning-correct);
  font-size: 1.35rem;
  font-weight: 950;
}
.chapter-complete-mark[hidden] { display: none; }
.textbook-tile:hover,
.textbook-tile:focus-visible { border-color: var(--mint-strong); transform: translateY(-3px); outline: 0; }
.textbook-tile[aria-disabled="true"] { cursor: default; opacity: .7; transform: none; }
.textbook-tile .tile-icon { display: grid; width: 56px; height: 56px; place-items: center; margin: 0 0 auto; border-radius: 17px; background: var(--surface); color: var(--mint-strong); font-size: 1.75rem; font-weight: 900; }
.textbook-tile strong { margin-top: 30px; font: 600 clamp(1.35rem, 2.6vw, 2rem)/1.15 Georgia, serif; }
.textbook-tile small { margin-top: 8px; color: var(--muted); font-weight: 700; }
.chapter-identity { display: grid; justify-items: start; margin-bottom: 20px; border: 1px solid var(--line); border-radius: 24px; background: var(--surface); padding: clamp(22px, 4vw, 34px); }
.chapter-identity > span { color: var(--mint-strong); font-size: .75rem; font-weight: 900; letter-spacing: .1em; text-transform: uppercase; }
.chapter-identity h2 { margin: 8px 0; }
.chapter-identity small { color: var(--muted); }
.breakdown-toolbar { display: flex; align-items: end; justify-content: space-between; gap: 16px; margin-bottom: 16px; }
.breakdown-toolbar h2 { margin-bottom: 0; }
.breakdown-toolbar > div:last-child { display: flex; flex-wrap: wrap; gap: 7px; }
.breakdown-toolbar .button { min-height: 40px; padding-inline: 15px; font-size: .75rem; }
.breakdown-sections { display: grid; gap: 12px; }
.breakdown-section { overflow: hidden; border: 1px solid var(--line); border-radius: 22px; background: var(--surface); box-shadow: 0 10px 34px rgba(37, 62, 44, .07); }
.breakdown-section > summary { display: grid; grid-template-columns: minmax(0, 1fr) auto; gap: 16px; align-items: center; cursor: pointer; padding: 20px clamp(20px, 4vw, 30px); list-style: none; }
.breakdown-section > summary::-webkit-details-marker { display: none; }
.breakdown-section > summary::after { content: "+"; display: grid; width: 34px; height: 34px; place-items: center; border-radius: 50%; background: var(--mint); color: var(--ink); font-weight: 900; }
.breakdown-section[open] > summary::after { content: "−"; }
.breakdown-section h3 { margin: 0; font: 600 clamp(1.05rem, 2vw, 1.45rem)/1.25 Georgia, serif; }
.breakdown-section summary small { display: block; margin-top: 5px; color: var(--muted); font-size: .68rem; font-weight: 800; }
.breakdown-content { display: grid; grid-template-columns: minmax(0, 1fr) minmax(300px, .72fr); gap: 24px; border-top: 1px solid var(--line); padding: clamp(20px, 4vw, 30px); }
.breakdown-content.no-figure { grid-template-columns: 1fr; }
.breakdown-summary { color: var(--mint-strong); font-weight: 800; line-height: 1.6; }
.breakdown-content ul { margin: 18px 0 0; padding-left: 20px; }
.breakdown-content li { margin: 0 0 10px; line-height: 1.58; }
.textbook-figures { display: grid; align-content: start; gap: 12px; }
.textbook-figure { margin: 0; overflow: hidden; border: 1px solid var(--line); border-radius: 16px; background: #fff; }
.textbook-figure img { display: block; width: 100%; height: auto; max-height: 560px; object-fit: contain; }
.textbook-figure figcaption { border-top: 1px solid var(--line); color: #465149; padding: 10px 12px; font-size: .72rem; font-weight: 700; }
.quiz-stat { display: inline-flex; align-items: baseline; gap: 8px; margin: 16px 0 24px; }
.quiz-stat strong { font-size: 2.3rem; }
.quiz-stat span { color: var(--muted); font-weight: 800; }
#quiz-setup .study-setup-panel > .button { margin: 4px 7px 0 0; }
:root[data-theme="dark"] .textbook-tile { background: linear-gradient(145deg, var(--surface), var(--mint)); }
:root[data-theme="dark"] .textbook-figure { background: #f7f7f4; }

@media (max-width: 820px) {
  .textbook-tiles,
  .textbook-book-tiles { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .chapter-tool-tiles > :last-child { grid-column: 1 / -1; }
  .breakdown-content { grid-template-columns: 1fr; }
}

@media (max-width: 560px) {
  .textbook-app-header h1 { font-size: 2rem; }
  .textbook-tiles,
  .textbook-book-tiles { grid-template-columns: 1fr; }
  .chapter-tool-tiles > :last-child { grid-column: auto; }
  .textbook-tile { min-height: 170px; }
  .breakdown-toolbar { align-items: stretch; flex-direction: column; }
  .breakdown-toolbar > div:last-child .button { flex: 1; }
  .breakdown-section > summary { padding: 17px; }
  .breakdown-content { padding: 17px; }
}

@media (prefers-reduced-motion: reduce) {
  .textbook-tile { transition: none; }
}