.lesson-header {
  margin-bottom: var(--space-4);
  padding: var(--space-5);
  border-top: 8px solid var(--strand-color);
  border-radius: var(--radius-lg);
  background: var(--color-surface);
  box-shadow: var(--shadow-soft);
}

.lesson-header__eyebrow {
  display: flex;
  align-items: center;
  margin: 0 0 var(--space-2);
  color: var(--color-text-muted);
  font-weight: 700;
}

.lesson-header__title {
  margin-bottom: var(--space-4);
}

.lesson-header__row {
  display: flex;
  flex-wrap: wrap;
  align-items: flex-start;
  gap: var(--space-3);
}

.lesson-header__row .mascot-bubble {
  flex: 1 1 320px;
}

.lesson-header__sgk {
  flex: none;
}

.lesson-tabs {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-2);
  margin-bottom: var(--space-5);
}

.lesson-tabs__item {
  display: inline-flex;
  align-items: center;
  gap: var(--space-1);
  min-height: var(--touch-target);
  padding: 0 var(--space-4);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-pill);
  background: var(--color-surface);
  color: var(--color-text);
  font-weight: 700;
  text-decoration: none;
}

a.lesson-tabs__item:hover {
  border-color: var(--strand-color, var(--color-primary));
}

.lesson-tabs__item.is-active {
  border-color: var(--strand-color, var(--color-primary));
  background: var(--color-surface-muted);
}

.lesson-tabs__item.is-locked {
  color: var(--color-text-muted);
  cursor: help;
}

.lesson-layout {
  display: grid;
  grid-template-columns: minmax(0, 1fr);
  gap: var(--space-5);
}

.lesson-toc {
  padding: var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
}

.lesson-toc__heading {
  margin: 0 0 var(--space-2);
  color: var(--color-text-muted);
  font-size: var(--font-size-sm);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.lesson-toc ul {
  margin: 0;
  padding: 0;
  list-style: none;
}

.lesson-toc a {
  display: block;
  padding: var(--space-1) 0;
  color: var(--color-text);
  text-decoration: none;
}

.lesson-toc a:hover {
  color: var(--color-primary);
}

.lesson-toc__level-3 {
  padding-left: var(--space-4);
  font-size: var(--font-size-sm);
}

.lesson-content .prose {
  max-width: 75ch;
}

/* Thuật ngữ gạch chân trong bài, bấm vào để xem định nghĩa (public/js/core/glossary-popover.js). */
.glossary-term {
  padding: 0;
  border: none;
  background: none;
  color: inherit;
  font: inherit;
  text-decoration: underline;
  text-decoration-style: dotted;
  text-decoration-color: var(--color-primary);
  text-underline-offset: 3px;
  cursor: pointer;
}

.glossary-popover {
  position: absolute;
  z-index: 20;
  max-width: min(320px, 80vw);
  padding: var(--space-3);
  border: 2px solid var(--color-primary);
  border-radius: var(--radius-md);
  background: var(--color-surface);
  box-shadow: var(--shadow-lift);
  font-size: var(--font-size-sm);
}

/* Câu hỏi tự kiểm tra dạng lật thẻ (tab Kiến thức). */
.self-check-card {
  margin: var(--space-3) 0;
  padding: var(--space-4);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
}

.self-check-card__prompt {
  margin-bottom: var(--space-2);
}

.self-check-card__reveal summary {
  display: inline-flex;
  align-items: center;
  min-height: var(--touch-target);
  padding: 0 var(--space-4);
  border: 2px solid var(--color-primary);
  border-radius: var(--radius-pill);
  color: var(--color-primary);
  font-weight: 700;
  cursor: pointer;
}

.self-check-card__reveal[open] summary {
  margin-bottom: var(--space-3);
  background: var(--color-surface-muted);
}

@media (min-width: 900px) {
  .lesson-layout {
    grid-template-columns: 220px minmax(0, 1fr);
    align-items: start;
  }

  .lesson-toc {
    position: sticky;
    top: calc(61px + var(--space-4));
  }
}

@media (max-width: 560px) {
  .lesson-header {
    padding: var(--space-4) var(--space-3);
  }

  .lesson-tabs {
    flex-wrap: nowrap;
    overflow-x: auto;
  }
}