/* ==========================================================================
   OgbonMath — Lesson / Section Navigation Cards
   --------------------------------------------------------------------------
   Card-based "Previous / Up next" pattern at the bottom of every tutor
   lesson and every textbook section. Modeled on Stripe / Linear / Notion
   docs nav: each side is a full card with destination context, animated
   arrow, and a distinctive treatment when the destination is in a
   different module (tutor) or chapter (textbook).

   Used by renderLessonNavCards() in app.js. Click dispatch by data
   attribute: data-lesson-prev / data-lesson-next → loadLesson(),
   data-tb-prev / data-tb-next → renderTextbookSection().
   ========================================================================== */

.lesson-nav {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--sp-4);
  margin: var(--sp-8) 0 var(--sp-6);
  padding-top: var(--sp-6);
  border-top: 1px solid var(--bdr);
  max-width: 920px;
  margin-left: auto;
  margin-right: auto;
}

.lesson-nav-card {
  display: flex;
  flex-direction: column;
  gap: 6px;
  padding: var(--sp-5) var(--sp-6);
  background: var(--bg2);
  border: 1px solid var(--bdr);
  border-radius: var(--radius-lg);
  cursor: pointer;
  text-align: left;
  font-family: inherit;
  color: var(--tx);
  min-height: 96px;
  transition: transform var(--dur-fast) var(--ease-out),
              border-color var(--dur-fast) var(--ease-out),
              box-shadow var(--dur-fast) var(--ease-out),
              background var(--dur-fast) var(--ease-out);
}

.lesson-nav-card:hover {
  transform: translateY(-2px);
  border-color: var(--ac);
  box-shadow: var(--shadow-lg);
  background: var(--bg3);
}

.lesson-nav-card:focus-visible {
  outline: 3px solid var(--ac-ring);
  outline-offset: 2px;
}

/* Right-justify the "next" card so direction-of-travel reads naturally */
.lesson-nav-next {
  text-align: right;
  align-items: flex-end;
}

.lesson-nav-empty {
  /* Holds the grid slot when there's no prev/next — no border, no card. */
  background: transparent;
  border: 1px solid transparent;
  cursor: default;
  pointer-events: none;
  min-height: 0;
}

/* --- Direction label ("Previous" / "Up next") with animated arrow ------- */
.lesson-nav-direction {
  font-family: var(--font-sans);
  font-size: 11px;
  font-weight: var(--fw-semibold);
  letter-spacing: 1.5px;
  text-transform: uppercase;
  color: var(--tx3);
}

.lesson-nav-arrow {
  display: inline-block;
  font-weight: var(--fw-semibold);
  transition: transform var(--dur-fast) var(--ease-out);
}

.lesson-nav-prev:hover .lesson-nav-arrow {
  transform: translateX(-4px);
}

.lesson-nav-next:hover .lesson-nav-arrow {
  transform: translateX(4px);
}

/* --- Module/chapter context (only shown when crossing a boundary) ------- */
.lesson-nav-context {
  font-family: var(--font-sans);
  font-size: 11px;
  font-weight: var(--fw-semibold);
  color: var(--ac);
  letter-spacing: 0.4px;
  text-transform: uppercase;
}

/* --- Destination title — display font for premium feel ------------------ */
.lesson-nav-title {
  font-family: var(--font-display, var(--font-sans));
  font-size: var(--fs-md, 16px);
  font-weight: var(--fw-semibold);
  color: var(--tx);
  letter-spacing: var(--ls-tight, -0.01em);
  line-height: 1.3;
  display: -webkit-box;
  -webkit-line-clamp: 2;
  line-clamp: 2;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

/* --- Boundary-crossing treatment ----------------------------------------
   When the next/prev destination lives in a different module/chapter, the
   card gets an accent border-left and a soft accent gradient so the user
   sees they're moving into new territory. */
.lesson-nav-card.is-boundary {
  border-left: 4px solid var(--ac);
  background: linear-gradient(135deg, var(--ac-soft) 0%, var(--bg2) 70%);
  padding-left: calc(var(--sp-6) - 3px);
}

.lesson-nav-card.is-boundary:hover {
  background: linear-gradient(135deg, var(--ac-soft) 0%, var(--bg3) 70%);
}

/* When the boundary card is on the "next" side, the visual accent on the
   left edge feels backward (moving forward, accent on the trailing side).
   Mirror it. */
.lesson-nav-next.is-boundary {
  border-left: 1px solid var(--bdr);
  border-right: 4px solid var(--ac);
  padding-left: var(--sp-6);
  padding-right: calc(var(--sp-6) - 3px);
}

/* --- Mobile: stack cards vertically; always-left text alignment for
   readability since right-justified text on narrow screens reads awkwardly. */
@media (max-width: 640px) {
  .lesson-nav {
    grid-template-columns: 1fr;
    gap: var(--sp-3);
    margin-top: var(--sp-6);
  }
  .lesson-nav-card {
    padding: var(--sp-4);
    min-height: auto;
  }
  .lesson-nav-next {
    text-align: left;
    align-items: flex-start;
  }
  .lesson-nav-empty {
    display: none;
  }
}