/* ── Design tokens ──────────────────────────────────────────────────────── */
/* Default accent = blue (Lambda Calculus). Subject pages override :root    */
/* for their own theme (e.g. Term Functor Logic uses purple).               */

:root {
  --accent:         #3b82f6;
  --accent-hover:   #2563eb;
  --accent-code:    #1d4ed8;
  --accent-bg:      #eff6ff;
  --accent-border:  #bfdbfe;
  --accent-text:    #1e40af;
  --accent-code-bg: #dbeafe;
  --page-bg:        #f0f4f8;

  --dark:           #0f172a;
  --text:           #1e293b;
  --text-body:      #334155;
  --text-secondary: #475569;
  --muted:          #64748b;
  --border:         #e2e8f0;

  --green:          #22c55e;
  --green-dark:     #16a34a;
  --green-bg:       #f0fdf4;
  --green-text:     #166534;
  --red:            #f87171;
  --red-dark:       #dc2626;
  --red-bg:         #fff1f1;
}

/* ── Reset & base ──────────────────────────────────────────────────────── */

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  background: var(--page-bg);
  color: var(--text);
  line-height: 1.65;
  min-height: 100vh;
}

code {
  font-family: 'Courier New', Courier, monospace;
  background: var(--accent-bg);
  color: var(--accent-code);
  padding: 0.1em 0.4em;
  border-radius: 4px;
  font-size: 0.92em;
}

/* ── Layout ────────────────────────────────────────────────────────────── */

#app {
  max-width: 720px;
  margin: 0 auto;
  padding: 0 1.25rem 4rem;
}

/* ── Header ────────────────────────────────────────────────────────────── */

header {
  text-align: center;
  padding: 3rem 0 2.5rem;
  border-bottom: 2px solid var(--border);
  margin-bottom: 2.5rem;
}

.header-icon {
  font-size: 3.5rem;
  line-height: 1;
  color: var(--accent);
  font-weight: 900;
  margin-bottom: 0.5rem;
  letter-spacing: -0.05em;
}

header h1 {
  font-size: 2rem;
  font-weight: 800;
  letter-spacing: -0.025em;
  color: var(--dark);
}

header p {
  color: var(--muted);
  font-size: 1rem;
  margin-top: 0.35rem;
}

/* ── Lesson navigation ─────────────────────────────────────────────────── */

#lesson-nav {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.4rem;
  margin-bottom: 1.75rem;
}

.nav-lesson {
  display: flex;
  flex-direction: row;
  align-items: center;
  gap: 0.5rem;
  padding: 0.4rem 0.8rem;
  border-radius: 8px;
  border: 1.5px solid var(--border);
  background: white;
  cursor: pointer;
  text-align: left;
  transition: border-color 0.15s, background 0.15s, transform 0.1s;
}

.nav-lesson:hover:not(.active) {
  border-color: var(--accent-border);
  background: var(--accent-bg);
}

.nav-lesson:active { transform: scale(0.97); }

.nav-lesson.active {
  background: var(--accent);
  border-color: var(--accent);
}

.nav-lesson-num {
  font-size: 0.65rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: #94a3b8;
}

.nav-lesson.active .nav-lesson-num { color: rgba(255,255,255,0.7); }

.nav-lesson-title {
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--text);
}

.nav-lesson.active .nav-lesson-title { color: white; }

.nav-check {
  font-size: 0.65rem;
  font-weight: 700;
  color: var(--green-dark);
  margin-left: auto;
  flex-shrink: 0;
}

.nav-lesson.active .nav-check { color: rgba(255,255,255,0.8); }

/* ── Lesson header ─────────────────────────────────────────────────────── */

.lesson-header { margin-bottom: 1.75rem; }

.lesson-header h2 {
  font-size: 1.6rem;
  font-weight: 800;
  color: var(--dark);
  letter-spacing: -0.02em;
}

.lesson-desc {
  color: var(--muted);
  margin-top: 0.2rem;
}

/* ── Blocks (shared) ───────────────────────────────────────────────────── */

.block {
  background: #ffffff;
  border-radius: 14px;
  padding: 1.75rem 2rem;
  margin-bottom: 1.25rem;
  border: 1px solid var(--border);
  box-shadow: 0 1px 4px rgba(0, 0, 0, 0.06);
  animation: fadeUp 0.3s ease-out both;
}

.block--hidden { display: none; }

@keyframes fadeUp {
  from { opacity: 0; transform: translateY(14px); }
  to   { opacity: 1; transform: translateY(0); }
}

.block-title {
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--dark);
  margin-bottom: 1rem;
}

/* ── Concept block content ─────────────────────────────────────────────── */

.block-content p {
  color: var(--text-body);
  margin-bottom: 0.75rem;
}

.block-content ul,
.block-content ol {
  color: var(--text-body);
  margin: 0.5rem 0 0.75rem 1.5rem;
}

.block-content li { margin-bottom: 0.3rem; }

/* Parts list (abstraction breakdown) */
.parts-list {
  list-style: none;
  margin: 0.5rem 0 0.75rem 0;
  padding: 0;
}

.parts-list li {
  padding: 0.25rem 0;
  color: var(--text-body);
  display: flex;
  gap: 0.5rem;
  align-items: baseline;
}

/* Syntax display box */
.syntax-box {
  background: var(--accent-bg);
  border: 1px solid var(--accent-border);
  border-left: 4px solid var(--accent);
  border-radius: 0 8px 8px 0;
  padding: 0.7rem 1rem;
  margin: 0.9rem 0;
  font-family: 'Courier New', Courier, monospace;
  font-size: 1.2rem;
  color: var(--accent-code);
}

.syntax-box code {
  background: none;
  padding: 0;
  font-size: inherit;
  color: inherit;
}

.syntax-box table {
  border-collapse: collapse;
  font-size: 1rem;
  width: 100%;
}

.syntax-box td {
  padding: 0.2rem 0.75rem 0.2rem 0;
  vertical-align: baseline;
}

.syntax-box td:first-child {
  font-weight: 700;
  min-width: 7rem;
}

.syntax-box td:last-child {
  color: var(--muted);
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  font-size: 0.88rem;
}

/* Info boxes */
.callout-note {
  background: var(--accent-bg);
  border: 1px solid var(--accent-border);
  border-left: 4px solid var(--accent);
  border-radius: 0 8px 8px 0;
  padding: 0.65rem 1rem;
  margin-top: 1rem;
  font-size: 0.88rem;
  color: var(--accent-text);
  display: flex;
  gap: 0.65rem;
  align-items: baseline;
  flex-wrap: wrap;
}

.grammar-rule {
  background: #f0fdf4;
  border: 1px solid #bbf7d0;
  border-left: 4px solid var(--green);
  border-radius: 0 8px 8px 0;
  padding: 0.65rem 1rem;
  margin-top: 1rem;
  font-size: 0.88rem;
  color: var(--green-text);
  display: flex;
  gap: 0.65rem;
  align-items: baseline;
  flex-wrap: wrap;
}

/* Box label chips */
.cn-label,
.g-label {
  font-weight: 700;
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: white;
  padding: 0.1em 0.45em;
  border-radius: 4px;
  flex-shrink: 0;
}

.cn-label { background: var(--accent); }
.g-label  { background: var(--green-dark); }

.callout-note code {
  background: var(--accent-code-bg);
  color: var(--accent-text);
  padding: 0.05em 0.25em;
  border-radius: 3px;
}

.grammar-rule code {
  background: #dcfce7;
  color: var(--green-text);
}

/* Example table */
.ex-table {
  margin: 0.75rem 0;
  display: flex;
  flex-direction: column;
  gap: 0.45rem;
}

.ex-row {
  display: flex;
  align-items: baseline;
  gap: 1.25rem;
}

.ex-row code {
  font-size: 1rem;
  min-width: 9rem;
  flex-shrink: 0;
}

.ex-row span {
  color: var(--muted);
  font-size: 0.88rem;
}

.ex-row span code { font-size: 0.88em; }

/* Section divider inside concept blocks */
.section-divider {
  border: none;
  border-top: 1px solid var(--border);
  margin: 1.25rem 0;
}

/* ── Proof box ─────────────────────────────────────────────────────────── */

.proof-box {
  background: #fafafa;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.85rem 1.2rem;
  margin: 0.9rem 0;
  font-family: 'Courier New', Courier, monospace;
}

.proof-row {
  display: flex;
  align-items: baseline;
  gap: 1.5rem;
  padding: 0.15rem 0;
}

.proof-row code {
  font-size: 1rem;
  min-width: 8rem;
  background: none;
  color: var(--accent-code);
  padding: 0;
}

.proof-row .proof-note {
  color: var(--muted);
  font-size: 0.82rem;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  font-style: italic;
}

.proof-divider {
  border: none;
  border-top: 1.5px solid #94a3b8;
  margin: 0.4rem 0;
}

.proof-row.proof-conclusion code {
  font-weight: 700;
  color: var(--accent);
}

.proof-therefore {
  color: var(--accent);
  font-weight: 700;
  font-size: 1.1rem;
  margin-right: 0.35rem;
}

/* ── Step traces ───────────────────────────────────────────────────────── */

.step-trace {
  margin: 0.9rem 0;
  padding: 0.8rem 1rem;
  background: #f8fafc;
  border: 1px solid var(--border);
  border-radius: 8px;
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
}

.step {
  display: flex;
  align-items: baseline;
  gap: 0.75rem;
  padding-left: 1.4rem;
}

.step:first-child { padding-left: 0; }

.step-reduce { position: relative; }

.step-reduce::before {
  content: '→';
  position: absolute;
  left: 0;
  color: var(--accent);
  font-weight: 700;
}

.step code { font-size: 1rem; }

.step-note {
  color: #94a3b8;
  font-size: 0.82rem;
  font-style: italic;
}

/* ── Syllogism display ─────────────────────────────────────────────────── */

.syllogism-display {
  font-family: 'Courier New', Courier, monospace;
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
  margin-bottom: 0.2rem;
}

.syllogism-display code {
  background: none;
  padding: 0;
  font-size: 1rem;
  color: var(--accent-code);
}

.syl-row {
  display: flex;
  align-items: baseline;
  gap: 1rem;
}

.syl-label {
  color: var(--muted);
  font-size: 0.8rem;
  font-style: italic;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  min-width: 5.5rem;
}

.syl-divider {
  border: none;
  border-top: 1.5px solid #94a3b8;
  margin: 0.25rem 0;
  width: 12rem;
}

.syl-row.syl-conclusion code { font-weight: 700; }

.syl-therefore {
  color: var(--accent);
  font-weight: 700;
}

.syl-text {
  font-size: 0.95rem;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
}

/* ── Buttons (shared base) ─────────────────────────────────────────────── */

.block-footer {
  margin-top: 1.5rem;
  display: flex;
  justify-content: flex-end;
}

.btn-continue,
.btn-next-lesson {
  background: var(--accent);
  color: white;
  border: none;
  border-radius: 8px;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, transform 0.1s;
}

.btn-continue:hover,
.btn-next-lesson:hover  { background: var(--accent-hover); }

.btn-continue:active,
.btn-next-lesson:active { transform: scale(0.97); }

.btn-continue {
  padding: 0.55rem 1.35rem;
  font-size: 0.92rem;
}

/* ── Exercise block ────────────────────────────────────────────────────── */

.exercise-block {
  background: #fafbfc;
  border-color: #d1d9e0;
}

.exercise-title {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  flex-wrap: wrap;
}

.title-badge {
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  background: var(--accent);
  color: white;
  padding: 0.15em 0.5em;
  border-radius: 5px;
  flex-shrink: 0;
}

.title-badge.final { background: #059669; }

.exercise-instruction {
  color: var(--text-secondary);
  font-size: 0.95rem;
  margin-bottom: 1.25rem;
}

/* Exercise items */
.exercise-items {
  display: flex;
  flex-direction: column;
  gap: 0.65rem;
}

.exercise-item {
  border: 1.5px solid var(--border);
  border-radius: 10px;
  padding: 0.9rem 1.1rem;
  background: white;
  transition: border-color 0.2s, background 0.2s;
}

.exercise-item.correct {
  border-color: var(--green);
  background: var(--green-bg);
}

.exercise-item.incorrect {
  border-color: var(--red);
  background: var(--red-bg);
}

.expr-display { margin-bottom: 0.7rem; }

.expr-display code {
  font-size: 1.1rem;
  padding: 0.25em 0.6em;
}

.answer-buttons {
  display: flex;
  gap: 0.65rem;
}

.btn-answer {
  padding: 0.35rem 1rem;
  border-radius: 6px;
  font-size: 0.87rem;
  font-weight: 600;
  cursor: pointer;
  border: 2px solid;
  transition: background 0.12s, transform 0.08s;
}

.btn-answer:active:not(:disabled) { transform: scale(0.96); }

.btn-valid {
  color: var(--green-dark);
  border-color: var(--green);
  background: white;
}

.btn-valid:hover:not(:disabled) { background: var(--green-bg); }

.btn-invalid {
  color: var(--red-dark);
  border-color: var(--red);
  background: white;
}

.btn-invalid:hover:not(:disabled) { background: var(--red-bg); }

.btn-answer:disabled {
  opacity: 0.45;
  cursor: not-allowed;
}

/* Feedback */
.feedback {
  display: none;
  margin-top: 0.65rem;
  padding: 0.45rem 0.75rem;
  border-radius: 6px;
  font-size: 0.87rem;
  gap: 0.45rem;
  align-items: flex-start;
}

.feedback.visible { display: flex; }

.feedback-correct  { background: #dcfce7; color: var(--green-text); }
.feedback-incorrect { background: #fee2e2; color: #991b1b; }

.fi {
  font-weight: 700;
  flex-shrink: 0;
  margin-top: 0.05em;
}

/* Score and footer */
.exercise-score {
  display: none;
  margin-top: 1.1rem;
  font-size: 0.88rem;
  font-weight: 600;
  color: var(--muted);
}

.exercise-score.visible { display: block; }

.exercise-footer {
  margin-top: 1.1rem;
  padding-top: 1.1rem;
  border-top: 1px solid var(--border);
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  align-items: flex-start;
}

.completion-msg {
  font-size: 0.95rem;
  color: var(--text-secondary);
}

.completion-msg.perfect {
  color: var(--green-dark);
  font-weight: 600;
}

/* Skip button */
.btn-skip {
  background: white;
  color: #94a3b8;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  padding: 0.55rem 1.35rem;
  font-size: 0.92rem;
  font-weight: 500;
  cursor: pointer;
  transition: background 0.15s, transform 0.1s;
}

.btn-skip:hover {
  background: #f8fafc;
  color: var(--muted);
  border-color: #cbd5e1;
}

/* ── Lesson complete ────────────────────────────────────────────────────── */

.lesson-complete {
  margin-top: 1.5rem;
  background: linear-gradient(135deg, var(--accent-bg) 0%, #f0fdf4 100%);
  border: 1px solid var(--accent-border);
  border-radius: 14px;
  padding: 2.5rem;
  text-align: center;
  animation: fadeUp 0.4s ease-out;
}

.complete-icon {
  font-size: 3rem;
  color: var(--accent);
  font-weight: 900;
  line-height: 1;
  margin-bottom: 0.75rem;
}

.lesson-complete h3 {
  font-size: 1.5rem;
  font-weight: 800;
  color: var(--dark);
  margin-bottom: 0.6rem;
}

.lesson-complete p {
  color: var(--text-secondary);
  margin-bottom: 0.45rem;
  font-size: 0.97rem;
}

/* ── Multiple choice exercises ─────────────────────────────────────────── */

.mc-question {
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  gap: 0.4rem;
  margin-bottom: 0.8rem;
}

.mc-prompt {
  color: var(--muted);
  font-size: 0.88rem;
  flex-shrink: 0;
}

.mc-expr {
  font-size: 1.05rem;
  padding: 0.2em 0.5em;
}

.mc-choices {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.5rem;
}

.btn-choice {
  padding: 0.55rem 0.85rem;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  background: white;
  cursor: pointer;
  text-align: left;
  transition: border-color 0.12s, background 0.12s, transform 0.08s;
  font-size: 0.9rem;
  line-height: 1.3;
}

.btn-choice code {
  background: none;
  color: var(--text);
  font-size: 0.97rem;
  padding: 0;
}

.btn-choice:hover:not(:disabled) {
  border-color: var(--accent);
  background: var(--accent-bg);
}

.btn-choice:active:not(:disabled) { transform: scale(0.97); }

.btn-choice.choice-correct {
  border-color: var(--green);
  background: var(--green-bg);
}

.btn-choice.choice-correct code { color: var(--green-dark); }

.btn-choice.choice-wrong {
  border-color: var(--red);
  background: var(--red-bg);
}

.btn-choice.choice-wrong code { color: var(--red-dark); }

.btn-choice:disabled {
  opacity: 0.55;
  cursor: not-allowed;
}

/* ── Next-lesson button ────────────────────────────────────────────────── */

.next-up { font-size: 0.95rem; }

.btn-next-lesson {
  display: inline-block;
  margin-top: 1rem;
  padding: 0.65rem 1.5rem;
  font-size: 1rem;
  font-weight: 700;
}

/* ── Subject catalog (root hub page) ──────────────────────────────────── */

.subject-catalog {
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
  margin-top: 0.5rem;
}

.subject-card {
  display: flex;
  align-items: flex-start;
  gap: 1.5rem;
  background: white;
  border: 2px solid var(--border);
  border-radius: 14px;
  padding: 1.75rem 2rem;
  text-decoration: none;
  color: inherit;
  transition: border-color 0.15s, box-shadow 0.15s, transform 0.1s;
}

.subject-card:hover {
  border-color: var(--accent);
  box-shadow: 0 4px 16px rgba(59, 130, 246, 0.12);
  transform: translateY(-2px);
}

.subject-card-icon {
  font-size: 2.5rem;
  font-weight: 900;
  color: var(--accent);
  line-height: 1;
  flex-shrink: 0;
  padding-top: 0.15rem;
}

.subject-card-body { flex: 1; }

.subject-card-tag {
  display: inline-block;
  font-size: 0.78rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--accent);
  background: var(--accent-bg);
  border-radius: 99px;
  padding: 0.2em 0.75em;
  margin-bottom: 0.75rem;
}

.subject-card-title {
  font-size: 1.4rem;
  font-weight: 800;
  color: var(--dark);
  margin-bottom: 0.45rem;
}

.subject-card-desc {
  font-size: 0.95rem;
  color: var(--text-secondary);
  line-height: 1.6;
  margin-bottom: 1rem;
}

/* Purple accent for the TFL card on the neutral home page */
.subject-card--purple .subject-card-icon,
.subject-card--purple .subject-card-tag,
.subject-card--purple .subject-card-cta {
  color: #7c3aed;
}
.subject-card--purple .subject-card-tag { background: #ede9fe; }
.subject-card--purple:hover {
  border-color: #7c3aed;
  box-shadow: 0 4px 16px rgba(124, 58, 237, 0.12);
}

.subject-card-cta {
  font-size: 0.9rem;
  font-weight: 700;
  color: var(--accent);
}

/* ── Course catalog (subject hub page) ─────────────────────────────────── */

.course-catalog {
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
  margin-top: 0.5rem;
}

.course-card {
  display: block;
  background: white;
  border: 2px solid var(--border);
  border-radius: 14px;
  padding: 1.75rem 2rem;
  text-decoration: none;
  color: inherit;
  transition: border-color 0.15s, box-shadow 0.15s, transform 0.1s;
}

.course-card:hover {
  border-color: var(--accent);
  box-shadow: 0 4px 16px rgba(59, 130, 246, 0.12);
  transform: translateY(-2px);
}

.course-card-tag {
  display: inline-block;
  font-size: 0.78rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--accent);
  background: var(--accent-bg);
  border-radius: 99px;
  padding: 0.2em 0.75em;
  margin-bottom: 0.75rem;
}

.course-card-title {
  font-size: 1.4rem;
  font-weight: 800;
  color: var(--dark);
  margin-bottom: 0.45rem;
}

.course-card-desc {
  font-size: 0.95rem;
  color: var(--text-secondary);
  line-height: 1.6;
  margin-bottom: 1rem;
}

.course-card-cta {
  font-size: 0.9rem;
  font-weight: 700;
  color: var(--accent);
}

/* ── Back navigation links ──────────────────────────────────────────────── */

.back-to-subjects,
.back-to-courses {
  display: inline-block;
  margin-top: 1.5rem;
  font-size: 0.88rem;
  font-weight: 600;
  color: var(--muted);
  text-decoration: none;
  transition: color 0.12s;
}

.back-to-subjects:hover,
.back-to-courses:hover { color: var(--accent); }

/* ── Home: subject nav bar ─────────────────────────────────────────────── */

.subject-nav {
  display: flex;
  align-items: center;
  justify-content: center;
  border-bottom: 1px solid var(--border);
  background: var(--page-bg);
}

.subject-nav-link {
  display: flex;
  align-items: center;
  gap: 0.45rem;
  padding: 0.72rem 2rem;
  font-size: 0.86rem;
  font-weight: 500;
  letter-spacing: 0.02em;
  color: var(--muted);
  text-decoration: none;
  transition: color 0.13s, background 0.13s;
}

.subject-nav-link:hover {
  color: var(--text);
  background: rgba(0,0,0,0.03);
}

.subject-nav-link--purple:hover { color: #6d28d9; }

.subject-nav-sym {
  font-size: 1.05rem;
  font-weight: 700;
  opacity: 0.7;
}

.subject-nav-sep {
  display: block;
  width: 1px;
  height: 1.1rem;
  background: var(--border);
  flex-shrink: 0;
}

/* ── Home: hero ─────────────────────────────────────────────────────────── */

.home-hero {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 5.5rem 1rem 4rem;
  text-align: center;
}

.home-title {
  font-size: 2.6rem;
  font-weight: 800;
  letter-spacing: -0.035em;
  color: var(--dark);
  line-height: 1.1;
}

.home-tagline {
  font-size: 0.95rem;
  color: var(--muted);
}

/* ── Responsive ────────────────────────────────────────────────────────── */

@media (max-width: 520px) {
  .block { padding: 1.25rem 1.1rem; }
  .ex-row { flex-direction: column; gap: 0.1rem; }
  .ex-row code { min-width: 0; }
  .answer-buttons { flex-wrap: wrap; }
  .mc-choices { grid-template-columns: 1fr; }
  .proof-row { flex-direction: column; gap: 0.1rem; }
  .syl-label { min-width: 0; }
  .subject-card { flex-direction: column; gap: 0.75rem; }
}

/* ── Lambda Lab panel ──────────────────────────────────────────────────── */

.lab-toggle {
  position: fixed;
  bottom: 1.25rem;
  right: 1.25rem;
  z-index: 60;
  border: none;
  border-radius: 999px;
  padding: 0.75rem 1.3rem;
  background: var(--accent);
  color: #fff;
  font-size: 0.95rem;
  font-weight: 700;
  cursor: pointer;
  box-shadow: 0 4px 14px rgba(0, 0, 0, 0.18);
  transition: transform 0.1s, background 0.15s;
}
.lab-toggle:hover { background: var(--accent-hover); transform: translateY(-1px); }
body.lab-open .lab-toggle { display: none; }

.lab {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  z-index: 50;
  width: 400px;
  max-width: 100vw;
  background: #fff;
  border-left: 2px solid var(--border);
  box-shadow: -6px 0 24px rgba(0, 0, 0, 0.06);
  display: flex;
  flex-direction: column;
  transform: translateX(102%);
  transition: transform 0.22s ease;
}
body.lab-open .lab { transform: none; }

/* Wide screens: shift the page over instead of covering it */
@media (min-width: 1200px) {
  body { transition: padding-right 0.22s ease; }
  body.lab-open { padding-right: 400px; }
}

.lab-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0.9rem 1.1rem;
  border-bottom: 2px solid var(--border);
}
.lab-title { font-weight: 800; font-size: 1.05rem; color: var(--dark); }
.lab-close {
  border: none;
  background: none;
  font-size: 1.5rem;
  line-height: 1;
  color: var(--muted);
  cursor: pointer;
  padding: 0 0.2rem;
}
.lab-close:hover { color: var(--dark); }

.lab-body {
  flex: 1;
  overflow-y: auto;
  padding: 1rem 1.1rem 2rem;
  display: flex;
  flex-direction: column;
  gap: 0.8rem;
}

.lab-editor {
  width: 100%;
  min-height: 150px;
  resize: vertical;
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.9rem;
  line-height: 1.55;
  padding: 0.7rem 0.8rem;
  border: 1.5px solid var(--border);
  border-radius: 10px;
  color: var(--dark);
}
.lab-editor:focus { outline: none; border-color: var(--accent-border); }

.lab-actions { display: flex; align-items: center; gap: 0.75rem; }
.lab-run {
  border: none;
  border-radius: 8px;
  padding: 0.5rem 1.1rem;
  background: var(--accent);
  color: #fff;
  font-weight: 700;
  cursor: pointer;
}
.lab-run:hover { background: var(--accent-hover); }
.lab-hint { font-size: 0.75rem; color: var(--muted); }

.lab-output { display: flex; flex-direction: column; gap: 0.6rem; }
.lab-readback {
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 1.35rem;
  font-weight: 800;
  color: var(--dark);
}
.lab-code {
  display: block;
  font-size: 0.85rem;
  background: #f6f8fa;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  padding: 0.6rem 0.7rem;
  overflow-x: auto;
  white-space: pre-wrap;
  overflow-wrap: anywhere;
}
.lab-statusline { font-size: 0.78rem; color: var(--muted); }
.lab-note { font-size: 0.85rem; color: var(--text-secondary); }
.lab-warning {
  font-size: 0.85rem;
  color: #92400e;
  background: #fffbeb;
  border: 1.5px solid #fde68a;
  border-radius: 8px;
  padding: 0.6rem 0.7rem;
}
.lab-error {
  font-size: 0.85rem;
  color: #b91c1c;
  background: #fef2f2;
  border: 1.5px solid #fecaca;
  border-radius: 8px;
  padding: 0.6rem 0.7rem;
  white-space: pre-wrap;
}

.lab-steps-btn, .lab-more {
  align-self: flex-start;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  padding: 0.35rem 0.8rem;
  background: #fff;
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--text-secondary);
  cursor: pointer;
}
.lab-steps-btn:hover, .lab-more:hover {
  border-color: var(--accent-border);
  color: var(--accent);
}
.lab-more { display: inline-block; margin-left: 0.4rem; }

.lab-trace {
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
  border-top: 1.5px solid var(--border);
  padding-top: 0.6rem;
}
.lab-step { display: flex; gap: 0.5rem; align-items: baseline; font-size: 0.82rem; }
.lab-step-badge {
  flex-shrink: 0;
  width: 1.4rem;
  text-align: right;
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.72rem;
  font-weight: 700;
  color: var(--muted);
}
.lab-step code { overflow-wrap: anywhere; }
mark.lab-mark { background: #fde68a; border-radius: 3px; padding: 0 1px; }

/* "Try it" chips injected into runnable lesson code boxes (lambda pages) */
.lab-try {
  float: right;
  margin: -0.15rem -0.35rem 0.2rem 0.75rem;
  border: 1px solid var(--accent-border);
  border-radius: 6px;
  padding: 0.15rem 0.55rem;
  background: #fff;
  color: var(--accent);
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  font-size: 0.72rem;
  font-weight: 700;
  cursor: pointer;
  transition: background 0.12s, color 0.12s;
}
.lab-try:hover { background: var(--accent); color: #fff; }

.lab-examples {
  align-self: flex-start;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  padding: 0.3rem 0.5rem;
  background: #fff;
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--text-secondary);
  cursor: pointer;
}
.lab-examples:focus { outline: none; border-color: var(--accent-border); }

.lab-caret {
  margin-top: 0.45rem;
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.85rem;
  line-height: 1.35;
  color: #7f1d1d;
  overflow-x: auto;
}

/* Mobile polish for the lab */
.lab-toggle { bottom: calc(1.25rem + env(safe-area-inset-bottom, 0px)); }
@media (max-width: 520px) {
  .lab-editor { font-size: 16px; } /* prevents iOS focus auto-zoom */
  .lab-body { padding-bottom: calc(2rem + env(safe-area-inset-bottom, 0px)); }
}

/* Write-expression exercises */
.we-row { display: flex; gap: 0.5rem; align-items: center; flex-wrap: wrap; }
.we-input {
  flex: 1;
  min-width: 12rem;
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.95rem;
  padding: 0.5rem 0.7rem;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  color: var(--dark);
}
.we-input:focus { outline: none; border-color: var(--accent-border); }
.we-input:disabled { background: #f8fafc; color: var(--muted); }
.we-check {
  border: none;
  border-radius: 8px;
  padding: 0.5rem 1rem;
  background: var(--accent);
  color: #fff;
  font-weight: 700;
  cursor: pointer;
}
.we-check:hover:not(:disabled) { background: var(--accent-hover); }
.we-check:disabled { opacity: 0.5; cursor: default; }
.we-reveal {
  border: 1.5px solid var(--border);
  border-radius: 8px;
  padding: 0.5rem 0.9rem;
  background: #fff;
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--muted);
  cursor: pointer;
}
.we-reveal:hover { border-color: var(--accent-border); color: var(--accent); }
.feedback .lab-try { float: none; margin: 0 0 0 0.6rem; }
@media (max-width: 520px) { .we-input { font-size: 16px; } }
.we-prompt {
  color: var(--text-body);
  font-size: 0.92rem;
  margin-bottom: 0.7rem;
}

/* ── TFL Lab (D6) ───────────────────────────────────────────────────────── */
/* Reuses the .lab / .lab-* panel chrome above; these are the TFL-specific   */
/* controls (palette, banner, derivation pane, verdicts, square) shared by   */
/* the course-page panel and the standalone full page.                       */

.tfl-lab-core { display: flex; flex-direction: column; gap: 0.7rem; }

.tfl-toolbar { display: flex; flex-wrap: wrap; gap: 0.5rem; align-items: center; }
.tfl-io {
  border: 1.5px solid var(--border);
  border-radius: 8px;
  padding: 0.3rem 0.7rem;
  background: #fff;
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--text-secondary);
  cursor: pointer;
}
.tfl-io:hover { border-color: var(--accent-border); color: var(--accent); }

.tfl-label {
  font-size: 0.72rem;
  font-weight: 800;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: -0.35rem;
}

.tfl-editor { min-height: 130px; }

.tfl-palette { display: flex; flex-wrap: wrap; gap: 0.35rem; }
.tfl-key {
  min-width: 2rem;
  border: 1.5px solid var(--border);
  border-radius: 7px;
  padding: 0.28rem 0.5rem;
  background: #fff;
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 1rem;
  color: var(--dark);
  cursor: pointer;
}
.tfl-key:hover { border-color: var(--accent-border); background: var(--accent-bg); color: var(--accent-text); }

.tfl-banner {
  font-size: 0.82rem;
  border-radius: 8px;
  min-height: 0;
}
.tfl-banner:empty { display: none; }
.tfl-banner-ok {
  padding: 0.45rem 0.7rem;
  color: #166534;
  background: #f0fdf4;
  border: 1.5px solid #bbf7d0;
}
.tfl-banner-bad {
  padding: 0.45rem 0.7rem;
  color: #b91c1c;
  background: #fef2f2;
  border: 1.5px solid #fecaca;
  display: flex;
  align-items: center;
  gap: 0.6rem;
  flex-wrap: wrap;
}
.tfl-banner-error {
  padding: 0.45rem 0.7rem;
  color: #92400e;
  background: #fffbeb;
  border: 1.5px solid #fde68a;
}
.tfl-banner-line { font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace; font-size: 0.78rem; }
.tfl-banner-bad .tfl-steps { margin: 0; }

.tfl-query-row { display: flex; gap: 0.5rem; align-items: stretch; }
.tfl-query {
  flex: 1;
  min-width: 0;
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.9rem;
  padding: 0.5rem 0.7rem;
  border: 1.5px solid var(--border);
  border-radius: 9px;
  color: var(--dark);
}
.tfl-query:focus { outline: none; border-color: var(--accent-border); }
.tfl-ask { white-space: nowrap; }
.tfl-hint { margin-top: -0.2rem; }

.tfl-output { gap: 0.55rem; }
.tfl-note { color: var(--text-secondary); }

.tfl-verdict {
  align-self: flex-start;
  font-weight: 800;
  font-size: 0.95rem;
  padding: 0.25rem 0.7rem;
  border-radius: 999px;
}
.tfl-verdict-yes     { color: #166534; background: #dcfce7; }
.tfl-verdict-no      { color: #b91c1c; background: #fee2e2; }
.tfl-verdict-unknown { color: #6b7280; background: #f1f5f9; }

.tfl-question {
  font-weight: 700;
  color: var(--dark);
  font-size: 0.95rem;
}
.tfl-explain {
  color: var(--text-body);
  font-size: 0.9rem;
  line-height: 1.5;
}

.tfl-prop {
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.92rem;
  font-weight: 700;
  color: var(--accent-code);
}

.tfl-answer {
  display: flex;
  gap: 0.6rem;
  align-items: baseline;
  flex-wrap: wrap;
  padding: 0.3rem 0.55rem;
  border-left: 3px solid var(--accent-border);
  background: var(--accent-bg);
  border-radius: 0 7px 7px 0;
}
.tfl-reading { color: var(--text-secondary); font-size: 0.85rem; }
.tfl-eq-rule {
  margin-left: auto;
  font-size: 0.72rem;
  font-weight: 700;
  color: var(--muted);
  font-style: italic;
}

.tfl-extra {
  font-size: 0.85rem;
  line-height: 1.45;
  padding: 0.4rem 0.6rem;
  border-radius: 7px;
  color: var(--text-body);
}
.tfl-stronger { background: #eff6ff; border: 1.5px solid #bfdbfe; color: #1e40af; }
.tfl-perhaps  { background: #fefce8; border: 1.5px solid #fde68a; color: #854d0e; }
.tfl-naf      { background: #f8fafc; border: 1.5px solid var(--border); color: var(--text-secondary); }

.tfl-suggest {
  display: flex;
  gap: 0.5rem;
  align-items: baseline;
  flex-wrap: wrap;
  font-size: 0.85rem;
}
.tfl-suggest-note { color: var(--text-secondary); }
.tfl-add {
  border: 1.5px solid var(--accent-border);
  border-radius: 6px;
  padding: 0.12rem 0.5rem;
  background: #fff;
  color: var(--accent);
  font-size: 0.72rem;
  font-weight: 700;
  cursor: pointer;
}
.tfl-add:hover { background: var(--accent); color: #fff; }

/* Derivation pane */
.tfl-deriv {
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
  border-top: 1.5px solid var(--border);
  padding-top: 0.5rem;
  margin-top: 0.1rem;
}
.tfl-deriv-line { display: flex; gap: 0.55rem; align-items: baseline; font-size: 0.85rem; }
.tfl-deriv-n {
  flex-shrink: 0;
  width: 1.6rem;
  text-align: right;
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.75rem;
  color: var(--muted);
}
.tfl-deriv-rule {
  margin-left: auto;
  font-size: 0.72rem;
  font-weight: 700;
  color: var(--accent-text);
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  white-space: nowrap;
}

/* Square of opposition */
.tfl-square {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.5rem;
  margin-top: 0.4rem;
  padding-top: 0.6rem;
  border-top: 1.5px solid var(--border);
}
.tfl-sq-title {
  grid-column: 1 / -1;
  font-weight: 800;
  font-size: 0.8rem;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: var(--muted);
}
.tfl-sq-cell {
  display: flex;
  flex-direction: column;
  gap: 0.2rem;
  padding: 0.5rem 0.6rem;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  background: #fff;
}
.tfl-sq-label {
  font-size: 0.68rem;
  font-weight: 800;
  letter-spacing: 0.05em;
  text-transform: uppercase;
  color: var(--accent);
}

/* Standalone full page: give the fact base and derivations room */
.tfl-lab-page { display: block; }
.tfl-lab-page .tfl-lab-core {
  max-width: 780px;
  margin: 0 auto;
}
.tfl-lab-page .tfl-editor { min-height: 260px; }
.tfl-lab-page .tfl-query { font-size: 0.95rem; }

@media (max-width: 520px) {
  .tfl-editor, .tfl-query { font-size: 16px; } /* prevents iOS focus auto-zoom */
  .tfl-square { grid-template-columns: 1fr; }
}

/* TFL Lab — numerical syllogism condition checklist (D9) */
.tfl-conds { display: flex; flex-direction: column; gap: 0.25rem; }
.tfl-cond { font-size: 0.85rem; }
.tfl-cond-ok { color: #166534; }
.tfl-cond-no { color: #b91c1c; }

/* Stacked syntax box: a multi-line formula/argument that both displays as
   lines and parses line-by-line (so the TFL Lab "▸ try" chip reads it as an
   argument). Used for numerical syllogisms in Course 4. */
.syntax-box.stacked {
  white-space: pre-line;
  line-height: 1.75;
}

/* Fallback shown when a lesson block fails to render (engine.js hardening) —
   keeps the rest of the lesson usable instead of blanking the page. */
.block-error {
  border-left: 4px solid #f59e0b;
  background: #fffbeb;
}
.block-error-detail {
  font-family: 'SF Mono', ui-monospace, Menlo, Consolas, monospace;
  font-size: 0.8rem;
  color: #92400e;
  white-space: pre-wrap;
  overflow-x: auto;
  margin-top: 0.5rem;
}