.chapter-page main { overflow: visible; }

.chapter-links .is-current {
  color: var(--accent);
  border-color: var(--accent);
  background: var(--bg-soft);
}

.chapter-hero,
.chapter-section,
.chapter-pagination {
  width: min(var(--max), calc(100% - 2rem));
  margin-inline: auto;
}

.chapter-hero {
  padding: clamp(2rem, 5vw, 4.5rem) 0 clamp(3rem, 7vw, 6rem);
}

.chapter-kicker {
  display: flex; flex-wrap: wrap; gap: .55rem; align-items: center;
  margin-bottom: 1.5rem; color: var(--muted); font-size: .78rem;
}
.chapter-kicker a { color: var(--muted); text-decoration: none; }
.chapter-kicker a:hover { color: var(--accent); }
.chapter-kicker strong { color: var(--accent); }

.chapter-hero-grid {
  display: grid; grid-template-columns: .8fr 1.2fr;
  align-items: center; gap: clamp(2rem, 5vw, 5rem);
}
.chapter-hero h1 {
  margin: 0;
  font-size: clamp(3.2rem, 7vw, 7rem);
  line-height: .9; letter-spacing: -.055em;
}
.chapter-hero h1 span { color: var(--accent); }
.chapter-lede {
  max-width: 680px; margin: 1.4rem 0 0;
  color: var(--muted); font-size: clamp(1rem, 1.3vw, 1.16rem);
}
.assumption-row {
  display: flex; flex-wrap: wrap; gap: .45rem; margin-top: 1.5rem;
}
.assumption-row span {
  padding: .45rem .65rem;
  border: 1px solid var(--line); border-radius: 999px;
  color: var(--muted); background: var(--panel);
  font: 700 .7rem ui-monospace, SFMono-Regular, Menlo, monospace;
}
.chapter-concept {
  margin: 0; padding: .65rem;
  border: 1px solid var(--line); border-radius: 22px;
  background: var(--panel); box-shadow: 0 24px 60px var(--shadow);
}
.chapter-concept img { width: 100%; border-radius: 16px; }

.chapter-section {
  padding: clamp(3.5rem, 7vw, 6.5rem) 0;
  border-top: 1px solid var(--line);
}

.concept-grid,
.insight-grid {
  display: grid; grid-template-columns: repeat(3, 1fr);
  gap: 1rem; margin-top: 2rem;
}
.concept-card,
.insight-grid article {
  padding: 1.1rem;
  border: 1px solid var(--line); border-radius: var(--radius);
  background: var(--panel);
}
.concept-card h3, .insight-grid h3 { margin: .45rem 0 .6rem; }
.concept-card p, .insight-grid p { margin: .55rem 0 0; color: var(--muted); font-size: .9rem; }
.formula {
  padding: .75rem .85rem;
  border: 1px solid var(--line); border-radius: 10px;
  background: var(--bg-soft); color: var(--accent);
  font: 700 .92rem ui-monospace, SFMono-Regular, Menlo, monospace;
}

.lab-section .section-heading { max-width: 900px; }
.lab-shell {
  display: grid; grid-template-columns: minmax(0, 1fr) 300px;
  gap: 1rem; margin-top: 2rem;
}
.lab-main,
.control-panel {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
}
.lab-main { min-width: 0; overflow: hidden; }
.lab-toolbar {
  display: flex; justify-content: space-between; align-items: center;
  gap: 1rem; padding: .8rem 1rem;
  border-bottom: 1px solid var(--line);
}
.lab-toolbar-title { display: grid; grid-template-columns: auto 1fr; column-gap: .55rem; align-items: center; }
.lab-toolbar-title .status-dot { grid-row: 1 / 3; }
.lab-toolbar-title small { color: var(--muted); font-size: .68rem; }
.status-dot { width: 9px; height: 9px; border-radius: 50%; background: var(--accent); box-shadow: 0 0 0 4px color-mix(in srgb, var(--accent) 15%, transparent); }
.lab-actions { display: flex; gap: .45rem; }
.lab-button {
  min-height: 36px; padding: .45rem .7rem;
  border: 1px solid var(--line); border-radius: 9px;
  background: var(--bg-soft); color: var(--text);
  font: inherit; font-size: .76rem; font-weight: 750; cursor: pointer;
}
.lab-button:hover { border-color: var(--line-strong); }
.lab-button:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }

.plot-wrap { padding: .75rem; }
#trajectoryPlot { width: 100%; height: auto; display: block; }
.plot-bg { fill: var(--nav-bg); stroke: var(--line); }
.grid-line { stroke: var(--line); stroke-width: 1; opacity: .75; }
.axis-line { stroke: var(--line-strong); stroke-width: 1.3; }
.axis-text, .plot-text {
  fill: var(--muted); font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
  font-size: 12px;
}
.trajectory-path {
  fill: none; stroke: var(--accent); stroke-width: 4;
  stroke-linecap: round; stroke-linejoin: round;
}
.flat-path {
  fill: none; stroke: var(--muted); stroke-width: 2;
  stroke-dasharray: 8 8; opacity: .65;
}
.state-marker { fill: var(--text); stroke: var(--nav-bg); stroke-width: 3; }
.apogee-marker { fill: var(--accent-2); stroke: var(--nav-bg); stroke-width: 2; }
.vector { stroke-width: 3; stroke-linecap: round; }
.velocity-vector { stroke: var(--accent); }
.gravity-vector { stroke: var(--accent-2); }

.timeline-control { padding: .2rem 1rem .85rem; }
.timeline-control label {
  display: flex; justify-content: space-between; gap: 1rem;
  color: var(--muted); font-size: .75rem; margin-bottom: .45rem;
}
.timeline-control output { color: var(--text); font-variant-numeric: tabular-nums; }
.timeline-control input,
.control input[type="range"] { width: 100%; accent-color: var(--accent); }

.lab-legend {
  display: flex; flex-wrap: wrap; gap: .75rem 1rem;
  padding: .75rem 1rem 1rem; color: var(--muted); font-size: .7rem;
}
.lab-legend span { display: inline-flex; align-items: center; gap: .42rem; }
.legend-line { width: 24px; height: 0; border-top: 3px solid var(--accent); }
.legend-line.compare { border-top: 2px dashed var(--muted); }
.legend-dot { width: 8px; height: 8px; border-radius: 50%; background: var(--text); }
.legend-vector { width: 18px; height: 0; border-top: 3px solid var(--accent); }
.legend-vector.gravity { border-color: var(--accent-2); }

.control-panel { padding: 1rem; }
.control-head h3 { margin: -.25rem 0 1rem; }
.control {
  padding: .85rem 0; border-top: 1px solid var(--line);
}
.control label {
  display: flex; justify-content: space-between; gap: .75rem;
  margin-bottom: .55rem; color: var(--muted); font-size: .75rem;
}
.control output { color: var(--text); font-weight: 750; font-variant-numeric: tabular-nums; }
.toggle-row {
  display: flex; align-items: flex-start; gap: .7rem;
  padding: .9rem 0; border-top: 1px solid var(--line); cursor: pointer;
}
.toggle-row input { margin-top: .2rem; accent-color: var(--accent); }
.toggle-row span { display: grid; }
.toggle-row b { font-size: .8rem; }
.toggle-row small { color: var(--muted); font-size: .68rem; margin-top: .16rem; }
.control-note {
  margin-top: .8rem; padding: .8rem;
  border: 1px solid var(--line); border-radius: 10px;
  background: var(--bg-soft);
}
.control-note strong { font-size: .76rem; color: var(--accent-2); }
.control-note p { margin: .35rem 0 0; color: var(--muted); font-size: .72rem; }

.metric-grid {
  display: grid; grid-template-columns: repeat(6, 1fr);
  gap: .65rem; margin-top: .7rem;
}
.metric-grid article {
  min-width: 0; padding: .8rem;
  border: 1px solid var(--line); border-radius: 12px;
  background: var(--panel);
}
.metric-grid span, .metric-grid small {
  display: block; color: var(--muted); font-size: .66rem;
}
.metric-grid strong {
  display: block; margin: .3rem 0;
  font-size: clamp(1rem, 1.5vw, 1.25rem);
  font-variant-numeric: tabular-nums;
}

.interpretation-panel {
  display: grid; grid-template-columns: .75fr 1.25fr;
  gap: 1.5rem; align-items: end;
  margin-top: .7rem; padding: 1rem;
  border: 1px solid var(--line); border-radius: var(--radius);
  background: var(--bg-soft);
}
.interpretation-panel h3 { margin: -.2rem 0 0; }
.interpretation-panel > p { margin: 0; color: var(--muted); }

.chapter-pagination {
  display: grid; grid-template-columns: 1fr 1fr; gap: 1rem;
  padding: 0 0 4rem;
}
.chapter-pagination a {
  display: grid; padding: 1rem;
  border: 1px solid var(--line); border-radius: 13px;
  background: var(--panel); text-decoration: none;
}
.chapter-pagination a:last-child { text-align: right; }
.chapter-pagination span { color: var(--muted); font-size: .7rem; }
.chapter-pagination strong { margin-top: .25rem; }
.chapter-pagination a:not(.disabled):hover { border-color: var(--accent); }
.chapter-pagination .disabled { opacity: .45; cursor: default; }

@media (max-width: 1040px) {
  .chapter-hero-grid { grid-template-columns: 1fr; }
  .chapter-concept { max-width: 820px; }
  .lab-shell { grid-template-columns: 1fr; }
  .control-panel {
    display: grid; grid-template-columns: repeat(3, 1fr); gap: 0 1rem;
  }
  .control-head, .control-note { grid-column: 1 / -1; }
  .toggle-row { border-top: 1px solid var(--line); }
  .metric-grid { grid-template-columns: repeat(3, 1fr); }
}

@media (max-width: 720px) {
  .chapter-hero,
  .chapter-section,
  .chapter-pagination {
    width: min(100% - 1.2rem, var(--max));
  }
  .chapter-hero { padding-top: 1.4rem; }
  .chapter-hero h1 { font-size: clamp(3rem, 15vw, 5rem); }
  .chapter-concept { padding: .4rem; border-radius: 16px; }
  .concept-grid, .insight-grid { grid-template-columns: 1fr; }
  .lab-toolbar { align-items: flex-start; }
  .lab-toolbar-title small { max-width: 180px; }
  .plot-wrap { padding: .35rem; overflow-x: auto; }
  #trajectoryPlot { min-width: 680px; }
  .control-panel { display: block; }
  .metric-grid { grid-template-columns: 1fr 1fr; }
  .interpretation-panel { grid-template-columns: 1fr; align-items: start; }
  .chapter-pagination { grid-template-columns: 1fr; }
  .chapter-pagination a:last-child { text-align: left; }
}

@media (max-width: 460px) {
  .metric-grid { grid-template-columns: 1fr; }
  .lab-actions { flex-direction: column; }
}