* {
  box-sizing: border-box;
}

:root {
  --bg: var(--site-bg);
  --card: var(--site-surface);
  --ink: var(--site-text);
  --muted: var(--site-muted);
  --line: var(--site-border);
  --soft-line: var(--site-border-soft);
  --panel-bg: var(--site-surface-soft);
  --control-fill: var(--site-green);
  --control-text: var(--site-on-accent);
  /* functional colors (match canvas drawing) */
  --compress: #2f6fd6;
  --stretch: #d64545;
  --danger: #ef8a17;
  --newbond: #16a34a;
  --clamp: var(--site-muted);
  /* canvas colours, read by js/main.js */
  --canvas-bg: var(--site-surface);
  --canvas-ink: var(--site-text);
  --canvas-muted: var(--site-muted);
  --canvas-grid: var(--site-border-soft);
  --canvas-axis: var(--site-border);
  --canvas-marker: var(--site-muted);
  --canvas-clamp: var(--site-surface-soft);
  --canvas-clamp-hatch: color-mix(in srgb, var(--site-text) 14%, transparent);
  --canvas-clamp-edge: var(--site-border);
  --canvas-clamp-atom: var(--site-muted);
  --canvas-tag-bg: color-mix(in srgb, var(--site-surface) 92%, transparent);
  --canvas-bond: 190, 196, 204;
  --canvas-broken: color-mix(in srgb, var(--site-danger) 75%, transparent);
  --canvas-break-text: var(--site-danger);
  --canvas-blue-text: var(--site-info);
  --canvas-orange-text: var(--site-amber);
  --canvas-arrow: var(--site-amber);
}

body {
  margin: 0;
  min-height: 100vh;
  color: var(--ink);
  background: var(--bg);
  font-family: Inter, -apple-system, BlinkMacSystemFont, "Segoe UI", Arial, sans-serif;
}

button,
input {
  font: inherit;
}

.topbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 24px;
  width: min(1280px, calc(100% - 48px));
  margin: 0 auto;
    padding: 24px 0 18px;
}

.home-button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  flex: none;
  width: 44px;
  height: 44px;
  padding: 0;
  border: 1px solid var(--line);
  border-radius: 50%;
  background: var(--card);
  color: var(--ink);
  text-decoration: none;
  font-size: 13px;
  font-weight: 600;
  transition: background 0.15s ease, border-color 0.15s ease;
}

.home-button svg {
  display: block;
}

.home-button:hover,
.home-button:focus-visible {
  background: var(--soft-line);
  border-color: var(--control-fill);
  outline: none;
}

.eyebrow {
  margin: 0 0 8px;
  color: var(--muted);
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.12em;
  text-transform: uppercase;
}

h1,
h2,
h3,
p {
  margin-top: 0;
}

h1 {
  margin-bottom: 10px;
  font-size: clamp(26px, 3vw, 34px);
  line-height: 1.12;
  font-weight: 700;
  letter-spacing: -0.01em;
}

.lede {
  max-width: 780px;
  margin-bottom: 0;
  color: var(--muted);
  line-height: 1.6;
  font-size: 14px;
}

.app {
  display: grid;
  grid-template-columns: minmax(280px, 360px) minmax(0, 1fr);
  gap: 18px;
  width: min(1280px, calc(100% - 48px));
  margin: 0 auto;
  padding: 0 0 36px;
}

.panel,
.stage-panel {
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--card);
}

.controls-panel {
  display: flex;
  flex-direction: column;
  gap: 16px;
  padding: 16px;
}

.mode-tabs {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 6px;
  padding: 4px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--panel-bg);
}

.mode-button {
  min-height: 40px;
  border: 0;
  border-radius: 6px;
  color: var(--muted);
  background: transparent;
  font-weight: 700;
  cursor: pointer;
}

.mode-button.active {
  color: var(--control-text);
  background: var(--control-fill);
}

.control-block {
  display: grid;
  gap: 14px;
  padding: 14px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--panel-bg);
}

.block-heading {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
}

.block-heading h2,
.stage-header h2,
.explanation-panel h2 {
  margin: 0;
  font-size: 17px;
  line-height: 1.25;
  font-weight: 700;
}

.status-pill {
  flex: 0 0 auto;
  padding: 5px 9px;
  border-radius: 999px;
  color: var(--site-bg);
  background: var(--site-muted);
  font-size: 12px;
  font-weight: 700;
}

.status-pill.on {
  background: var(--current);
}

.status-pill.neutral {
  color: var(--ink);
  background: var(--soft-line);
}

.field-switch,
.secondary-button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 10px;
  min-height: 42px;
  border: 1px solid var(--line);
  border-radius: 6px;
  color: var(--ink);
  background: var(--card);
  font-weight: 700;
  cursor: pointer;
}

.secondary-button:hover {
  background: var(--soft-line);
}

.field-switch.active {
  color: var(--control-text);
  border-color: var(--control-fill);
  background: var(--control-fill);
}

.switch-track {
  position: relative;
  width: 38px;
  height: 20px;
  border-radius: 999px;
  background: var(--site-border);
}

.switch-thumb {
  position: absolute;
  top: 3px;
  left: 3px;
  width: 14px;
  height: 14px;
  border-radius: 50%;
  background: var(--site-text);
  transition: transform 0.18s ease;
}

.field-switch.active .switch-track {
  background: color-mix(in srgb, var(--control-text) 35%, transparent);
}

.field-switch.active .switch-thumb {
  background: var(--control-text);
  transform: translateX(18px);
}

.slider-row {
  display: grid;
  grid-template-columns: 112px minmax(0, 1fr) 64px;
  align-items: center;
  gap: 10px;
  color: var(--ink);
  font-size: 14px;
  font-weight: 600;
}

.slider-row strong {
  text-align: right;
  font-variant-numeric: tabular-nums;
}

input[type="range"] {
  width: 100%;
  accent-color: var(--ink);
}

.metrics {
  display: grid;
  gap: 10px;
}

.metric {
  display: grid;
  gap: 4px;
  padding: 11px 12px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--panel-bg);
}

.metric span {
  color: var(--muted);
  font-size: 12px;
  font-weight: 600;
}

.metric strong {
  font-size: 16px;
  line-height: 1.25;
}

.stage-panel {
  min-width: 0;
  overflow: hidden;
}

.stage-header {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 18px;
  padding: 16px 18px 12px;
  border-bottom: 1px solid var(--soft-line);
}

.stage-header p {
  max-width: 760px;
  margin: 6px 0 0;
  color: var(--muted);
  line-height: 1.45;
}

.legend {
  display: flex;
  flex-wrap: wrap;
  justify-content: flex-end;
  gap: 10px 14px;
  color: var(--muted);
  font-size: 13px;
  font-weight: 600;
}

.legend span {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  white-space: nowrap;
}

.legend-dot {
  width: 12px;
  height: 12px;
  border-radius: 50%;
  display: inline-block;
}

.legend-line {
  width: 22px;
  height: 3px;
  border-radius: 999px;
}

canvas {
  display: block;
  width: 100%;
  background: var(--canvas-bg);
}

#latticeCanvas {
  height: min(54vh, 560px);
  min-height: 360px;
}

.readout-strip {
  display: flex;
  justify-content: space-between;
  gap: 14px;
  padding: 12px 18px;
  border-top: 1px solid var(--soft-line);
  color: var(--muted);
  background: var(--card);
  font-size: 13px;
  font-weight: 600;
}

.explanation-panel {
  grid-column: 1 / -1;
  padding: 18px;
}

.concept-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 14px;
  margin-top: 14px;
}

.concept-grid > div {
  padding: 14px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--panel-bg);
}

.concept-grid h3 {
  margin-bottom: 8px;
  font-size: 15px;
  font-weight: 700;
}

.concept-grid p {
  margin-bottom: 0;
  color: var(--muted);
  line-height: 1.55;
}

@media (max-width: 900px) {
  .topbar,
  .stage-header,
  .readout-strip {
    flex-direction: column;
  }

  .topbar {
    align-items: flex-start;
  }

  .app {
    grid-template-columns: 1fr;
    width: calc(100% - 28px);
  }

  .concept-grid {
    grid-template-columns: 1fr;
  }

  .legend {
    justify-content: flex-start;
  }

  #latticeCanvas {
    height: 420px;
  }
}

@media (max-width: 520px) {
  .slider-row {
    grid-template-columns: 1fr 62px;
  }

  .slider-row span {
    grid-column: 1 / -1;
  }

  #latticeCanvas {
    min-height: 300px;
    height: 340px;
  }
}

/* ---- Deformation in Collisions: additions ---- */

.app {
  grid-template-rows: auto auto auto;
}

.controls-panel {
  grid-row: 1 / span 2;
  align-self: start;
}

.primary-button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 44px;
  border: 1px solid var(--control-fill);
  border-radius: 6px;
  color: var(--control-text);
  background: var(--control-fill);
  font-weight: 700;
  cursor: pointer;
}

.primary-button:hover {
  opacity: 0.88;
}

.button-row {
  display: grid;
  grid-template-columns: minmax(0, 1.4fr) minmax(0, 1fr);
  gap: 10px;
}

.preset-row {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 8px;
}

.preset-button {
  display: grid;
  gap: 2px;
  min-height: 48px;
  padding: 6px 4px;
  border: 1px solid var(--line);
  border-radius: 6px;
  color: var(--ink);
  background: var(--card);
  font-weight: 700;
  cursor: pointer;
}

.preset-button small {
  color: var(--muted);
  font-size: 11px;
  font-weight: 600;
  line-height: 1.25;
}

.preset-button:hover {
  background: var(--soft-line);
}

.preset-button.active {
  color: var(--control-text);
  border-color: var(--control-fill);
  background: var(--control-fill);
}

.preset-button.active small {
  color: inherit;
  opacity: 0.75;
}

.mode-button {
  padding: 6px 4px;
  line-height: 1.25;
}

.mode-button small {
  display: block;
  font-size: 11px;
  font-weight: 600;
  opacity: 0.8;
}

.check-row {
  display: flex;
  align-items: center;
  gap: 10px;
  color: var(--ink);
  font-size: 14px;
  font-weight: 600;
  cursor: pointer;
}

.check-row input {
  width: 16px;
  height: 16px;
  accent-color: var(--ink);
}

.hint-text {
  margin: 0;
  color: var(--muted);
  font-size: 12px;
  line-height: 1.6;
}

kbd {
  padding: 1px 5px;
  border: 1px solid var(--line);
  border-bottom-width: 2px;
  border-radius: 4px;
  background: var(--card);
  font-family: inherit;
  font-size: 11px;
}

button:disabled {
  opacity: 0.5;
  cursor: default;
}

.status-pill.elastic { color: var(--site-info); background: color-mix(in srgb, var(--site-info) 12%, var(--card)); }
.status-pill.plastic { color: var(--site-success); background: var(--site-tint-green); }
.status-pill.crack,
.status-pill.fracture { color: var(--site-danger); background: color-mix(in srgb, var(--site-danger) 12%, var(--card)); }

.stage-panel {
  align-self: start;
}

#timeReadout {
  flex: none;
  white-space: nowrap;
}

.metric.verdict strong {
  font-size: 14px;
  line-height: 1.45;
}

.legend em {
  font-style: normal;
}

.legend-line.compress { background: var(--compress); }
.legend-line.stretch { background: var(--stretch); }
.legend-line.danger { background: var(--danger); height: 4px; }
.legend-line.newbond { background: var(--newbond); }
.legend-line.broken {
  background: repeating-linear-gradient(90deg, var(--stretch) 0 4px, transparent 4px 7px);
}
.legend-dot.clamp {
  border-radius: 2px;
  background: var(--clamp);
}

.graphs-panel {
  display: grid;
  grid-template-columns: minmax(0, 1.15fr) minmax(0, 1fr);
  min-width: 0;
  overflow: hidden;
}

.graph-card {
  min-width: 0;
}

.graph-card + .graph-card {
  border-left: 1px solid var(--soft-line);
}

.graph-card .stage-header {
  padding: 12px 16px 10px;
}

.graph-card canvas {
  height: 270px;
}

.concept-grid.two {
  grid-template-columns: repeat(2, minmax(0, 1fr));
}

.concept-grid h3 .tag {
  display: inline-block;
  margin-right: 6px;
  padding: 1px 7px;
  border-radius: 999px;
  color: var(--site-on-accent);
  font-size: 12px;
  vertical-align: 1px;
}

.tag.elastic { background: var(--site-info); }
.tag.plastic { background: var(--site-success); }
.tag.fracture { background: var(--site-danger); }

@media (max-width: 900px) {
  .controls-panel {
    grid-row: auto;
  }

  .graphs-panel,
  .concept-grid.two {
    grid-template-columns: 1fr;
  }

  .graph-card + .graph-card {
    border-left: 0;
    border-top: 1px solid var(--soft-line);
  }
}

/* wide screens (projector): controls, particles and graphs side by side */
@media (min-width: 1500px) {
  .topbar,
  .app {
    width: min(1720px, calc(100% - 48px));
  }

  .app {
    grid-template-columns: 330px minmax(0, 1fr) minmax(360px, 26vw);
    grid-template-rows: auto auto;
  }

  .controls-panel {
    grid-row: 1;
  }

  .stage-panel {
    grid-column: 2;
    grid-row: 1;
    align-self: start;
  }

  .graphs-panel {
    grid-column: 3;
    grid-row: 1;
    grid-template-columns: 1fr;
    align-self: start;
  }

  .graph-card + .graph-card {
    border-left: 0;
    border-top: 1px solid var(--soft-line);
  }

  .graph-card canvas {
    height: min(27vh, 280px);
    min-height: 220px;
  }

  #latticeCanvas {
    height: min(56vh, 580px);
  }
}

/* The renderer builds rgba() from this numeric bond triplet. */
@media (prefers-color-scheme: dark) {
  :root { --canvas-bond: 86, 94, 106; }
}