/* Palette: cobalt glass (what chemists look through at a flame test)
   with flame-test colours: copper, strontium, sodium, potassium and barium for the
   interface, and seven more so each of the twelve domains has its own. */
:root {
  --bg: #1b1f5e;
  --bg-deep: #12154a;
  --panel: #252a78;
  --line: #3d4399;
  --ink: #f2f3ff;
  --dim: #a9aee6;
  --cu: #2fe0b5;
  --sr: #ff5d73;
  --na: #ffc83d;
  --k: #c39bff;
  --ba: #b6f36a;
  --ca: #ff9a4d;
  --cs: #62c4ff;
  --in: #8a97ff;
  --li: #ff8fd8;
  --fe: #f0b890;
  --b: #e8eaff;
  --tl: #74dd8c;
  --tone: var(--cu);
  --display: "Bricolage Grotesque", "Avenir Next", system-ui, sans-serif;
  --body: "Atkinson Hyperlegible Next", "Atkinson Hyperlegible", system-ui, sans-serif;
  --ease: cubic-bezier(.2, .8, .2, 1);
  --spring: cubic-bezier(.3, 1.6, .5, 1);
}

* { box-sizing: border-box; }
html, body { height: 100%; margin: 0; }
body {
  background: var(--bg);
  color: var(--ink);
  font: 400 17px/1.5 var(--body);
  -webkit-font-smoothing: antialiased;
  overflow: hidden;
}
button { font: inherit; color: inherit; cursor: pointer; }
button:focus-visible, input:focus-visible {
  outline: 3px solid var(--na);
  outline-offset: 3px;
}
#app { height: 100dvh; }

/* ───────────── Map ───────────── */
.map {
  position: relative;
  height: 100%;
  overflow: hidden;
  touch-action: none;
  /* A drag moves the map; it must not also select tile names or raise the callout. */
  -webkit-user-select: none;
  user-select: none;
  -webkit-touch-callout: none;
  contain: layout paint;
  cursor: grab;
  background:
    radial-gradient(circle at 1px 1px, rgb(255 255 255 / .09) 1.2px, transparent 1.6px) 0 0 / 28px 28px,
    radial-gradient(120% 90% at 20% 40%, #262b85 0%, var(--bg) 55%, var(--bg-deep) 100%);
}
.map.dragging { cursor: grabbing; }
.world { position: absolute; left: 0; top: 0; transform-origin: 0 0; will-change: transform; }
.world.glide { transition: transform .6s var(--ease); }
.bonds { position: absolute; left: 0; top: 0; overflow: visible; pointer-events: none; }
.bond { stroke: var(--line); stroke-width: 5; stroke-linecap: round; transition: stroke .5s; }
.bond[data-state="open"] { stroke: var(--dim); stroke-dasharray: 2 12; animation: flow 1.2s steps(14) infinite; }
.bond[data-state="done"] { stroke: var(--ink); }
.bond[data-state="ghost"] { stroke-width: 4; stroke-dasharray: 1 10; opacity: .7; }
/* Each step of the flowing dashes repaints the map: they move a pixel at a time (twelve
   repaints a second, not sixty) and rest while the map is being moved. */
.map.moving .bond { animation-play-state: paused; }
@keyframes flow { to { stroke-dashoffset: -14; } }

.tile {
  --size: 92px;
  position: absolute;
  width: var(--size);
  height: var(--size);
  margin: calc(var(--size) / -2) 0 0 calc(var(--size) / -2);
  padding: 0;
  display: grid;
  place-items: center;
  border: 2.5px solid var(--tone);
  border-radius: 16px;
  background: var(--bg-deep);
  opacity: var(--fade, 1);
  transition: transform .25s var(--spring), background .4s, box-shadow .3s, opacity .3s;
}
.tile:hover, .tile.picked, .tile:focus-visible { transform: scale(1.08); opacity: 1; }
.tile .sym { font: 800 38px/1 var(--display); letter-spacing: -.02em; }
.tile .mark { position: absolute; top: 6px; right: 8px; font-size: 13px; line-height: 1; color: var(--tone); }
.tile .name {
  position: absolute;
  top: calc(100% + 9px);
  left: 50%;
  width: 150px;
  translate: -50% 0;
  font: 600 14.5px/1.2 var(--body);
  color: var(--ink);
  text-wrap: balance;
  pointer-events: none;
}
.tile[data-state="done"] { background: var(--tone); color: var(--bg-deep); }
.tile[data-state="done"] .mark { color: var(--bg-deep); }
/* The pulsing ring is a layer of its own for the browser to composite, so only tiles on
   screen (.live) have one. It is ::after so that it is painted last: anything painted over
   a layer needs a layer too, and ::before put the name and the mark over it. (Rings as
   separate elements in one group after the tiles measured slower in Chrome.) */
.tile.live[data-state="open"]::after {
  content: "";
  position: absolute;
  inset: -9px;
  border-radius: 22px;
  border: 2px solid var(--tone);
  animation: ring 2.2s var(--ease) infinite;
}
@keyframes ring {
  from { opacity: .9; transform: scale(.94); }
  to { opacity: 0; transform: scale(1.16); }
}
.tile[data-state="open"] { background: color-mix(in srgb, var(--tone) 18%, var(--bg-deep)); box-shadow: 0 0 26px color-mix(in srgb, var(--tone) 45%, transparent); }
.tile[data-state="locked"] { border-color: var(--line); color: color-mix(in srgb, var(--dim) 55%, var(--bg)); background: color-mix(in srgb, var(--bg-deep) 60%, var(--bg)); }
.tile[data-state="locked"] .name { opacity: .7; }
.tile[data-state="locked"] .name, .tile[data-state="ghost"] .name { color: var(--dim); }
.tile[data-state="locked"] .mark { color: var(--dim); }
.tile[data-state="ghost"] {
  --faint: color-mix(in srgb, var(--tone) 50%, var(--line));
  border: 2.5px dashed var(--faint);
  color: var(--faint);
  background: var(--bg);
}
.map.far .tile .name { opacity: 0; }
.tile .name { transition: opacity .3s; }
.tile.picked { box-shadow: 0 0 0 5px color-mix(in srgb, var(--tone) 30%, transparent); }

.masthead {
  position: absolute;
  left: 12px;
  top: 10px;
  max-width: 354px;
  padding: 10px 14px 14px 12px;
  border-radius: 18px;
  /* No backdrop blur: Safari would blur the map again on every frame of a drag. */
  background: color-mix(in srgb, var(--bg) 90%, transparent);
  pointer-events: none;
}
.masthead h1 { margin: 0; font: 800 46px/.95 var(--display); letter-spacing: -.035em; }
.masthead p { margin: 10px 0 0; color: var(--dim); }
.tally {
  margin-top: 14px;
  display: inline-flex;
  gap: 10px;
  align-items: center;
  font-weight: 600;
}
.tally-bar { width: 120px; height: 8px; border-radius: 8px; background: var(--line); overflow: hidden; }
.tally-bar i { display: block; height: 100%; background: var(--cu); border-radius: 8px; transition: width .8s var(--ease); }

.zoomer { position: absolute; right: 18px; top: 18px; display: flex; gap: 6px; }
.zoomer button, .legend {
  background: color-mix(in srgb, var(--bg-deep) 85%, transparent);
  border: 1.5px solid var(--line);
  border-radius: 12px;
}
.zoomer button { width: 42px; height: 42px; font: 700 20px/1 var(--display); }
.zoomer button svg { display: block; margin: auto; }
.zoomer button.wide { width: auto; padding: 0 14px; font-size: 15px; }
.legend {
  position: absolute;
  left: 24px;
  bottom: 20px;
  display: flex;
  flex-wrap: wrap;
  gap: 0 4px;
  padding: 5px 6px;
  font-size: 14px;
  color: var(--dim);
  max-width: min(660px, calc(100% - 48px));
}
.legend button {
  display: inline-flex;
  align-items: center;
  gap: 7px;
  min-height: 30px;
  padding: 0 8px;
  border: 0;
  border-radius: 8px;
  background: none;
  color: inherit;
}
.legend button:hover { color: var(--ink); background: var(--panel); }
.legend i { width: 11px; height: 11px; border-radius: 4px; background: var(--tone); }

.card {
  position: absolute;
  right: 18px;
  bottom: 18px;
  width: min(380px, calc(100% - 36px));
  padding: 20px;
  background: var(--panel);
  border: 1.5px solid var(--line);
  border-radius: 22px;
  box-shadow: 0 18px 50px rgb(8 10 50 / .55);
  cursor: default;
  -webkit-user-select: text;
  user-select: text;
  animation: rise .35s var(--spring);
}
@keyframes rise { from { opacity: 0; transform: translateY(18px) scale(.97); } }
.card h2 { margin: 0 0 6px; font: 700 26px/1.1 var(--display); letter-spacing: -.02em; }
.card p { margin: 0 0 14px; color: var(--dim); }
.card .where { color: var(--tone); font-weight: 600; font-size: 15px; margin-bottom: 4px; }
.card .note { color: var(--dim); font-size: 15px; margin: 0; }

.go {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 50px;
  padding: 0 24px;
  border: 0;
  border-radius: 14px;
  background: var(--tone);
  color: var(--bg-deep);
  font: 700 18px/1 var(--display);
  transition: transform .2s var(--spring), opacity .3s, filter .2s;
}
.go:hover:not(:disabled) { transform: translateY(-2px) scale(1.02); filter: brightness(1.08); }
.go:active:not(:disabled) { transform: scale(.97); }
.go:disabled { opacity: .35; cursor: not-allowed; }
.quiet {
  min-height: 50px;
  padding: 0 16px;
  border: 1.5px solid var(--line);
  border-radius: 14px;
  background: transparent;
  color: var(--dim);
  font-weight: 600;
}
.quiet:hover { color: var(--ink); border-color: var(--dim); }

/* ───────────── Lesson ───────────── */
.lesson {
  height: 100%;
  display: grid;
  grid-template: "bar bar" auto "stage talk" minmax(0, 1fr) / minmax(0, 1.55fr) minmax(330px, 1fr);
  background: var(--bg-deep);
}
.lbar {
  grid-area: bar;
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 10px 16px;
  border-bottom: 1.5px solid var(--line);
}
.lbar .back { min-height: 40px; padding: 0 14px; }
.lsym {
  width: 40px;
  height: 40px;
  flex: none;
  display: grid;
  place-items: center;
  border-radius: 10px;
  background: var(--tone);
  color: var(--bg-deep);
  font: 800 19px/1 var(--display);
  view-transition-name: tile;
}
.lbar h1 { margin: 0; font: 700 20px/1.1 var(--display); letter-spacing: -.01em; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.lprog { flex: 1; min-width: 60px; height: 8px; border-radius: 8px; background: var(--line); overflow: hidden; }
.lprog i { display: block; height: 100%; width: 0; background: var(--tone); border-radius: 8px; transition: width .6s var(--ease); }

.stage {
  grid-area: stage;
  position: relative;
  display: flex;
  flex-direction: column;
  min-height: 0;
  background: var(--bg);
  border-right: 1.5px solid var(--line);
}
.stage-view { position: relative; flex: 1; min-height: 0; overflow: hidden; }
.stage-controls {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: center;
  gap: 10px 14px;
  padding: 12px 16px;
  border-top: 1.5px solid var(--line);
  background: var(--bg-deep);
  min-height: 68px;
}
.stage-controls:empty { display: none; }
.kit-canvas { position: absolute; inset: 0; width: 100%; height: 100%; display: block; touch-action: none; }

.talk { grid-area: talk; display: flex; flex-direction: column; min-height: 0; }
.talk-body {
  flex: 1;
  position: relative;
  overflow-y: auto;
  padding: 30px 28px 34px;
  /* --fit is lowered by the lesson runner when the text would overflow the pane. */
  font-size: calc(1em * var(--fit, 1));
  /* Fades the last lines when the text overflows, as a cue that the pane scrolls. */
  -webkit-mask-image: linear-gradient(to bottom, #000 calc(100% - 30px), transparent);
  mask-image: linear-gradient(to bottom, #000 calc(100% - 30px), transparent);
}
.stage.resting .stage-view { pointer-events: none; }
.stage-finish { position: absolute; inset: 0; display: grid; place-content: center; justify-items: center; gap: 4px; text-align: center; }
.stage-finish p { margin: 0; font: 700 22px/1.2 var(--display); max-width: 16ch; text-wrap: balance; }
.stage-finish .big-tile {
  width: clamp(96px, 22vmin, 168px);
  height: clamp(96px, 22vmin, 168px);
  font-size: clamp(42px, 10vmin, 76px);
  border-radius: 24px;
  margin-bottom: 18px;
}
.talk-body > * { animation: beat-in .5s var(--ease) backwards; }
.talk-body > :nth-child(2) { animation-delay: .06s; }
.talk-body > :nth-child(3) { animation-delay: .12s; }
.talk-body > :nth-child(4) { animation-delay: .18s; }
@keyframes beat-in { from { opacity: 0; transform: translateY(14px); } }
.talk .count { color: var(--tone); font-weight: 600; font-size: 15px; margin: 0 0 8px; }
.talk h2 { margin: 0 0 14px; font: 700 calc(32px * var(--fit, 1))/1.05 var(--display); letter-spacing: -.025em; text-wrap: balance; }
.talk p { margin: 0 0 14px; max-width: 46ch; }
.talk strong { color: var(--tone); font-weight: 700; }
.goal {
  display: flex;
  gap: 12px;
  align-items: flex-start;
  padding: 12px 14px;
  border: 1.5px dashed var(--tone);
  border-radius: 14px;
  font-weight: 600;
  transition: background .4s, border-color .4s;
}
.goal .tick {
  flex: none;
  width: 24px;
  height: 24px;
  border-radius: 50%;
  border: 2px solid var(--tone);
  display: grid;
  place-items: center;
  font-size: 14px;
  color: transparent;
  transition: background .3s, color .3s, transform .4s var(--spring);
}
.goal.met { background: color-mix(in srgb, var(--tone) 16%, transparent); border-style: solid; }
.goal.met .tick { background: var(--tone); color: var(--bg-deep); transform: scale(1.15); }
.talk-foot { display: grid; gap: 12px; padding: 12px 28px 22px; }
.foot-row { display: flex; gap: 10px; align-items: center; }
.foot-row .go { flex: 1; }
.goal-slot:empty { display: none; }
.stage.resting .stage-controls { display: none; }
.talk-foot .skip { border: 0; background: none; color: var(--dim); text-decoration: underline; font-size: 15px; padding: 8px; }

.options { display: grid; gap: 10px; margin-top: 18px; }
.option {
  text-align: left;
  padding: 14px 16px;
  border: 1.5px solid var(--line);
  border-radius: 14px;
  background: var(--panel);
  font-weight: 600;
  transition: transform .2s var(--spring), border-color .2s, background .3s;
}
.option:hover:not(:disabled) { border-color: var(--tone); transform: translateX(4px); }
.option.right { background: var(--tone); border-color: var(--tone); color: var(--bg-deep); }
.option.wrong { border-color: var(--sr); animation: shake .35s; opacity: .6; }
.option.wrong { text-decoration: line-through; text-decoration-thickness: 2px; }
.option.right::before { content: "✓ "; }
@keyframes shake { 25% { transform: translateX(-6px); } 75% { transform: translateX(6px); } }
.why { margin-top: 16px; color: var(--dim); }

.finish { text-align: left; }
.big-tile {
  width: 104px;
  height: 104px;
  display: grid;
  place-items: center;
  border-radius: 20px;
  background: var(--tone);
  color: var(--bg-deep);
  font: 800 46px/1 var(--display);
  margin-bottom: 20px;
  animation: pop .7s var(--spring) backwards;
  box-shadow: 0 0 0 0 var(--tone);
}
@keyframes pop {
  0% { transform: scale(.3) rotate(-18deg); opacity: 0; }
  60% { box-shadow: 0 0 0 22px color-mix(in srgb, var(--tone) 0%, transparent); }
}
.nexts { display: grid; gap: 10px; margin-top: 8px; }
.next {
  display: grid;
  grid-template-columns: 54px 1fr;
  gap: 14px;
  align-items: center;
  text-align: left;
  padding: 10px 14px 10px 10px;
  border: 1.5px solid var(--line);
  border-radius: 16px;
  background: var(--panel);
  transition: transform .2s var(--spring), border-color .2s;
}
.next:hover { transform: translateX(5px); border-color: var(--tone); }
.next .mini {
  width: 54px;
  height: 54px;
  display: grid;
  place-items: center;
  border-radius: 12px;
  border: 2.5px solid var(--tone);
  font: 800 23px/1 var(--display);
}
.next b { display: block; font: 700 18px/1.15 var(--display); }
.next small { color: var(--dim); font-size: 14.5px; }

/* ───────────── Practice ───────────── */
/* On the map a practice node is round and carries ten pips, one per problem. */
.tile.practice { --size: 66px; border-radius: 50%; border-style: dotted; border-width: 3px; }
.tile.practice[data-state="open"], .tile.practice[data-state="done"] { border-style: solid; }
.tile.practice[data-state="open"]::after { border-radius: 50%; }
.tile.practice[data-state="done"] { background: color-mix(in srgb, var(--tone) 22%, var(--bg-deep)); color: var(--ink); }
.tile.practice .name { top: calc(100% + 7px); font-size: 13.5px; font-style: italic; }
.pips { display: grid; grid-template-columns: repeat(5, 7px); grid-auto-rows: 7px; gap: 5px 3px; }
.pips i { border-radius: 50%; border: 1.5px solid currentColor; opacity: .75; }
.pips i.on { background: var(--tone); border-color: var(--tone); opacity: 1; }
.tie { stroke: var(--tone); stroke-width: 3; stroke-linecap: round; stroke-dasharray: 1 9; opacity: .9; animation: tie-in .4s var(--ease) backwards; }
.tie.met { stroke-dasharray: none; }
@keyframes tie-in { from { opacity: 0; } }
.tile.needed { opacity: 1; box-shadow: 0 0 0 4px color-mix(in srgb, var(--ink) 55%, transparent); }
.card .best, .finish .best { color: var(--tone); font-weight: 600; }
.tally-more { margin-top: 4px; font-size: 14.5px; color: var(--dim); font-weight: 600; }

/* A problem with no model takes the whole page as one column. */
.practice.solo { grid-template: "bar" auto "talk" minmax(0, 1fr) / minmax(0, 1fr); }
.practice.solo .stage { display: none; }
.practice.solo .talk { width: min(100%, 720px); margin: 0 auto; }
.practice.solo .talk-body { padding-top: clamp(18px, 7vh, 64px); }
.practice .lbar h1 { flex: 1; min-width: 0; }
.ppips { display: flex; gap: 7px; flex: none; }
.ppips i, .big-pips i { width: 14px; height: 14px; border-radius: 50%; border: 2px solid var(--line); transition: background .4s, border-color .4s, transform .4s var(--spring); }
.ppips i.now { border-color: var(--tone); transform: scale(1.2); }
.ppips i.first, .big-pips i.first { background: var(--tone); border-color: var(--tone); }
.ppips i.helped, .big-pips i.helped { border-color: var(--tone); background: color-mix(in srgb, var(--tone) 28%, transparent); }
.big-pips { display: grid; grid-template-columns: repeat(5, max-content); gap: 14px; margin-bottom: 26px; }
.big-pips i { width: 44px; height: 44px; border-width: 3px; animation: pop .6s var(--spring) backwards; }
.big-pips i:nth-child(2) { animation-delay: .08s; }
.big-pips i:nth-child(3) { animation-delay: .16s; }
.big-pips i:nth-child(4) { animation-delay: .24s; }
.big-pips i:nth-child(5) { animation-delay: .32s; }
.big-pips i:nth-child(6) { animation-delay: .4s; }
.big-pips i:nth-child(7) { animation-delay: .48s; }
.big-pips i:nth-child(8) { animation-delay: .56s; }
.big-pips i:nth-child(9) { animation-delay: .64s; }
.big-pips i:nth-child(10) { animation-delay: .72s; }

.prompt { font-size: 1.12em; line-height: 1.45; margin-bottom: 18px; max-width: 56ch; }
.prompt p { max-width: none; }
.prompt > :last-child { margin-bottom: 0; }
.prompt .eq { display: block; margin: 10px 0; font: 700 1.1em/1.3 var(--display); }
.prompt table { border-collapse: collapse; margin: 12px 0; font-size: .92em; font-variant-numeric: tabular-nums; }
.prompt th, .prompt td { padding: 5px 14px; border-bottom: 1.5px solid var(--line); text-align: left; }
.prompt th { color: var(--dim); font-weight: 600; }
.practice { --good: var(--ba); }
.practice .why { max-width: 56ch; min-height: 1.5em; margin-top: 16px; }
.practice .why:empty { min-height: 0; margin: 0; }
.practice .options { margin-top: 0; }
.practice .option.right { background: var(--good); border-color: var(--good); }

.blanks { display: flex; flex-wrap: wrap; gap: 12px 22px; align-items: flex-end; }
.blank { display: inline-flex; flex-wrap: wrap; align-items: baseline; gap: 6px 10px; font-weight: 600; }
.blank .lab { flex-basis: 100%; color: var(--dim); font-size: .9em; }
.blank .unit { color: var(--dim); }
.blank input {
  min-height: 52px;
  max-width: 100%;
  padding: 0 14px;
  border: 1.5px solid var(--line);
  border-radius: 14px;
  background: var(--panel);
  color: var(--ink);
  font: 700 22px/1 var(--display);
  font-variant-numeric: tabular-nums;
  transition: border-color .2s, background .3s;
}
.blank input:focus { border-color: var(--tone); outline: none; }
.blank input.bad { border-color: var(--sr); animation: shake .35s; }
.blank input.good { border-color: var(--good); background: color-mix(in srgb, var(--good) 18%, var(--panel)); }

.equation { display: flex; flex-wrap: wrap; align-items: center; gap: 12px 10px; font: 700 26px/1 var(--display); }
.equation .op { color: var(--dim); }
.term { display: inline-flex; align-items: center; gap: 8px; }
.term sub, .term sup { font-size: .62em; }
.stepper { display: grid; justify-items: center; border: 1.5px solid var(--line); border-radius: 12px; background: var(--panel); overflow: hidden; }
.stepper button { width: 44px; height: 36px; border: 0; background: none; color: var(--dim); font: 700 20px/1 var(--display); }
.stepper button:hover { color: var(--bg-deep); background: var(--tone); }
.stepper b { font-size: 24px; line-height: 1.2; color: var(--ink); font-variant-numeric: tabular-nums; }
.balance.locked .stepper button { visibility: hidden; height: 6px; }
.atom-tally { display: inline-grid; grid-template-columns: repeat(3, auto); gap: 4px 22px; margin-top: 18px; font-weight: 600; font-variant-numeric: tabular-nums; }
.atom-tally[hidden] { display: none; }
.atom-tally .head { color: var(--dim); font-size: .88em; }
.atom-tally span { color: var(--sr); }
.atom-tally span:nth-child(3n + 1), .atom-tally .head { color: var(--dim); }
.atom-tally .level { color: var(--good); }

.order-wrap { max-width: 460px; }
.order-wrap .end { color: var(--dim); font-size: .88em; font-weight: 600; margin: 4px 2px; }
.order { list-style: none; margin: 0; padding: 0; display: grid; gap: 8px; }
.order li {
  display: grid;
  grid-template-columns: auto 1fr auto auto;
  align-items: center;
  gap: 8px;
  min-height: 50px;
  padding: 4px 6px 4px 12px;
  border: 1.5px solid var(--line);
  border-radius: 14px;
  background: var(--panel);
  font-weight: 600;
  cursor: grab;
  touch-action: none;
  user-select: none;
  -webkit-user-select: none;
}
.order li.lifted { border-color: var(--tone); box-shadow: 0 10px 30px rgb(8 10 50 / .6); position: relative; z-index: 1; cursor: grabbing; }
.order .grip { color: var(--dim); letter-spacing: -3px; }
.order button { width: 40px; height: 40px; border: 0; border-radius: 10px; background: none; color: var(--dim); font-size: 18px; }
.order button:hover { background: var(--tone); color: var(--bg-deep); }
.order.locked li { cursor: default; border-color: var(--good); }
.order.locked button, .order.locked .grip { visibility: hidden; }

.sort { display: grid; gap: 8px; max-width: 560px; }
.sort-row {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 6px 12px;
  padding: 6px 6px 6px 14px;
  border: 1.5px solid var(--line);
  border-radius: 14px;
  background: var(--panel);
  font-weight: 600;
}
.sort-row.bad { border-color: var(--sr); }
.sort-row.good { border-color: var(--good); }
.sort-row .bins { display: inline-flex; gap: 4px; margin-left: auto; }
.sort-row .bins button { min-height: 38px; padding: 0 12px; border: 1.5px solid var(--line); border-radius: 10px; background: transparent; color: var(--dim); font-weight: 700; font-size: .92em; }
.sort-row .bins button[aria-pressed="true"] { background: var(--tone); border-color: var(--tone); color: var(--bg-deep); }
.next.practice .mini { border-radius: 50%; }

/* ───────────── Kit controls (used inside lessons) ───────────── */
.kit-slider { display: grid; gap: 4px; min-width: 190px; flex: 1 1 190px; max-width: 360px; font-size: 15px; }
.kit-slider .row { display: flex; justify-content: space-between; gap: 10px; color: var(--dim); font-weight: 600; }
.kit-slider output { color: var(--ink); font-variant-numeric: tabular-nums; }
.kit-slider input {
  -webkit-appearance: none;
  appearance: none;
  width: 100%;
  height: 26px;
  margin: 0;
  background: transparent;
  cursor: pointer;
}
.kit-slider input::-webkit-slider-runnable-track { height: 8px; border-radius: 8px; background: var(--line); }
.kit-slider input::-moz-range-track { height: 8px; border-radius: 8px; background: var(--line); }
.kit-slider input::-webkit-slider-thumb {
  -webkit-appearance: none;
  width: 26px;
  height: 26px;
  margin-top: -9px;
  border-radius: 50%;
  border: 0;
  background: var(--tone);
  box-shadow: 0 2px 10px rgb(0 0 0 / .4);
  transition: transform .15s var(--spring);
}
.kit-slider input::-moz-range-thumb { width: 26px; height: 26px; border-radius: 50%; border: 0; background: var(--tone); }
.kit-slider input:active::-webkit-slider-thumb { transform: scale(1.2); }
.kit-btn {
  min-height: 44px;
  padding: 0 18px;
  border: 1.5px solid var(--tone);
  border-radius: 12px;
  background: transparent;
  color: var(--ink);
  font-weight: 700;
  transition: transform .2s var(--spring), background .2s, color .2s;
}
/* Hover only where there is a pointer that hovers: on a touch screen it would stick to the last tap. */
@media (hover: hover) {
  .kit-btn:hover:not(:disabled) { background: var(--tone); color: var(--bg-deep); transform: translateY(-1px); }
}
.kit-btn:active:not(:disabled) { transform: scale(.95); }
.kit-btn:disabled { opacity: .4; cursor: not-allowed; }
.kit-btn.solid { background: var(--tone); color: var(--bg-deep); }
/* A question on the stage: the prompt, then answer buttons that all look alike. */
.kit-quiz { flex: 1 1 100%; min-width: 0; }
.kit-ask { display: grid; justify-items: center; gap: 8px; animation: ask-in .4s var(--ease) backwards; }
.kit-ask.leaving { opacity: 0; transform: translateY(-6px); transition: opacity .2s, transform .2s; }
.kit-ask-q { margin: 0; text-align: center; font-weight: 700; line-height: 1.25; text-wrap: balance; max-width: 60ch; }
.kit-ask-step { display: block; color: var(--tone); font-size: 13px; font-weight: 700; letter-spacing: .04em; text-transform: uppercase; }
.kit-ask-step:empty { display: none; }
.kit-ask-opts { display: flex; flex-wrap: wrap; justify-content: center; gap: 8px 10px; }
.kit-ask-opts .kit-btn { animation: ask-in .4s var(--ease) backwards; }
.kit-ask-opts .kit-btn:nth-child(2) { animation-delay: .05s; }
.kit-ask-opts .kit-btn:nth-child(3) { animation-delay: .1s; }
.kit-ask-opts .kit-btn:nth-child(n+4) { animation-delay: .15s; }
.kit-ask-opts .kit-btn { position: relative; }
.kit-ask-opts .kit-btn.right { background: var(--cu); border-color: var(--cu); color: var(--bg-deep); opacity: 1; }
.kit-ask-opts .kit-btn.wrong, .kit-ask-opts .kit-btn.out, .option.out {
  color: var(--dim); text-decoration: line-through; text-decoration-thickness: 2px; opacity: .5;
}
.kit-ask-opts .kit-btn.out { border-color: var(--line); }
/* The learner's own wrong pick stays told apart from the choices that were merely crossed out. */
.kit-ask-opts .kit-btn.wrong { border-color: var(--sr); opacity: .8; }
/* The tick and the cross are corner badges, so marking a choice never changes its width. */
.kit-ask-opts .kit-btn.right::after, .kit-ask-opts .kit-btn.wrong::after {
  content: "✓"; position: absolute; top: -8px; right: -8px; width: 20px; height: 20px; border-radius: 50%;
  display: grid; place-items: center; font: 700 12px/1 var(--body); text-decoration: none;
  background: var(--ink); color: var(--bg-deep); box-shadow: 0 0 0 2px var(--bg-deep);
}
.kit-ask-opts .kit-btn.wrong::after { content: "✕"; background: var(--sr); }
.kit-ask-note { margin: 0; font-size: 14px; color: var(--na); font-weight: 600; text-align: center; }
.kit-ask-note { min-height: 1.35em; }
.kit-ask-note:empty:not(.kept) { display: none; }
.kit-ask-done { margin: 0; font-weight: 700; color: var(--cu); animation: ask-in .4s var(--ease) backwards; }
.kit-ask-done::before { content: "✓ "; }
@keyframes ask-in { from { opacity: 0; transform: translateY(8px); } }

/* An explanation. It rises over the tray, so the model above stays in view, and waits to be closed. */
.kit-pop {
  position: absolute; inset: 0; z-index: 3;
  display: grid; align-items: end; justify-items: center; padding: 12px;
  background: linear-gradient(to top, rgb(18 21 74 / .85) 30%, rgb(18 21 74 / 0) 75%);
  animation: pop-fade .2s backwards;
}
.kit-pop-card {
  --edge: var(--line);
  position: relative; width: min(100%, 460px); max-height: 100%; overflow: auto;
  display: grid; gap: 8px; padding: 14px 16px 14px;
  border: 1.5px solid var(--edge); border-radius: 16px; background: var(--panel);
  box-shadow: 0 14px 40px rgb(0 0 0 / .45);
  animation: pop-in .32s var(--spring) backwards;
}
.kit-pop.good .kit-pop-card { --edge: var(--cu); }
.kit-pop.bad .kit-pop-card { --edge: var(--sr); }
.kit-pop h3 { margin: 0 36px 0 0; display: flex; align-items: center; gap: 8px; font: 700 19px/1.15 var(--display); }
.kit-pop h3 i {
  flex: none; width: 24px; height: 24px; border-radius: 50%; display: grid; place-items: center;
  background: var(--edge); color: var(--bg-deep); font: 700 14px/1 var(--body);
}
.kit-pop p { margin: 0; line-height: 1.4; }
.kit-pop .kit-btn { justify-self: end; margin-top: 2px; }
.kit-pop-x {
  position: absolute; top: 6px; right: 6px; width: 36px; height: 36px;
  border: 0; border-radius: 10px; background: none; color: var(--dim); font-size: 24px; line-height: 1;
}
.kit-pop-x:hover { color: var(--ink); background: rgb(255 255 255 / .08); }
.kit-pop.leaving { opacity: 0; transition: opacity .18s; pointer-events: none; }
.stage.resting .kit-pop { display: none; }
@keyframes pop-fade { from { opacity: 0; } }
@keyframes pop-in { from { opacity: 0; transform: translateY(24px) scale(.96); } }

.kit-chips { display: inline-flex; padding: 4px; gap: 4px; border-radius: 14px; background: var(--panel); flex-wrap: wrap; justify-content: center; }
.kit-chips button {
  min-height: 38px;
  padding: 0 14px;
  border: 0;
  border-radius: 10px;
  background: transparent;
  color: var(--dim);
  font-weight: 700;
  transition: background .25s, color .25s;
}
.kit-chips button[aria-pressed="true"] { background: var(--tone); color: var(--bg-deep); }
.kit-readout { display: grid; font-size: 14px; color: var(--dim); font-weight: 600; line-height: 1.2; text-align: center; min-width: 84px; }
.kit-readout b { font: 700 22px/1.1 var(--display); color: var(--ink); font-variant-numeric: tabular-nums; }

/* ───────────── Small screens ───────────── */
/* ───────────── Periodic table pop-up ───────────── */
.pt-open { min-height: 40px; padding: 0 14px; white-space: nowrap; flex: none; }
.pt-open .short { display: none; }
.zoomer .pt-open { min-height: 42px; color: var(--ink); }
.ptable {
  width: min(1120px, calc(100vw - 24px));
  max-height: calc(100dvh - 24px);
  padding: 16px 18px 14px;
  border: 1.5px solid var(--line);
  border-radius: 22px;
  background: var(--bg-deep);
  color: var(--ink);
  box-shadow: 0 24px 70px rgb(8 10 50 / .7);
  overflow: auto;
}
.ptable[open] { animation: rise .3s var(--spring); }
.ptable::backdrop { background: rgb(8 10 50 / .6); backdrop-filter: blur(3px); }
.ptable header { display: flex; align-items: center; justify-content: space-between; margin-bottom: 10px; }
.ptable h2 { margin: 0; font: 700 24px/1 var(--display); letter-spacing: -.02em; }
.ptable header .quiet { min-height: 40px; }
.pt-body { position: relative; }
.pt-grid {
  display: grid;
  grid-template-columns: repeat(18, minmax(0, 1fr));
  grid-template-rows: repeat(7, auto) 10px repeat(2, auto);
  gap: 3px;
}
[data-cat="a"] { --cat: var(--sr); }
[data-cat="e"] { --cat: var(--ca); }
[data-cat="t"] { --cat: var(--na); }
[data-cat="p"] { --cat: var(--tl); }
[data-cat="m"] { --cat: var(--cu); }
[data-cat="n"] { --cat: var(--cs); }
[data-cat="h"] { --cat: var(--k); }
[data-cat="g"] { --cat: var(--li); }
[data-cat="l"] { --cat: var(--fe); }
[data-cat="c"] { --cat: var(--in); }
[data-cat="u"] { --cat: var(--dim); }
.pt-cell {
  aspect-ratio: 1;
  min-width: 0;
  padding: 0;
  display: grid;
  place-items: center;
  position: relative;
  border: 1.5px solid color-mix(in srgb, var(--cat) 55%, transparent);
  border-radius: 8px;
  background: color-mix(in srgb, var(--cat) 14%, var(--bg-deep));
  color: var(--ink);
  transition: transform .18s var(--spring), background .2s;
}
.pt-cell small { position: absolute; top: 2px; left: 4px; font: 600 10px/1.2 var(--body); color: var(--dim); }
.pt-cell b { font: 700 clamp(13px, 1.75vw, 21px)/1 var(--display); margin-top: 6px; }
.pt-cell:hover, .pt-cell[aria-current] { transform: scale(1.12); z-index: 1; }
.pt-cell[aria-current] { background: var(--cat); color: var(--bg-deep); }
.pt-cell[aria-current] small { color: var(--bg-deep); }
.pt-gap {
  display: grid;
  place-items: center;
  border: 1.5px dashed color-mix(in srgb, var(--cat) 55%, transparent);
  border-radius: 8px;
  font: 600 10px/1 var(--body);
  color: var(--dim);
}
/* The detail panel sits in the empty space above the transition metals. */
.pt-detail {
  position: absolute;
  left: calc(100% * 2 / 18 + 8px);
  top: 0;
  width: calc(100% * 10 / 18 - 16px);
  aspect-ratio: 10 / 3.05;
  display: flex;
  align-items: center;
  gap: 18px;
  pointer-events: none;
}
.pt-big {
  height: 82%;
  aspect-ratio: 1;
  flex: none;
  display: grid;
  place-items: center;
  position: relative;
  border-radius: 16px;
  background: var(--cat);
  color: var(--bg-deep);
}
.pt-big small { position: absolute; top: 7px; left: 10px; font: 700 15px/1 var(--body); }
.pt-big b { font: 800 clamp(30px, 4.6vw, 58px)/1 var(--display); letter-spacing: -.02em; margin-top: 8px; }
.pt-facts h3 { margin: 0 0 2px; font: 700 24px/1.1 var(--display); letter-spacing: -.01em; }
.pt-facts p { margin: 0; font-size: 15px; line-height: 1.35; color: var(--dim); }
.pt-facts .pt-cat { color: var(--cat); font-weight: 600; }
.pt-legend { display: flex; flex-wrap: wrap; gap: 4px 16px; margin-top: 12px; font-size: 13.5px; color: var(--dim); }
.pt-legend span { display: inline-flex; align-items: center; gap: 6px; }
.pt-legend i { width: 11px; height: 11px; border-radius: 4px; background: var(--cat); }

@media (max-width: 820px) {
  .lesson { grid-template: "bar" auto "stage" minmax(0, 47fr) "talk" minmax(0, 53fr) / minmax(0, 1fr); }
  /* The questions and the finish screen need the room more than the resting model does. */
  .lesson:has(.stage.resting) { grid-template-rows: auto minmax(0, 34fr) minmax(0, 66fr); }
  .stage { border-right: 0; border-bottom: 1.5px solid var(--line); }
  .why { margin-top: 10px; }
  body { font-size: 16px; }
  .talk-body { padding: 14px 18px 30px; }
  .talk p { margin-bottom: 10px; }
  .next { padding: 7px 12px 7px 7px; }
  .next .mini { width: 46px; height: 46px; font-size: 20px; }
  .next small { display: none; }
  .talk h2 { font-size: calc(25px * var(--fit, 1)); margin-bottom: 8px; }
  .talk-foot { padding: 8px 18px 12px; gap: 8px; }
  .go, .quiet { min-height: 46px; }
  .goal { padding: 8px 12px; font-size: 15.5px; }
  .talk .count { margin-bottom: 4px; }
  .tally { font-size: 14.5px; white-space: nowrap; }
  .tally-bar { width: 70px; }
  .stage-controls { padding: 8px 10px; min-height: 0; gap: 6px 10px; }
  .kit-ask { gap: 6px; }
  /* Room for two lines always, so the model does not jump when a longer question arrives. */
  .kit-ask-q { font-size: 15.5px; min-height: 2.5em; display: grid; align-content: center; }
  .kit-ask-step { font-size: 12px; }
  .kit-ask-opts .kit-btn { padding: 0 14px; }
  .kit-pop { padding: 8px; }
  .kit-pop-card { padding: 12px 14px; gap: 6px; }
  .kit-pop h3 { font-size: 17px; }
  .kit-pop p { font-size: 15.5px; }
  .masthead { left: 8px; top: 8px; max-width: 60%; padding: 8px 12px 10px; }
  .masthead h1 { font-size: 34px; }
  .masthead p { display: none; }
  .legend { display: none; }
  /* Long titles wrap onto a second line; the progress bar becomes the header's bottom edge. */
  .lbar { position: relative; gap: 10px; }
  .lbar h1 { flex: 1; min-width: 0; font-size: 16px; white-space: normal; text-wrap: balance; display: -webkit-box; -webkit-box-orient: vertical; -webkit-line-clamp: 2; }
  .lprog { position: absolute; left: 0; right: 0; bottom: -1.5px; height: 3px; border-radius: 0; background: none; }
  .lprog i { border-radius: 0; }
  .tile { --size: 84px; }
  .tile.practice { --size: 60px; }
  .practice.solo .talk-body { padding-top: 16px; }
  .prompt { font-size: 1.05em; margin-bottom: 12px; }
  /* Ten pips in one row would crowd out the title: two rows of five. */
  .ppips { display: grid; grid-template-columns: repeat(5, 9px); gap: 4px; }
  .ppips i { width: 9px; height: 9px; border-width: 1.5px; }
  .equation { font-size: 21px; gap: 10px 6px; }
  .stepper button { width: 40px; height: 32px; }
  .blank input { font-size: 20px; min-height: 48px; }
  .big-pips { gap: 10px; }
  .big-pips i { width: 36px; height: 36px; }
  .pt-open { padding: 0 10px; }
  .pt-open .full { display: none; }
  .pt-open .short { display: inline; }
  .zoomer .pt-open { display: none; }
  .ptable { padding: 12px 12px 10px; }
  /* The table is wider than a phone: it scrolls sideways under a fixed detail panel. */
  .pt-detail { position: static; width: auto; aspect-ratio: auto; height: 96px; margin-bottom: 10px; gap: 12px; }
  .pt-big { height: 84px; }
  .pt-big b { font-size: 38px; }
  .pt-facts h3 { font-size: 20px; }
  .pt-facts p { font-size: 13.5px; }
  .pt-scroll { overflow-x: auto; margin: 0 -12px; padding: 6px 12px; }
  .pt-grid { width: 700px; }
  .pt-cell b { font-size: 15px; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: .01ms !important; animation-iteration-count: 1 !important; transition-duration: .01ms !important; }
}
::view-transition-old(root), ::view-transition-new(root) { animation-duration: .35s; }
::view-transition-group(tile) { animation-duration: .5s; animation-timing-function: cubic-bezier(.2, .8, .2, 1); }