/* Layout: a textbook column with live figures that break out wider; a sticky
   chapter rail on the left tracks position and quiz progress. */
:root {
  /* neutrals lean slightly blue, like graph paper */
  --bg: #f4f5f8;
  --surface: #ffffff;
  --surface-2: #eef0f4;
  --ink: #151a24;
  --ink-2: #474f60;
  --ink-3: #6f7788;
  --rule: #d9dde5;
  --grid: #e8ebf0;
  /* data colors: one job each, used the same way in every figure */
  --s1: #2a78d6;   /* the data */
  --s2: #eb6834;   /* first direction */
  --s3: #1baf7a;   /* second direction */
  --div-neg: #2a78d6;
  --div-pos: #e34948;
  --div-mid: #f0efec;
  /* status, quizzes only */
  --good: #0a7d0a;
  --good-bg: #e6f4e6;
  --bad: #c22f2f;
  --bad-bg: #fbe9e9;

  --font-body: "Literata", "Iowan Old Style", "Palatino Linotype", Georgia, serif;
  --font-ui: "IBM Plex Sans", "Segoe UI", system-ui, -apple-system, sans-serif;
  --font-mono: "IBM Plex Mono", ui-monospace, "Cascadia Mono", Consolas, monospace;

  --text-w: 43rem;
  --fig-w: 60rem;
  --rail-w: 15rem;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0e1116; --surface: #161a21; --surface-2: #1d222b;
    --ink: #e8ebf0; --ink-2: #b1b8c5; --ink-3: #8a92a1;
    --rule: #2c323d; --grid: #222833;
    --s1: #3987e5; --s2: #d95926; --s3: #199e70;
    --div-neg: #3987e5; --div-pos: #e66767; --div-mid: #383835;
    --good: #35b535; --good-bg: #14301a; --bad: #ee6f6f; --bad-bg: #381a1c;
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #0e1116; --surface: #161a21; --surface-2: #1d222b;
  --ink: #e8ebf0; --ink-2: #b1b8c5; --ink-3: #8a92a1;
  --rule: #2c323d; --grid: #222833;
  --s1: #3987e5; --s2: #d95926; --s3: #199e70;
  --div-neg: #3987e5; --div-pos: #e66767; --div-mid: #383835;
  --good: #35b535; --good-bg: #14301a; --bad: #ee6f6f; --bad-bg: #381a1c;
  color-scheme: dark;
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font: 400 1.0625rem/1.65 var(--font-body);
}
[hidden] { display: none !important; }
a { color: inherit; text-decoration-color: var(--ink-3); text-underline-offset: 0.18em; }
a:hover { text-decoration-color: var(--ink); }
:focus-visible { outline: 2px solid var(--ink); outline-offset: 2px; border-radius: 2px; }

/* ---------- page frame ---------- */
.page {
  display: grid;
  grid-template-columns: var(--rail-w) minmax(0, 1fr);
  gap: 3rem;
  max-width: 84rem;
  margin: 0 auto;
  padding-block: 0 6rem;
  padding-left: max(16px, env(safe-area-inset-left, 0px));
  padding-right: max(16px, env(safe-area-inset-right, 0px));
}
main { min-width: 0; overflow-x: clip; }  /* MathJax's hidden screen-reader copies can poke past the edge */
main > * , main section > * { max-width: var(--text-w); }
main section > .fig, main section > .quiz, main section > .wide, main > .fig { max-width: var(--fig-w); }
main section { max-width: none; }

/* ---------- rail ---------- */
.rail {
  position: sticky; top: 0; align-self: start;
  max-height: 100vh; overflow-y: auto;
  padding-block: 1.5rem 1.5rem;
  font: 400 0.84rem/1.4 var(--font-ui);
}
.rail-title { font-weight: 600; letter-spacing: 0.01em; margin: 0 0 0.9rem; }
.rail ol { list-style: none; margin: 0; padding: 0; }
.rail-part { margin-bottom: 0.75rem; }
.rail-part > a {
  display: block; font-weight: 600; text-decoration: none;
  padding: 0.2rem 0; color: var(--ink);
}
.rail-part ol a {
  display: flex; gap: 0.5rem; align-items: baseline;
  padding: 0.12rem 0 0.12rem 0.75rem; text-decoration: none;
  color: var(--ink-3); border-left: 1px solid var(--rule);
}
.rail-part > a.active { text-decoration: underline; text-underline-offset: 0.3em; text-decoration-thickness: 2px; }
.rail-part ol a:hover { color: var(--ink); }
.rail-part ol a.active { color: var(--ink); border-left-color: var(--ink); font-weight: 500; }
.rail-part ol a .score { margin-left: auto; font: 500 0.72rem var(--font-mono); color: var(--ink-3); white-space: nowrap; }
.rail-part ol a .score.full { color: var(--good); }
.rail-tools { display: flex; flex-wrap: wrap; gap: 0.5rem; margin-top: 1rem; }
.rail-total { margin: 0.9rem 0 0; color: var(--ink-2); }
.rail-total b { font: 600 0.84rem var(--font-mono); color: var(--ink); }
.meter { height: 4px; background: var(--rule); border-radius: 2px; margin-top: 0.4rem; overflow: hidden; }
.meter > i { display: block; height: 100%; width: 0; background: var(--ink); transition: width 0.3s; }

/* ---------- type ---------- */
header.top { padding-block: 4.5rem 1rem; }
.eyebrow {
  font: 600 0.78rem/1 var(--font-ui); letter-spacing: 0.12em; text-transform: uppercase;
  color: var(--ink-3); margin: 0 0 0.9rem;
}
h1, h2, h3, h4 { font-family: var(--font-ui); text-wrap: balance; line-height: 1.15; }
h1 { font-size: clamp(2.3rem, 5.2vw, 3.6rem); font-weight: 600; letter-spacing: -0.025em; margin: 0 0 1rem; }
.lede { font-size: 1.22rem; line-height: 1.55; color: var(--ink-2); margin: 0 0 1.5rem; }
h2.part {
  font-size: clamp(1.8rem, 3.6vw, 2.5rem); font-weight: 600; letter-spacing: -0.02em;
  margin: 0 0 0.6rem;
}
.part-head { margin-top: 6rem; padding-top: 2rem; border-top: 2px solid var(--ink); max-width: var(--fig-w); }
.part-head p { max-width: var(--text-w); color: var(--ink-2); font-size: 1.12rem; margin: 0; }
h3 { font-size: 1.42rem; font-weight: 600; letter-spacing: -0.01em; margin: 3.6rem 0 0.8rem; scroll-margin-top: 1.5rem; }
h3 .num { font: 500 0.95rem var(--font-mono); color: var(--ink-3); margin-right: 0.6rem; }
h4 { font-size: 1rem; font-weight: 600; margin: 1.8rem 0 0.4rem; }
p { margin: 0 0 1.05rem; }
ul, ol { padding-left: 1.3rem; margin: 0 0 1.05rem; }
li { margin-bottom: 0.35rem; }
strong { font-weight: 650; }
code, .mono { font-family: var(--font-mono); font-size: 0.9em; }
mjx-container[display="true"] { overflow-x: auto; overflow-y: hidden; margin: 1.1rem 0 !important; padding: 2px 0; }
.src {
  font: 400 0.82rem/1.45 var(--font-ui); color: var(--ink-3);
  margin: -0.3rem 0 1.1rem;
}
.key {
  border-left: 2px solid var(--ink); padding: 0.1rem 0 0.1rem 1rem; margin: 1.4rem 0;
  font-family: var(--font-ui); font-size: 1rem; line-height: 1.5;
}
.key p:last-child { margin-bottom: 0; }
details.aside {
  border-top: 1px solid var(--rule); border-bottom: 1px solid var(--rule);
  margin: 1.6rem 0; font-size: 0.98rem;
}
details.aside summary {
  cursor: pointer; padding: 0.7rem 0; font: 600 0.92rem var(--font-ui);
}
details.aside[open] summary { margin-bottom: 0.3rem; }
.chip {
  display: inline-block; width: 0.68em; height: 0.68em; border-radius: 50%;
  margin: 0 0.28em 0 0.1em; vertical-align: baseline;
}
.chip.s1 { background: var(--s1); } .chip.s2 { background: var(--s2); } .chip.s3 { background: var(--s3); }
.chip.ink { background: var(--ink); }

/* ---------- tables ---------- */
.table-wrap { overflow-x: auto; margin: 1.4rem 0; max-width: var(--fig-w); }
table { border-collapse: collapse; font: 400 0.92rem/1.45 var(--font-ui); min-width: 34rem; width: 100%; }
th, td { text-align: left; vertical-align: top; padding: 0.6rem 0.9rem 0.6rem 0; border-bottom: 1px solid var(--rule); }
thead th { font-weight: 600; border-bottom: 1px solid var(--ink); }
tbody th { font-weight: 600; white-space: nowrap; }

/* ---------- figures ---------- */
.fig {
  margin: 2rem 0 2.2rem; background: var(--surface);
  border: 1px solid var(--rule); border-radius: 4px;
  font-family: var(--font-ui);
}
.fig-head {
  display: flex; flex-wrap: wrap; gap: 0.4rem 1rem; align-items: baseline;
  padding: 0.75rem 1.1rem; border-bottom: 1px solid var(--rule);
}
.fig-head .fig-num { font: 500 0.78rem var(--font-mono); color: var(--ink-3); }
.fig-head .fig-title { font-weight: 600; font-size: 0.98rem; }
.fig-body { padding: 1.1rem; display: grid; gap: 1.1rem; }
.fig-body.split { grid-template-columns: minmax(0, 1.15fr) minmax(0, 1fr); align-items: start; }
.fig-body.halves { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.fig-body.thirds { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.fig-side { display: grid; gap: 0.95rem; align-content: start; min-width: 0; }
.panel { min-width: 0; }
.panel-label {
  font: 600 0.72rem/1.2 var(--font-ui); letter-spacing: 0.08em; text-transform: uppercase;
  color: var(--ink-3); margin: 0 0 0.4rem;
}
canvas { display: block; width: 100%; max-width: 100%; border-radius: 2px; }
canvas.draggable { touch-action: none; cursor: grab; }
canvas.plot { background:
  linear-gradient(var(--surface), var(--surface)); }
figcaption, .fig-cap {
  padding: 0.8rem 1.1rem 0.95rem; border-top: 1px solid var(--rule);
  font-size: 0.88rem; line-height: 1.5; color: var(--ink-2);
}
.fig-cap b, figcaption b { color: var(--ink); font-weight: 600; }
.fig-note { font-size: 0.9rem; line-height: 1.5; color: var(--ink-2); margin: 0; }
.fig-note b { color: var(--ink); }

/* controls */
.row { display: flex; flex-wrap: wrap; gap: 0.5rem; align-items: center; }
.btn {
  font: 500 0.84rem/1 var(--font-ui); color: var(--ink);
  background: var(--surface); border: 1px solid var(--rule); border-radius: 4px;
  padding: 0.5rem 0.75rem; cursor: pointer;
}
.btn:hover { border-color: var(--ink-3); }
.btn.primary { background: var(--ink); color: var(--surface); border-color: var(--ink); }
.btn.primary:hover { opacity: 0.88; }
.btn[aria-pressed="true"], .seg .btn[aria-checked="true"] { background: var(--ink); color: var(--surface); border-color: var(--ink); }
.btn:disabled { opacity: 0.45; cursor: default; }
.seg { display: inline-flex; flex-wrap: wrap; gap: 0; }
.seg .btn { border-radius: 0; margin-left: -1px; }
.seg .btn:first-child { border-radius: 4px 0 0 4px; margin-left: 0; }
.seg .btn:last-child { border-radius: 0 4px 4px 0; }
.ctl { display: grid; gap: 0.25rem; }
.ctl label, .ctl .ctl-label {
  display: flex; justify-content: space-between; gap: 1rem;
  font-size: 0.82rem; color: var(--ink-2);
}
.ctl output, .val { font: 500 0.82rem var(--font-mono); color: var(--ink); font-variant-numeric: tabular-nums; }
input[type="range"] { width: 100%; accent-color: var(--ink); margin: 0; height: 1.4rem; }
select {
  font: 500 0.84rem var(--font-ui); color: var(--ink); background: var(--surface);
  border: 1px solid var(--rule); border-radius: 4px; padding: 0.42rem 0.5rem;
}

/* readouts */
.readout {
  display: grid; grid-template-columns: auto 1fr; gap: 0.3rem 1rem;
  font-size: 0.84rem; margin: 0; align-items: baseline;
}
.readout dt { color: var(--ink-2); }
.readout dd { margin: 0; font: 500 0.84rem var(--font-mono); font-variant-numeric: tabular-nums; text-align: right; }
.matrix {
  display: inline-grid; grid-template-columns: repeat(2, auto); gap: 0.15rem 0.9rem;
  font: 500 0.86rem var(--font-mono); font-variant-numeric: tabular-nums;
  padding: 0.3rem 0.6rem; border-left: 1.5px solid var(--ink); border-right: 1.5px solid var(--ink);
  border-radius: 3px; text-align: right;
}
.matrix-row { display: flex; align-items: center; gap: 0.6rem; font-size: 0.84rem; color: var(--ink-2); flex-wrap: wrap; }
.legend { display: flex; flex-wrap: wrap; gap: 0.3rem 1rem; font-size: 0.8rem; color: var(--ink-2); margin: 0; padding: 0; list-style: none; }
.legend li { margin: 0; display: flex; align-items: center; gap: 0.35rem; }
.swatch { width: 0.9rem; height: 3px; border-radius: 1px; display: inline-block; }
.swatch.dot { width: 0.6rem; height: 0.6rem; border-radius: 50%; }
.swatch.dash { background: repeating-linear-gradient(90deg, var(--ink-3) 0 4px, transparent 4px 7px) !important; }

/* stacked bar (variance vs error) */
.stack { display: flex; gap: 2px; height: 1.5rem; }
.stack > span { display: block; height: 100%; border-radius: 2px; min-width: 2px; }
.stack-labels { display: flex; justify-content: space-between; gap: 1rem; font-size: 0.8rem; color: var(--ink-2); margin-top: 0.3rem; }
.stack-labels b { font: 500 0.82rem var(--font-mono); color: var(--ink); }

/* stretch meter */
.range-meter { position: relative; height: 1.5rem; }
.range-meter .track { position: absolute; left: 0; right: 0; top: 50%; height: 2px; background: var(--rule); }
.range-meter .dot { position: absolute; top: 50%; width: 12px; height: 12px; margin: -6px 0 0 -6px; border-radius: 50%; background: var(--ink); border: 2px solid var(--surface); }
.range-ends { display: flex; justify-content: space-between; font: 500 0.78rem var(--font-mono); color: var(--ink-2); }

/* challenge strip inside figures */
.challenge {
  display: flex; gap: 0.6rem; align-items: baseline;
  font-size: 0.86rem; line-height: 1.45; padding: 0.6rem 0.75rem;
  background: var(--surface-2); border-radius: 4px; color: var(--ink-2);
}
.challenge .mark { font: 600 0.8rem var(--font-mono); color: var(--ink-3); white-space: nowrap; }
.challenge.done { background: var(--good-bg); color: var(--ink); }
.challenge.done .mark { color: var(--good); }

/* image tiles */
.tiles { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 0.9rem; }
.tile canvas { image-rendering: pixelated; aspect-ratio: 1; background: var(--surface-2); }
.pcs { display: grid; grid-template-columns: repeat(6, minmax(0, 1fr)); gap: 0.7rem; }
.pc canvas { image-rendering: pixelated; aspect-ratio: 1; }
.pc .ctl label { font-size: 0.74rem; }
.pc p { margin: 0.3rem 0 0.1rem; font-size: 0.74rem; color: var(--ink-2); }

/* signal rows */
.signals { display: grid; gap: 0.3rem; }
.sig-group { font: 600 0.72rem/1.2 var(--font-ui); letter-spacing: 0.08em; text-transform: uppercase; color: var(--ink-3); margin: 0.7rem 0 0.15rem; }
.sig-group:first-child { margin-top: 0; }
.sig { display: grid; grid-template-columns: 2.1rem 9.5rem minmax(0, 1fr); gap: 0.6rem; align-items: center; }
.sig canvas { height: 44px; }
.sig .name { font-size: 0.8rem; line-height: 1.25; color: var(--ink-2); }
.sig .name b { display: block; color: var(--ink); font-weight: 600; }
.play {
  width: 2.1rem; height: 2.1rem; border-radius: 50%; border: 1px solid var(--rule);
  background: var(--surface); color: var(--ink); cursor: pointer; font-size: 0.7rem; line-height: 1;
  display: grid; place-items: center; padding: 0;
}
.play:hover { border-color: var(--ink-3); }
.play.playing { background: var(--ink); color: var(--surface); border-color: var(--ink); }

/* tooltip */
.tip {
  position: fixed; z-index: 50; pointer-events: none;
  background: var(--ink); color: var(--surface);
  font: 500 0.76rem/1.35 var(--font-ui); padding: 0.35rem 0.55rem; border-radius: 3px;
  max-width: 16rem; white-space: nowrap;
}
.tip b { font-family: var(--font-mono); font-weight: 500; }

/* ---------- quizzes ---------- */
.quiz {
  margin: 3.2rem 0 1rem; background: var(--surface);
  border: 1px solid var(--ink); border-radius: 4px; font-family: var(--font-ui);
}
.quiz-head {
  display: flex; justify-content: space-between; align-items: baseline; gap: 1rem; flex-wrap: wrap;
  padding: 0.9rem 1.2rem; border-bottom: 1px solid var(--ink);
}
.quiz-head h3 { margin: 0; font-size: 1.1rem; }
.quiz-score { font: 500 0.84rem var(--font-mono); color: var(--ink-2); }
.quiz-foot { padding: 0.9rem 1.2rem; border-top: 1px solid var(--rule); display: flex; gap: 1rem; align-items: center; flex-wrap: wrap; font-size: 0.88rem; color: var(--ink-2); }
.q { padding: 1.15rem 1.2rem 1.25rem; border-bottom: 1px solid var(--rule); }
.quiz .q:last-of-type { border-bottom: 0; }
.q-text { font-size: 0.98rem; line-height: 1.5; margin: 0 0 0.75rem; font-weight: 500; }
.q-num { font: 500 0.78rem var(--font-mono); color: var(--ink-3); margin-right: 0.5rem; }
.opts { display: grid; gap: 0.4rem; }
.opt {
  display: grid; grid-template-columns: 1.4rem minmax(0, 1fr); gap: 0.5rem; align-items: baseline;
  text-align: left; width: 100%;
  font: 400 0.93rem/1.45 var(--font-ui); color: var(--ink);
  background: var(--surface); border: 1px solid var(--rule); border-radius: 4px;
  padding: 0.55rem 0.75rem; cursor: pointer;
}
.opt:hover:not(:disabled) { border-color: var(--ink-3); }
.opt .letter { font: 500 0.8rem var(--font-mono); color: var(--ink-3); }
.opt:disabled { cursor: default; color: var(--ink-2); }
.opt.right { border-color: var(--good); background: var(--good-bg); color: var(--ink); }
.opt.wrong { border-color: var(--bad); background: var(--bad-bg); color: var(--ink); }
.opt.right .letter { color: var(--good); } .opt.wrong .letter { color: var(--bad); }
.why { margin: 0.7rem 0 0; font-size: 0.92rem; line-height: 1.55; color: var(--ink-2); }
.why b.ok { color: var(--good); } .why b.no { color: var(--bad); }
.check {
  margin: 1.8rem 0; background: var(--surface); border: 1px solid var(--rule);
  border-radius: 4px; font-family: var(--font-ui); max-width: var(--text-w);
}
.check .q { border-bottom: 0; }
.check-label { font: 600 0.72rem/1 var(--font-ui); letter-spacing: 0.1em; text-transform: uppercase; color: var(--ink-3); padding: 0.9rem 1.2rem 0; margin: 0; }

footer.end { margin-top: 5rem; padding-top: 1.5rem; border-top: 1px solid var(--rule); font: 400 0.86rem/1.55 var(--font-ui); color: var(--ink-2); max-width: var(--fig-w); }
.sources { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 1.5rem; max-width: var(--fig-w); font: 400 0.9rem/1.5 var(--font-ui); }
.sources h4 { margin-top: 0; }
.sources ol { padding-left: 1.1rem; }
.sources li { margin-bottom: 0.7rem; }
.sources small { display: block; color: var(--ink-3); }

/* ---------- narrow ---------- */
@media (max-width: 62rem) {
  .page { grid-template-columns: minmax(0, 1fr); gap: 0; }
  .rail { position: static; max-height: none; padding-block: 1.2rem 0; border-bottom: 1px solid var(--rule); }
  .rail-title { margin-bottom: 0.6rem; }
  .rail nav > ol { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 0 1.5rem; }
  header.top { padding-top: 2.5rem; }
}
@media (max-width: 46rem) {
  body { font-size: 1rem; }
  .fig-body.split, .fig-body.halves, .fig-body.thirds { grid-template-columns: minmax(0, 1fr); }
  .tiles { gap: 0.5rem; }
  .pcs { grid-template-columns: repeat(3, minmax(0, 1fr)); }
  .sources { grid-template-columns: minmax(0, 1fr); }
  .sig { grid-template-columns: 2.1rem 6.2rem minmax(0, 1fr); gap: 0.4rem; }
  .rail nav > ol { grid-template-columns: minmax(0, 1fr); }
  .part-head { margin-top: 4rem; }
}