/* ==========================================================================
   MTH603 Study Companion — stylesheet
   Design: engineering notebook. Ruled-paper formula slips, ink palette,
   monospace for anything you would write by hand.
   Mobile-first. All breakpoints scale UP from 320px.
   ========================================================================== */

/* ---------- Tokens ---------- */
:root {
  /* Light ("paper") */
  --paper:        #F4F7FB;
  --surface:      #FFFFFF;
  --surface-2:    #EDF2F9;
  --ink:          #151A24;
  --ink-soft:     #4A5568;
  --ink-faint:    #7B8794;
  --rule:         #C9D8EC;
  --rule-soft:    #E2EAF4;

  --signal:       #3B4CD8;   /* electric indigo — links, active state */
  --signal-soft:  #E8EAFD;
  --critical:     #D6336C;   /* crimson — must-know */
  --critical-soft:#FCE8F0;
  --warm:         #B26A00;   /* amber — traps & warnings */
  --warm-soft:    #FDF2DC;
  --grow:         #1B7A5A;   /* green — correct answers, done */
  --grow-soft:    #E1F3ED;

  /* Type */
  --display: "Space Grotesk", "Segoe UI", system-ui, sans-serif;
  --body:    "IBM Plex Serif", Georgia, "Times New Roman", serif;
  --mono:    "IBM Plex Mono", "SF Mono", Consolas, monospace;

  /* Scale */
  --fs-base: 17px;
  --sp: 4px;
  --radius: 10px;
  --maxw: 760px;

  --shadow-sm: 0 1px 2px rgba(21,26,36,.06), 0 1px 3px rgba(21,26,36,.05);
  --shadow-md: 0 4px 14px rgba(21,26,36,.09);
}

[data-theme="dark"] {
  --paper:        #0E1219;
  --surface:      #161C26;
  --surface-2:    #1E2733;
  --ink:          #E6ECF4;
  --ink-soft:     #A9B6C6;
  --ink-faint:    #768396;
  --rule:         #2C3949;
  --rule-soft:    #222D3B;

  --signal:       #8B97FF;
  --signal-soft:  #1F2647;
  --critical:     #FF7BA8;
  --critical-soft:#3A1A28;
  --warm:         #F0B45E;
  --warm-soft:    #35280F;
  --grow:         #5FD3AC;
  --grow-soft:    #102E26;

  --shadow-sm: 0 1px 2px rgba(0,0,0,.4);
  --shadow-md: 0 4px 14px rgba(0,0,0,.5);
}

/* Font-size preference set by the Aa control */
html[data-fs="s"]  { --fs-base: 15px; }
html[data-fs="m"]  { --fs-base: 17px; }
html[data-fs="l"]  { --fs-base: 19px; }
html[data-fs="xl"] { --fs-base: 21px; }

/* ---------- Reset ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; }
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after { animation-duration: .001ms !important; transition-duration: .001ms !important; }
}

body {
  margin: 0;
  background: var(--paper);
  color: var(--ink);
  font-family: var(--body);
  font-size: var(--fs-base);
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
  overflow-wrap: break-word;
  padding-bottom: 72px; /* room for bottom bar */
}

img { max-width: 100%; }

:focus-visible {
  outline: 3px solid var(--signal);
  outline-offset: 2px;
  border-radius: 4px;
}

/* ---------- Top bar ---------- */
.topbar {
  position: sticky; top: 0; z-index: 40;
  background: var(--surface);
  border-bottom: 1px solid var(--rule);
  box-shadow: var(--shadow-sm);
}
.topbar__row {
  display: flex; align-items: center; gap: 8px;
  max-width: var(--maxw); margin: 0 auto;
  padding: 10px 14px;
}
.brand {
  font-family: var(--display);
  font-weight: 700; font-size: .95rem; letter-spacing: -.01em;
  line-height: 1.15; margin-right: auto;
}
.brand small {
  display: block; font-family: var(--mono); font-weight: 400;
  font-size: .62rem; letter-spacing: .08em; text-transform: uppercase;
  color: var(--ink-faint);
}
.iconbtn {
  flex: 0 0 auto;
  min-width: 40px; height: 40px; padding: 0 10px;
  display: inline-flex; align-items: center; justify-content: center; gap: 4px;
  background: var(--surface-2); color: var(--ink);
  border: 1px solid var(--rule); border-radius: 8px;
  font-family: var(--display); font-size: .85rem; font-weight: 600;
  cursor: pointer;
}
.iconbtn:active { transform: translateY(1px); }

/* Search */
.search { max-width: var(--maxw); margin: 0 auto; padding: 0 14px 10px; }
.search input {
  width: 100%; height: 42px; padding: 0 12px;
  font-family: var(--body); font-size: .95rem;
  color: var(--ink); background: var(--surface-2);
  border: 1px solid var(--rule); border-radius: 8px;
}
.search input::placeholder { color: var(--ink-faint); }

/* Progress */
.progress {
  max-width: var(--maxw); margin: 0 auto; padding: 0 14px 10px;
  display: flex; align-items: center; gap: 10px;
  font-family: var(--mono); font-size: .68rem; color: var(--ink-faint);
  letter-spacing: .04em;
}
.progress__track {
  flex: 1; height: 6px; border-radius: 99px;
  background: var(--surface-2); border: 1px solid var(--rule); overflow: hidden;
}
.progress__fill {
  height: 100%; width: 0%; background: var(--grow);
  transition: width .35s ease;
}

/* ---------- Layout ---------- */
main { max-width: var(--maxw); margin: 0 auto; padding: 18px 14px 40px; }

/* ---------- Hero ---------- */
.hero {
  background: var(--surface);
  border: 1px solid var(--rule);
  border-radius: var(--radius);
  padding: 22px 18px;
  margin-bottom: 20px;
  position: relative; overflow: hidden;
}
.hero::before {
  /* graph-paper wash */
  content: ""; position: absolute; inset: 0; pointer-events: none; opacity: .5;
  background-image:
    linear-gradient(var(--rule-soft) 1px, transparent 1px),
    linear-gradient(90deg, var(--rule-soft) 1px, transparent 1px);
  background-size: 22px 22px;
  mask-image: linear-gradient(to bottom, rgba(0,0,0,.9), transparent);
  -webkit-mask-image: linear-gradient(to bottom, rgba(0,0,0,.9), transparent);
}
.hero > * { position: relative; }
.hero h1 {
  font-family: var(--display); font-weight: 700;
  font-size: 1.75rem; line-height: 1.1; letter-spacing: -.02em;
  margin: 6px 0 8px;
}
.hero p { margin: 0 0 14px; color: var(--ink-soft); font-size: .95rem; }
.eyebrow {
  font-family: var(--mono); font-size: .66rem; letter-spacing: .14em;
  text-transform: uppercase; color: var(--signal); font-weight: 500;
}
.hero__cta { display: flex; flex-wrap: wrap; gap: 8px; }
.btn {
  display: inline-flex; align-items: center; gap: 6px;
  padding: 11px 16px; border-radius: 8px;
  font-family: var(--display); font-weight: 600; font-size: .88rem;
  border: 1px solid var(--signal); background: var(--signal); color: #fff;
  text-decoration: none; cursor: pointer;
}
.btn--ghost { background: transparent; color: var(--signal); }
.btn:active { transform: translateY(1px); }

/* ---------- Section (accordion) ---------- */
.sec {
  background: var(--surface);
  border: 1px solid var(--rule);
  border-radius: var(--radius);
  margin-bottom: 12px;
  overflow: hidden;
  scroll-margin-top: 120px;
}
.sec__head {
  display: flex;
  list-style: none; align-items: flex-start; gap: 10px;
  width: 100%; text-align: left;
  padding: 14px 14px;
  background: none; border: 0; cursor: pointer;
  color: inherit; font: inherit;
}
.sec__num {
  flex: 0 0 auto;
  font-family: var(--mono); font-size: .7rem; font-weight: 500;
  color: var(--ink-faint); padding-top: 4px; min-width: 26px;
}
.sec__titlewrap { flex: 1; min-width: 0; }
.sec__title {
  font-family: var(--display); font-weight: 600; font-size: 1.02rem;
  line-height: 1.3; letter-spacing: -.01em; display: block;
}
.sec__meta { display: flex; flex-wrap: wrap; gap: 6px; margin-top: 6px; }
.sec__chev {
  flex: 0 0 auto; width: 26px; height: 26px; margin-top: 2px;
  display: grid; place-items: center;
  color: var(--ink-faint); transition: transform .25s ease;
}
.sec__head::-webkit-details-marker { display: none; }
.sec__head::marker { content: ""; }
.sec[open] .sec__chev { transform: rotate(90deg); }
.sec__body { padding: 0 14px 18px; }
.sec__body > *:first-child { margin-top: 0; }

/* Priority pills */
.pill {
  display: inline-flex; align-items: center; gap: 4px;
  font-family: var(--mono); font-size: .6rem; letter-spacing: .07em;
  text-transform: uppercase; font-weight: 500;
  padding: 3px 7px; border-radius: 99px; border: 1px solid;
}
.pill--must { color: var(--critical); background: var(--critical-soft); border-color: var(--critical); }
.pill--high { color: var(--warm);     background: var(--warm-soft);     border-color: var(--warm); }
.pill--mid  { color: var(--signal);   background: var(--signal-soft);   border-color: var(--signal); }
.pill--low  { color: var(--ink-faint);background: var(--surface-2);     border-color: var(--rule); }
.pill--done { color: var(--grow);     background: var(--grow-soft);     border-color: var(--grow); }

/* ---------- Typography inside sections ---------- */
.sec__body h3 {
  font-family: var(--display); font-weight: 600; font-size: 1rem;
  margin: 26px 0 8px; letter-spacing: -.01em;
  padding-bottom: 6px; border-bottom: 2px solid var(--rule-soft);
}
.sec__body h4 {
  font-family: var(--display); font-weight: 600; font-size: .9rem;
  margin: 18px 0 6px; color: var(--ink-soft);
}
.sec__body p { margin: 0 0 12px; }
.sec__body ul, .sec__body ol { margin: 0 0 14px; padding-left: 22px; }
.sec__body li { margin-bottom: 7px; }
.sec__body strong { font-weight: 700; }
code {
  font-family: var(--mono); font-size: .88em;
  background: var(--surface-2); padding: 1px 5px; border-radius: 4px;
  border: 1px solid var(--rule-soft);
}

/* ---------- Callout boxes ---------- */
.box {
  border-radius: 8px; padding: 13px 14px; margin: 0 0 14px;
  border: 1px solid; border-left-width: 4px;
  font-size: .93rem;
}
.box__label {
  display: block;
  font-family: var(--mono); font-size: .62rem; letter-spacing: .12em;
  text-transform: uppercase; font-weight: 500; margin-bottom: 6px;
}
.box p:last-child, .box ul:last-child, .box ol:last-child { margin-bottom: 0; }

.box--plain  { background: var(--signal-soft);  border-color: var(--signal); }
.box--plain  .box__label { color: var(--signal); }
.box--trap   { background: var(--warm-soft);    border-color: var(--warm); }
.box--trap   .box__label { color: var(--warm); }
.box--exam   { background: var(--critical-soft);border-color: var(--critical); }
.box--exam   .box__label { color: var(--critical); }
.box--tip    { background: var(--grow-soft);    border-color: var(--grow); }
.box--tip    .box__label { color: var(--grow); }

/* ---------- Formula slip (signature element) ---------- */
.slip {
  position: relative;
  background: var(--surface-2);
  border: 1px solid var(--rule);
  border-radius: 8px;
  margin: 0 0 14px;
  overflow: hidden;
}
.slip::before {
  /* ruled-paper lines behind the formula */
  content: ""; position: absolute; inset: 0; pointer-events: none; opacity: .55;
  background-image: repeating-linear-gradient(
    to bottom, transparent, transparent 27px, var(--rule-soft) 27px, var(--rule-soft) 28px
  );
}
.slip__tab {
  position: relative;
  display: inline-block;
  font-family: var(--mono); font-size: .6rem; letter-spacing: .12em;
  text-transform: uppercase; font-weight: 500;
  color: var(--surface); background: var(--ink-soft);
  padding: 3px 9px; border-radius: 0 0 6px 0;
}
.slip__body {
  position: relative;
  padding: 12px 14px 14px;
  font-family: var(--mono); font-size: .92rem; line-height: 1.9;
  overflow-x: auto;
}
.slip__body b { color: var(--signal); font-weight: 600; }
.slip--key { border-color: var(--critical); border-width: 2px; }
.slip--key .slip__tab { background: var(--critical); }

/* Symbol decoder */
.decoder { margin: 0 0 14px; }
.decoder dt {
  font-family: var(--mono); font-weight: 600; font-size: .88rem;
  color: var(--signal); margin-top: 8px;
}
.decoder dd { margin: 2px 0 0; font-size: .92rem; color: var(--ink-soft); }

/* ---------- Tables ---------- */
.tablewrap {
  overflow-x: auto; -webkit-overflow-scrolling: touch;
  margin: 0 0 14px; border: 1px solid var(--rule); border-radius: 8px;
}
table { border-collapse: collapse; width: 100%; font-size: .85rem; }
th, td {
  padding: 8px 10px; text-align: left;
  border-bottom: 1px solid var(--rule-soft);
  white-space: nowrap;
}
th {
  font-family: var(--display); font-weight: 600; font-size: .78rem;
  background: var(--surface-2); position: sticky; top: 0;
}
tbody tr:last-child td { border-bottom: 0; }
td.num, th.num { font-family: var(--mono); }
.tablewrap--wrap td { white-space: normal; min-width: 120px; }
.scrollhint {
  font-family: var(--mono); font-size: .62rem; color: var(--ink-faint);
  margin: -8px 0 14px; letter-spacing: .05em;
}

/* ---------- Step-by-step reveal ---------- */
.steps {
  border: 1px solid var(--rule); border-radius: 8px;
  background: var(--surface); margin: 0 0 14px; overflow: hidden;
}
.steps__head {
  padding: 11px 14px; background: var(--surface-2);
  border-bottom: 1px solid var(--rule);
  font-family: var(--display); font-weight: 600; font-size: .88rem;
  display: flex; align-items: center; gap: 8px; flex-wrap: wrap;
}
.steps__count {
  font-family: var(--mono); font-size: .65rem; color: var(--ink-faint);
  margin-left: auto; letter-spacing: .06em;
}
.steps__list { list-style: none; margin: 0; padding: 0; }
.steps__list li {
  display: none;
  padding: 13px 14px; border-bottom: 1px solid var(--rule-soft);
  margin: 0; font-size: .93rem;
  animation: stepIn .28s ease;
}
.steps__list li.shown { display: block; }
@keyframes stepIn { from { opacity: 0; transform: translateY(-4px); } to { opacity: 1; transform: none; } }
.steps__list li b {
  display: block; font-family: var(--mono); font-size: .66rem;
  letter-spacing: .1em; text-transform: uppercase;
  color: var(--signal); margin-bottom: 4px; font-weight: 500;
}
.steps__list .math {
  display: block; font-family: var(--mono); font-size: .88rem;
  background: var(--surface-2); padding: 8px 10px; border-radius: 6px;
  margin-top: 6px; overflow-x: auto; border: 1px solid var(--rule-soft);
}
.steps__foot { padding: 11px 14px; display: flex; gap: 8px; }
.steps__foot .btn { flex: 1; justify-content: center; }
.steps.done .steps__next { display: none; }

/* ---------- Inline quiz (per section) ---------- */
.q {
  border: 1px solid var(--rule); border-radius: 8px;
  padding: 13px 14px; margin: 0 0 12px; background: var(--surface);
}
.q__stem { font-weight: 700; margin: 0 0 10px; font-size: .93rem; }
.q__opts { list-style: none; margin: 0 0 0; padding: 0; }
.q__opts li { margin: 0 0 7px; }
.q__opt {
  display: flex; align-items: flex-start; gap: 9px; width: 100%;
  text-align: left; padding: 10px 12px;
  background: var(--surface-2); color: var(--ink);
  border: 1px solid var(--rule); border-radius: 7px;
  font: inherit; font-size: .9rem; cursor: pointer;
}
.q__opt .k {
  flex: 0 0 auto; font-family: var(--mono); font-weight: 600;
  color: var(--ink-faint); font-size: .8rem; padding-top: 1px;
}
.q__opt.correct { border-color: var(--grow); background: var(--grow-soft); }
.q__opt.correct .k { color: var(--grow); }
.q__opt.wrong   { border-color: var(--critical); background: var(--critical-soft); }
.q__opt.wrong .k { color: var(--critical); }
.q__why {
  display: none; margin-top: 10px; padding: 10px 12px;
  background: var(--surface-2); border-left: 3px solid var(--signal);
  border-radius: 0 6px 6px 0; font-size: .88rem;
}
.q__why.shown { display: block; }

/* ---------- Contents drawer ---------- */
.drawer {
  position: fixed; inset: 0; z-index: 60;
  display: none;
}
.drawer.open { display: block; }
.drawer__scrim { position: absolute; inset: 0; background: rgba(10,14,20,.55); }
.drawer__panel {
  position: absolute; left: 0; right: 0; bottom: 0;
  max-height: 78vh; overflow-y: auto; -webkit-overflow-scrolling: touch;
  background: var(--surface);
  border-radius: 16px 16px 0 0;
  border-top: 1px solid var(--rule);
  padding: 8px 0 24px;
  animation: slideUp .25s ease;
}
@keyframes slideUp { from { transform: translateY(100%); } to { transform: none; } }
.drawer__grip {
  display: block; width: 56px; height: 5px; padding: 0;
  border: 0; border-radius: 99px; background: var(--rule);
  margin: 8px auto 12px; cursor: pointer;
}
.drawer__grip:active { background: var(--ink-faint); }
.drawer__title {
  font-family: var(--display); font-weight: 700; font-size: .95rem;
  padding: 0 18px 10px; margin: 0;
}
.toc { list-style: none; margin: 0; padding: 0; }
.toc a {
  display: flex; align-items: center; gap: 10px;
  padding: 11px 18px; text-decoration: none; color: var(--ink);
  border-bottom: 1px solid var(--rule-soft); font-size: .92rem;
}
.toc a .n {
  font-family: var(--mono); font-size: .7rem; color: var(--ink-faint);
  min-width: 24px;
}
.toc a .dot { margin-left: auto; font-size: .7rem; }

/* ---------- Bottom bar ---------- */
.bottombar {
  position: fixed; left: 0; right: 0; bottom: 0; z-index: 50;
  background: var(--surface);
  border-top: 1px solid var(--rule);
  padding: 8px 12px calc(8px + env(safe-area-inset-bottom));
  display: flex; gap: 8px;
  box-shadow: 0 -2px 12px rgba(21,26,36,.07);
}
.bottombar .btn { flex: 1; justify-content: center; font-size: .84rem; padding: 11px 8px; }

/* ---------- Search results state ---------- */
body.searching .sec { display: none; }
body.searching .sec.match { display: block; }
body.searching .hero { display: none; }
.noresults {
  display: none; text-align: center; padding: 40px 20px; color: var(--ink-faint);
  font-family: var(--mono); font-size: .8rem;
}
body.searching .noresults.show { display: block; }

/* ---------- Back to top ---------- */
.totop {
  position: fixed; right: 14px; bottom: 78px; z-index: 45;
  width: 42px; height: 42px; border-radius: 50%;
  display: none; place-items: center;
  background: var(--ink); color: var(--paper);
  border: 0; cursor: pointer; box-shadow: var(--shadow-md);
  font-size: 1rem;
}
.totop.show { display: grid; }

/* ---------- Tablet / desktop ---------- */
@media (min-width: 720px) {
  :root { --fs-base: 18px; }
  body { padding-bottom: 0; }
  main { padding: 26px 24px 60px; }
  .hero { padding: 34px 30px; }
  .hero h1 { font-size: 2.3rem; }
  .sec__head { padding: 16px 20px; }
  .sec__body { padding: 0 20px 22px; }
  .bottombar {
    position: sticky; top: 0; bottom: auto;
    border-top: 0; border-bottom: 1px solid var(--rule);
    box-shadow: none; max-width: var(--maxw); margin: 0 auto;
    background: transparent; padding: 0 24px 14px;
  }
  .totop { bottom: 24px; }
  th, td { padding: 10px 14px; }
}

@media (min-width: 1040px) {
  :root { --maxw: 900px; }
}

/* ---------- Print ---------- */
@media print {
  .topbar, .bottombar, .totop, .drawer, .steps__foot { display: none !important; }
  .sec__body { display: block !important; }
  .steps__list li { display: block !important; }
  body { background: #fff; padding: 0; }
  .sec { break-inside: avoid; border-color: #999; }
}