/* ============================================================
   EE Learning Center — Shared Design System
   A dark "lab instrument" aesthetic: calm, precise, slick.
   Every topic page inherits these tokens, components, and layout.
   ============================================================ */

:root {
  /* ---- Color tokens ---- */
  --bg-0: #0a0e14;          /* page background */
  --bg-1: #111722;          /* panels */
  --bg-2: #182231;          /* raised surfaces / inputs */
  --bg-3: #22304399;        /* hover, translucent */
  --line: #2a3a52;          /* hairline borders */
  --line-soft: #1d2838;

  --txt-0: #e8eef7;         /* primary text */
  --txt-1: #aebacd;         /* secondary */
  --txt-2: #6b7a91;         /* muted / labels */

  --accent: #38e1c4;        /* primary accent — teal */
  --accent-dim: #1c8f7e;
  --accent-glow: #38e1c455;
  --accent-2: #6ea8ff;      /* secondary accent — blue */
  --warn: #ffb454;          /* caution */
  --bad: #ff6b6b;           /* fail / over-limit */
  --good: #5be37a;          /* pass / in-spec */

  /* ---- Typography ---- */
  --font-ui: "Inter", "Segoe UI", system-ui, -apple-system, sans-serif;
  --font-mono: "JetBrains Mono", "SF Mono", "Cascadia Code", ui-monospace, monospace;

  /* ---- Geometry ---- */
  --radius: 14px;
  --radius-sm: 9px;
  --gap: 22px;
  --shadow: 0 10px 30px -12px #000a, 0 2px 8px -4px #0008;
  --shadow-glow: 0 0 0 1px var(--accent-glow), 0 8px 26px -10px var(--accent-glow);
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  background:
    radial-gradient(1200px 700px at 80% -10%, #14304022, transparent 60%),
    radial-gradient(900px 600px at -10% 110%, #1a2a4022, transparent 55%),
    var(--bg-0);
  color: var(--txt-0);
  font-family: var(--font-ui);
  font-size: 15px;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
  letter-spacing: 0.1px;
}

a { color: var(--accent-2); text-decoration: none; }
a:hover { text-decoration: underline; }

h1, h2, h3 { line-height: 1.2; font-weight: 650; letter-spacing: -0.02em; }
h1 { font-size: clamp(1.8rem, 3vw, 2.6rem); margin: 0 0 .2em; }
h2 { font-size: 1.25rem; margin: 0 0 .5em; }
.muted { color: var(--txt-2); }
.mono { font-family: var(--font-mono); }

/* ---- Top navigation bar ---- */
.topbar {
  display: flex; align-items: center; gap: 16px;
  padding: 16px clamp(18px, 4vw, 48px);
  border-bottom: 1px solid var(--line-soft);
  background: linear-gradient(180deg, #0d121bdd, #0a0e1499);
  backdrop-filter: blur(10px);
  position: sticky; top: 0; z-index: 20;
}
.brand {
  display: flex; align-items: center; gap: 11px;
  font-weight: 700; letter-spacing: -0.01em; color: var(--txt-0);
}
a.brand { text-decoration: none; transition: color .14s ease; }
a.brand:hover { color: var(--accent); }
.brand .ver { color: var(--txt-2); font-weight: 500; font-size: .78rem; align-self: flex-end; padding-bottom: 1px; }
.brand .logo {
  width: 30px; height: 30px; border-radius: 8px;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  display: grid; place-items: center; color: #06121a; font-weight: 800;
  box-shadow: 0 0 18px -2px var(--accent-glow);
}
.topbar .crumbs { color: var(--txt-2); font-size: .92rem; }
.topbar .spacer { flex: 1; }

/* ---- Page shell ---- */
.wrap { max-width: 1320px; margin: 0 auto; padding: clamp(20px, 4vw, 40px); }
.lede { color: var(--txt-1); max-width: 70ch; font-size: 1.05rem; }

/* ---- Cards (hub) ---- */
.grid-cards {
  display: grid; gap: 14px;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  margin-top: 26px;
}
.card {
  display: block; position: relative; overflow: hidden;
  background: linear-gradient(180deg, var(--bg-1), #0e141d);
  border: 1px solid var(--line-soft);
  border-radius: var(--radius);
  padding: 14px 15px;
  transition: transform .18s ease, border-color .18s ease, box-shadow .18s ease;
}
.card:hover { transform: translateY(-3px); border-color: var(--accent-dim); box-shadow: var(--shadow); text-decoration: none; }
.card .tag {
  display: inline-block; font-size: .66rem; letter-spacing: .08em; text-transform: uppercase;
  color: var(--accent); background: #0e2620; border: 1px solid #18463c;
  padding: 2px 8px; border-radius: 999px; margin-bottom: 8px;
}
.card h3 { color: var(--txt-0); margin: 0 0 4px; font-size: 1rem; }
.card p { color: var(--txt-2); margin: 0; font-size: .82rem; line-height: 1.45; }
.card .thumb {
  height: 96px; margin: -3px -4px 10px; border-radius: 8px; overflow: hidden;
  background: radial-gradient(120% 120% at 0% 0%, #16263a, #0c1219);
  border: 1px solid var(--line-soft);
}
/* thumbnails are resized to the card and their art scale-fit by thumbCtx() in index.html */
.card .thumb canvas { display: block; width: 100%; height: 100%; }
.card.soon { opacity: .55; pointer-events: none; }
.card.soon::after {
  content: "Coming soon"; position: absolute; top: 14px; right: 14px;
  font-size: .68rem; color: var(--txt-2); border: 1px solid var(--line);
  padding: 2px 8px; border-radius: 999px;
}

/* ============================================================
   Topic page layout: controls | stage | explainer
   ============================================================ */
.lab {
  display: grid;
  grid-template-columns: 320px minmax(0, 1fr);
  gap: var(--gap);
  align-items: start;
}
@media (max-width: 960px) { .lab { grid-template-columns: 1fr; } }

.panel {
  background: linear-gradient(180deg, var(--bg-1), #0e141d);
  border: 1px solid var(--line-soft);
  border-radius: var(--radius);
  padding: 20px;
}
.panel + .panel { margin-top: var(--gap); }
.panel h2 .hint { font-weight: 400; font-size: .8rem; color: var(--txt-2); }
.panel-title {
  display: flex; align-items: center; gap: 9px; margin-bottom: 16px;
  font-size: .78rem; letter-spacing: .12em; text-transform: uppercase; color: var(--txt-2);
}
.panel-title::before {
  content: ""; width: 7px; height: 7px; border-radius: 50%;
  background: var(--accent); box-shadow: 0 0 10px var(--accent);
}

.stage { display: grid; gap: var(--gap); }

/* ---- Control: slider row ---- */
.control { margin-bottom: 20px; }
.control:last-child { margin-bottom: 0; }
.control .row { display: flex; justify-content: space-between; align-items: baseline; margin-bottom: 9px; }
.control .name { font-size: .9rem; color: var(--txt-1); }
.control .val {
  font-family: var(--font-mono); font-size: .9rem; color: var(--accent);
  background: #0c1a17; border: 1px solid #16382f; padding: 1px 8px; border-radius: 6px;
  min-width: 0; white-space: nowrap;
}
input[type="range"] {
  -webkit-appearance: none; appearance: none; width: 100%; height: 5px;
  background: linear-gradient(90deg, var(--accent-dim) var(--fill, 0%), var(--bg-2) var(--fill, 0%));
  border-radius: 999px; outline: none; cursor: pointer;
}
input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none; appearance: none; width: 18px; height: 18px; border-radius: 50%;
  background: radial-gradient(circle at 35% 30%, #eafff9, var(--accent));
  border: 2px solid #0a0e14; box-shadow: 0 0 0 1px var(--accent-dim), 0 2px 6px #0007;
  transition: transform .1s ease;
}
input[type="range"]::-webkit-slider-thumb:active { transform: scale(1.15); }
input[type="range"]::-moz-range-thumb {
  width: 16px; height: 16px; border-radius: 50%; border: 2px solid #0a0e14;
  background: var(--accent); box-shadow: 0 0 0 1px var(--accent-dim);
}

/* ---- Control: segmented toggle ---- */
.segmented { display: inline-flex; background: var(--bg-2); border: 1px solid var(--line); border-radius: 9px; padding: 3px; gap: 3px; }
.segmented button {
  border: 0; background: transparent; color: var(--txt-1); font: inherit; font-size: .85rem;
  padding: 5px 13px; border-radius: 6px; cursor: pointer; transition: all .14s ease;
}
.segmented button.active { background: var(--accent); color: #06121a; font-weight: 600; }
.segmented.fill { display: flex; width: 100%; }
.segmented.fill button { flex: 1; }

/* ---- Knob (SVG, draggable) ---- */
.knob-wrap { display: flex; flex-direction: column; align-items: center; gap: 6px; }
.knob { cursor: ns-resize; touch-action: none; user-select: none; }
.knob-label { font-size: .8rem; color: var(--txt-2); }
.knob-val { font-family: var(--font-mono); font-size: .85rem; color: var(--accent); }

/* ---- Editable number-input fields (solver pages) ---- */
.fields { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; }
.field { display: flex; flex-direction: column; gap: 5px; }
.field label { font-size: .78rem; color: var(--txt-2); }
.field .inwrap { display: flex; align-items: center; gap: 6px; background: var(--bg-2); border: 1px solid var(--line); border-radius: 8px; padding: 0 11px; transition: border-color .14s ease, box-shadow .14s ease; }
.field .inwrap:focus-within { border-color: var(--accent); box-shadow: 0 0 0 1px var(--accent-glow); }
.field input { width: 100%; min-width: 0; background: transparent; border: 0; color: var(--txt-0); font-family: var(--font-mono); font-size: 1rem; padding: 9px 0; outline: none; -moz-appearance: textfield; }
.field input::-webkit-outer-spin-button, .field input::-webkit-inner-spin-button { -webkit-appearance: none; margin: 0; }
.field .unit { font-family: var(--font-mono); color: var(--txt-2); font-size: .82rem; white-space: nowrap; }
.field input[type="range"] { margin-top: 3px; }

/* ---- Readouts / stats ---- */
.stats { display: grid; grid-template-columns: repeat(auto-fit, minmax(120px, 1fr)); gap: 12px; }
.stat { background: var(--bg-2); border: 1px solid var(--line-soft); border-radius: var(--radius-sm); padding: 12px 14px; }
.stat .k { font-size: .72rem; letter-spacing: .06em; text-transform: uppercase; color: var(--txt-2); }
.stat .v { font-family: var(--font-mono); font-size: 1.35rem; margin-top: 3px; }
.stat .v.good { color: var(--good); }
.stat .v.bad { color: var(--bad); }
.stat .v.warn { color: var(--warn); }

.verdict {
  display: inline-flex; align-items: center; gap: 8px; font-weight: 700;
  padding: 7px 15px; border-radius: 999px; font-size: .9rem; letter-spacing: .02em;
}
.verdict.pass { color: var(--good); background: #0e2417; border: 1px solid #1d4e30; }
.verdict.fail { color: var(--bad); background: #2a1314; border: 1px solid #5a2528; }
.verdict::before { content: ""; width: 9px; height: 9px; border-radius: 50%; background: currentColor; box-shadow: 0 0 8px currentColor; }

/* ---- Plot frame ---- */
.plot-frame { position: relative; width: 100%; background: #080c12; border: 1px solid var(--line-soft); border-radius: var(--radius-sm); overflow: hidden; }
.plot-frame canvas { display: block; width: 100%; }
.legend { display: flex; flex-wrap: wrap; gap: 16px; margin-top: 12px; font-size: .82rem; color: var(--txt-1); }
.legend .item { display: flex; align-items: center; gap: 7px; }
.legend .swatch { width: 14px; height: 3px; border-radius: 2px; }
.legend .swatch.dash { background: repeating-linear-gradient(90deg, currentColor 0 5px, transparent 5px 9px); height: 2px; }

/* ---- Explainer text ---- */
.explain { color: var(--txt-1); }
.explain p { margin: 0 0 12px; }
.explain .eq {
  font-family: var(--font-mono); background: var(--bg-2); border: 1px solid var(--line-soft);
  border-left: 3px solid var(--accent); border-radius: 6px; padding: 12px 14px; margin: 14px 0;
  color: var(--txt-0); font-size: .92rem; overflow-x: auto;
}
.explain .note { background: #11202c; border: 1px solid #1d3a4d; border-radius: 8px; padding: 12px 14px; color: var(--txt-1); font-size: .9rem; }
.explain strong { color: var(--txt-0); }
.kbd { font-family: var(--font-mono); font-size: .8rem; background: var(--bg-2); border: 1px solid var(--line); border-bottom-width: 2px; border-radius: 5px; padding: 1px 6px; }

.btn {
  font: inherit; font-size: .85rem; color: var(--txt-1); cursor: pointer;
  background: var(--bg-2); border: 1px solid var(--line); border-radius: 8px; padding: 7px 14px;
  transition: all .14s ease;
}
.btn:hover { border-color: var(--accent-dim); color: var(--txt-0); }

footer.site { color: var(--txt-2); font-size: .85rem; text-align: center; padding: 40px 0 20px; }

/* ---- Guided tour card + pinned-snapshot button ---- */
.tour-card {
  position: fixed; right: 22px; bottom: 22px; z-index: 60;
  width: min(400px, calc(100vw - 44px));
  background: linear-gradient(180deg, var(--bg-1), #0e141d);
  border: 1px solid var(--accent-dim); border-radius: var(--radius);
  padding: 16px 18px; box-shadow: var(--shadow);
}
.tour-cap { font-size: .92rem; color: var(--txt-1); line-height: 1.55; }
.tour-foot { display: flex; align-items: center; justify-content: space-between; gap: 10px; margin-top: 13px; }
.tour-dots i { display: inline-block; width: 7px; height: 7px; border-radius: 50%; background: var(--line); margin-right: 5px; }
.tour-dots i.on { background: var(--accent); }
.tour-btns { display: flex; gap: 6px; }
.tour-btns .btn { padding: 4px 10px; font-size: .8rem; }
.tour-btns .btn:disabled { opacity: .4; cursor: default; }
.btn.pinned { border-color: var(--accent); color: var(--accent); }

/* ---- Control feel (trial): glow while dragging ---- */
input[type="range"]:active::-webkit-slider-thumb { box-shadow: 0 0 0 1px var(--accent-dim), 0 0 16px 3px var(--accent-glow); }
input[type="range"]:active::-moz-range-thumb { box-shadow: 0 0 0 1px var(--accent-dim), 0 0 16px 3px var(--accent-glow); }
svg.knob { transition: filter .18s ease; }
svg.knob.dragging { filter: drop-shadow(0 0 10px var(--accent-glow)); }