/* Topic page. Builds on style.css; the formula boxes, one-line callout and JEE corner follow the theory book. */
:root { --teal: #0f6b5c; --teal-soft: #e6f3f0; --vec: #1f62c4; }
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) { --teal: #6fd6c2; --teal-soft: #14302b; --vec: #7fb1f5; }
}
:root[data-theme="dark"] { --teal: #6fd6c2; --teal-soft: #14302b; --vec: #7fb1f5; }

.eyebrow a { color: inherit; text-underline-offset: 3px; }
.title-block { min-width: 0; }
.weightage { margin: 8px 0 0; font-size: 15px; color: var(--muted); }

.nb { white-space: nowrap; }

/* Topics */
.topic-nav { display: flex; gap: 8px; padding-top: 16px; padding-bottom: 14px; overflow-x: auto; scrollbar-width: none; }
.topic-nav::-webkit-scrollbar { display: none; }
.topic-nav a {
  flex: none; display: flex; align-items: center; min-height: 38px; padding: 0 14px; border: 1px solid var(--line); border-radius: 980px;
  background: var(--surface); color: var(--muted); font-size: 13.5px; text-decoration: none;
}
.topic-nav a:hover { border-color: var(--accent); color: var(--ink); }
.topic-nav a[aria-current] { background: var(--ink); border-color: var(--ink); color: var(--bg); font-weight: 600; }

/* Tabs: an Apple-style segmented control that stays on screen */
.tabs-bar { position: sticky; top: 0; z-index: 5; padding: 10px 0; background: color-mix(in srgb, var(--bg) 82%, transparent); backdrop-filter: saturate(180%) blur(20px); -webkit-backdrop-filter: saturate(180%) blur(20px); border-bottom: 1px solid var(--line); }
.tabs { display: flex; gap: 2px; padding: 3px; background: color-mix(in srgb, var(--ink) 8%, transparent); border-radius: 12px; }
.tabs button {
  flex: 1; min-height: 40px; padding: 0 8px; background: none; border: 0; border-radius: 9px;
  color: var(--muted); font: 600 15px/1.2 var(--sys); cursor: pointer; transition: background .15s, color .15s;
}
.tabs button:hover { color: var(--ink); }
.tabs button[aria-selected="true"] { background: var(--surface); color: var(--ink); box-shadow: 0 1px 4px rgba(0, 0, 0, .12); }
.tab-count { color: var(--muted); font: 500 13px var(--sys); }
.panel { padding-top: 22px; }

/* Theory */
.one-line { margin: 0 0 8px; padding: 12px 14px; background: var(--note-soft); border-left: 4px solid var(--note); border-radius: 0 8px 8px 0; }
.one-line b { display: block; color: var(--note); font-size: 12px; letter-spacing: .08em; text-transform: uppercase; }
.one-line p { margin: 2px 0 0; }
.concept { margin-top: 26px; }
.concept h2, .block-head {
  margin: 0 0 10px; padding-bottom: 8px; border-bottom: 1px solid var(--line);
  font: 700 22px/1.25 var(--sys); letter-spacing: -0.02em; text-wrap: balance;
}
.concept h2 span { margin-right: 6px; color: var(--accent); }
.concept ul, .corner ul { margin: 8px 0; padding-left: 20px; }
.concept li, .corner li { margin: 6px 0; overflow-wrap: anywhere; }
.concept li::marker, .corner li::marker { color: var(--accent); }
.formula {
  width: fit-content; max-width: 100%; margin: 10px auto; padding: 2px 16px; overflow-x: auto;
  background: var(--surface); border: 1.5px solid var(--accent-soft); border-radius: 14px;
}
.formula .katex-display { margin: .5em 0; }
.table-scroll { margin: 12px 0; overflow-x: auto; background: var(--surface); border-radius: 14px; }
.table-scroll table { width: 100%; border-collapse: collapse; font-size: 14.5px; }
.table-scroll th { padding: 9px 12px; background: var(--accent-soft); color: var(--accent); font-size: 12.5px; font-weight: 600; text-align: left; }
.table-scroll td { padding: 9px 12px; border-top: 1px solid var(--line); vertical-align: top; }
.theory-fig { margin: 14px 0; text-align: center; color: var(--ink); }
.theory-fig svg { display: block; width: 100%; max-width: 480px; height: auto; margin: 0 auto; }
.theory-fig figcaption { margin-top: 4px; font-size: 13px; color: var(--muted); }
.theory-fig .path { fill: none; stroke: currentColor; stroke-width: 2; }
.theory-fig .v-arrow { stroke: var(--vec); fill: var(--vec); }
.theory-fig .a-arrow { stroke: var(--accent); fill: var(--accent); }
.theory-fig .v-label { fill: var(--vec); font: italic 600 15px Inter, sans-serif; }
.theory-fig .a-label { fill: var(--accent); font: italic 600 15px Inter, sans-serif; }
.theory-fig .axis-label { fill: currentColor; font: 500 13px var(--sys); }

.corner { margin-top: 30px; padding: 16px; background: var(--teal-soft); border-left: 4px solid var(--teal); border-radius: 0 10px 10px 0; }
.corner > h2 { margin: 0; color: var(--teal); font: 600 18px/1.3 var(--sys); }
.corner h3 { margin: 18px 0 4px; color: var(--teal); font-size: 15.5px; font-weight: 600; }
.corner .formula { border-color: var(--teal); }

/* Solved examples */
.solved-head { margin-top: 34px; }
.solved-head h2 { margin: 0; font: 700 26px/1.2 var(--sys); letter-spacing: -0.025em; }
.solved-head p { margin: 4px 0 12px; color: var(--muted); font-size: 14.5px; }
.solved { margin-bottom: 14px; padding: 20px; background: var(--surface); border-radius: 20px; }
.solved .meta b { color: var(--accent); font: 700 14px var(--sys); }
.solved-opts { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 6px 14px; margin: 0 0 8px; padding: 0; list-style: none; }
.solved-opts li { display: flex; gap: 6px; min-width: 0; overflow-wrap: anywhere; }
.solved-opts .opt-letter { flex: none; }
.solved summary { display: flex; align-items: center; min-height: 44px; color: var(--accent); font-size: 14px; font-weight: 500; cursor: pointer; text-decoration: underline; text-underline-offset: 3px; list-style: none; }
.solved summary::-webkit-details-marker { display: none; }
.solved details[open] summary { color: var(--muted); }

/* Practice */
.level-row { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 8px; }
.level-row button {
  min-height: 58px; padding: 6px 4px; background: var(--surface); border: 1px solid transparent; border-radius: 14px;
  font-size: 13.5px; font-weight: 600; line-height: 1.25; cursor: pointer;
}
.level-row button small { display: block; color: var(--muted); font-size: 12px; font-weight: 400; }
.level-row button:hover { border-color: var(--accent); }
.level-row button[aria-pressed="true"] { background: var(--accent-soft); border-color: var(--accent); color: var(--accent); }
.level-row button[aria-pressed="true"] small { color: var(--accent); }
.status-filter { display: flex; align-items: center; gap: 8px; font-size: 13px; font-weight: 600; color: var(--muted); }
.status-filter select { min-height: 44px; padding: 6px 10px; background: var(--surface); border: 1px solid var(--line); border-radius: 12px; color: var(--ink); font-weight: 400; }
.num-form { display: flex; flex-wrap: wrap; gap: 8px; }
.num-form input {
  width: 130px; min-height: 48px; padding: 8px 12px; background: var(--surface); border: 1px solid var(--line); border-radius: 12px;
  color: var(--ink); font: inherit;
}
.num-form input:disabled { color: var(--ink); opacity: 1; }
.num-form.correct input { border-color: var(--good); background: var(--good-soft); }
.num-form.wrong input { border-color: var(--bad); background: var(--bad-soft); }
.num-form button { min-height: 48px; padding: 0 20px; background: var(--accent); border: 0; border-radius: 980px; color: #fff; font-weight: 600; cursor: pointer; }
.num-hint { margin: 6px 0 0; color: var(--bad); font-size: 13.5px; }

.next-step { margin: 22px 0 8px; text-align: center; }
.cta { min-height: 46px; padding: 0 24px; background: var(--accent); border: 0; border-radius: 980px; color: #fff; font-weight: 600; cursor: pointer; }
.cta:hover { opacity: .9; }

/* Video */
.video-card {
  display: grid; grid-template-columns: 220px minmax(0, 1fr); gap: 18px; align-items: center; margin-bottom: 14px; padding: 14px;
  background: var(--surface); border: 1px solid transparent; border-radius: 20px; color: inherit; text-decoration: none;
}
.video-card:hover { border-color: var(--accent); }
.thumb { position: relative; display: block; aspect-ratio: 16 / 9; overflow: hidden; background: var(--accent-soft); border-radius: 12px; }
.thumb img { display: block; width: 100%; height: 100%; object-fit: cover; }
.thumb .len { position: absolute; right: 6px; bottom: 6px; padding: 1px 6px; background: #000c; border-radius: 4px; color: #fff; font-size: 12px; font-variant-numeric: tabular-nums; }
.video-card h3 { margin: 2px 0 4px; font: 600 16px/1.3 var(--sys); }
.video-card .kind { color: var(--accent); font-size: 12.5px; font-weight: 600; }
.video-card p { margin: 0; color: var(--muted); font-size: 14px; }
.video-card .opens { display: block; margin-top: 6px; color: var(--accent); font-size: 13.5px; font-weight: 500; }
.channel-line { margin: 16px 0 0; text-align: center; font-size: 14.5px; }
.channel-line a { color: var(--accent); text-underline-offset: 3px; }

.foot a { color: inherit; text-underline-offset: 3px; }

@media (max-width: 620px) {
  .level-row { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .video-card { grid-template-columns: minmax(0, 1fr); }
  .corner { padding: 14px 12px; }
}
@media (max-width: 400px) { .solved-opts { grid-template-columns: minmax(0, 1fr); } }

/* Exam + Show filters side by side */
.filter-pair { display: flex; flex-wrap: wrap; gap: 8px 16px; }

/* Topics whose theory is not on the site yet */
.soon-card { padding: 34px 24px; background: var(--surface); border-radius: 20px; text-align: center; }
.soon-tag { display: inline-block; margin: 0 0 10px; padding: 3px 10px; border-radius: 980px; background: var(--note-soft); color: var(--note); font-size: 12px; font-weight: 700; text-transform: uppercase; letter-spacing: .03em; }
.soon-card h2 { margin: 0 0 8px; font: 700 24px/1.2 var(--sys); letter-spacing: -0.02em; }
.soon-card p { max-width: 480px; margin: 0 auto; color: var(--muted); }
.soon-actions { display: flex; flex-wrap: wrap; justify-content: center; align-items: center; gap: 8px 18px; margin-top: 20px; }
.video-intro { margin: 0 0 14px; color: var(--muted); font-size: 15px; }
.crumb-here { margin: 0 !important; }