:root {
  --bg: #0d1117;
  --panel: #12171f;
  --panel-2: #171d26;
  --line: #232b36;
  --line-soft: #1b222c;
  --text: #dbe3ee;
  --text-dim: #8b98a9;
  --text-faint: #6a7686;
  --accent: #3ddc97;
  --accent-dim: #1f8f63;
  --accent-ink: #06140e;
  --warn: #ffb454;
  --bad: #ff6b6b;
  --good: #3ddc97;
  --link: #7cc7ff;
  --code-bg: #0a0e14;
  --radius: 6px;
  --mono: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, "Liberation Mono", monospace;
  --sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Inter, Roboto, Helvetica, Arial, sans-serif;
  --topbar-h: 52px;
}

html[data-theme="light"] {
  --bg: #f5f7fa;
  --panel: #ffffff;
  --panel-2: #f0f3f7;
  --line: #d8dee7;
  --line-soft: #e6eaf0;
  --text: #1b2330;
  --text-dim: #5a6675;
  --text-faint: #808c9b;
  --accent: #12855c;
  --accent-dim: #12855c;
  --accent-ink: #ffffff;
  --link: #0b64c0;
  --code-bg: #f0f3f7;
}

* { box-sizing: border-box; }

html, body { height: 100%; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 15px/1.6 var(--sans);
  -webkit-font-smoothing: antialiased;
}

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

/* ---------------------------------------------------------------- top bar */

.topbar {
  height: var(--topbar-h);
  display: flex;
  align-items: center;
  gap: 24px;
  padding: 0 16px;
  background: var(--panel);
  border-bottom: 1px solid var(--line);
  position: sticky;
  top: 0;
  z-index: 40;
}

.brand { display: flex; align-items: center; gap: 9px; color: var(--text); }
.brand:hover { text-decoration: none; }
.brand-mark {
  font-family: var(--mono);
  font-weight: 700;
  color: var(--accent-ink);
  background: var(--accent);
  padding: 3px 6px;
  border-radius: 4px;
  font-size: 13px;
  letter-spacing: -1px;
}
.brand-name { font-weight: 600; letter-spacing: -0.2px; white-space: nowrap; }

.tabs { display: flex; gap: 2px; }
.tabs a {
  color: var(--text-dim);
  padding: 6px 12px;
  border-radius: var(--radius);
  font-size: 14px;
  font-weight: 500;
}
.tabs a:hover { background: var(--panel-2); color: var(--text); text-decoration: none; }
.tabs a.on { color: var(--accent); background: var(--panel-2); }

.topbar-right { margin-left: auto; display: flex; align-items: center; gap: 10px; }

.seg { display: flex; border: 1px solid var(--line); border-radius: var(--radius); overflow: hidden; }
.seg button {
  background: transparent;
  border: 0;
  color: var(--text-dim);
  font: 500 13px var(--sans);
  padding: 5px 12px;
  cursor: pointer;
}
.seg button:hover { color: var(--text); }
.seg button.on { background: var(--accent); color: var(--accent-ink); }

.icon-btn {
  background: transparent;
  border: 1px solid var(--line);
  color: var(--text-dim);
  border-radius: var(--radius);
  width: 30px; height: 28px;
  cursor: pointer;
  font-size: 14px;
}
.icon-btn:hover { color: var(--text); }

/* ---------------------------------------------------------------- layout */

.shell { display: flex; min-height: calc(100vh - var(--topbar-h)); align-items: stretch; }

.sidebar {
  width: 268px;
  flex: 0 0 268px;
  background: var(--panel);
  border-right: 1px solid var(--line);
  padding: 16px 10px 40px;
  position: sticky;
  top: var(--topbar-h);
  height: calc(100vh - var(--topbar-h));
  overflow-y: auto;
}
.sidebar.hidden { display: none; }

.side-head {
  font: 600 11px var(--sans);
  letter-spacing: 0.09em;
  text-transform: uppercase;
  color: var(--text-faint);
  padding: 14px 10px 6px;
}
.side-head:first-child { padding-top: 0; }

.side-link {
  display: block;
  padding: 7px 10px;
  border-radius: var(--radius);
  color: var(--text-dim);
  font-size: 13.5px;
  line-height: 1.35;
  border-left: 2px solid transparent;
}
.side-link:hover { background: var(--panel-2); color: var(--text); text-decoration: none; }
.side-link.on { background: var(--panel-2); color: var(--accent); border-left-color: var(--accent); }
.side-link .tick { color: var(--accent); font-size: 11px; margin-left: 4px; }

.main { flex: 1; min-width: 0; padding: 28px 32px 80px; }
.main.flush { padding: 0; }

.wrap { max-width: 820px; }
.wrap-wide { max-width: 1100px; }

/* ---------------------------------------------------------------- text */

h1 { font-size: 26px; line-height: 1.25; margin: 0 0 6px; letter-spacing: -0.4px; }
h2 { font-size: 19px; margin: 34px 0 10px; letter-spacing: -0.2px; }
h3 { font-size: 16px; margin: 28px 0 8px; letter-spacing: -0.1px; }
h4 { font-size: 14px; margin: 20px 0 6px; color: var(--text-dim); }

.eyebrow {
  font: 600 11px var(--mono);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--text-faint);
  margin-bottom: 8px;
}

p { margin: 0 0 13px; }
ul, ol { margin: 0 0 13px; padding-left: 22px; }
li { margin: 4px 0; }

.lead { color: var(--text-dim); font-size: 15.5px; border-left: 2px solid var(--accent-dim); padding-left: 14px; }

code {
  font-family: var(--mono);
  font-size: 0.9em;
  background: var(--code-bg);
  border: 1px solid var(--line-soft);
  border-radius: 4px;
  padding: 1px 5px;
}

pre {
  background: var(--code-bg);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 13px 15px;
  overflow-x: auto;
  margin: 0 0 14px;
}
pre code { background: none; border: 0; padding: 0; font-size: 13px; line-height: 1.55; }

.expr {
  font-family: var(--mono);
  font-size: 15px;
  margin: 14px 0 16px;
  padding-left: 18px;
  letter-spacing: 0.02em;
}

.cases { display: flex; align-items: center; gap: 12px; margin: 14px 0 16px; padding-left: 18px;
         font-family: var(--mono); font-size: 14px; }
.cases .fn { white-space: nowrap; }
.cases .rows { border-left: 2px solid var(--text-faint); padding-left: 14px; }
.cases .rows > div { display: flex; gap: 26px; padding: 2px 0; }
.cases .rows > div > span:first-child { min-width: 160px; }
.cases .rows > div > span:last-child { color: var(--text-dim); }

.subq { font-family: var(--mono); font-size: 13.5px; margin: 12px 0 14px; padding-left: 18px;
        display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 6px 20px; }
.subq span b { color: var(--text-dim); font-weight: 600; margin-right: 8px; }

.tbl { border-collapse: collapse; margin: 0 0 16px; font-size: 13.5px; width: auto; }
.tbl th, .tbl td { border: 1px solid var(--line); padding: 5px 12px; text-align: left; }
.tbl th { background: var(--panel-2); font-weight: 600; color: var(--text-dim); }
.tbl td { font-family: var(--mono); font-size: 12.5px; }
.tbl-scroll { overflow-x: auto; }

/* ---------------------------------------------------------------- cards */

.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(280px, 1fr)); gap: 12px; }

.card {
  display: block;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 14px 16px;
  color: var(--text);
}
a.card:hover { border-color: var(--accent-dim); text-decoration: none; }
.card h3 { margin: 0 0 5px; font-size: 15px; }
.card p { margin: 0; color: var(--text-dim); font-size: 13.5px; line-height: 1.5; }
.card-foot { margin-top: 10px; display: flex; gap: 8px; align-items: center; }

.chip {
  font: 600 10.5px var(--mono);
  letter-spacing: 0.05em;
  text-transform: uppercase;
  color: var(--text-faint);
  border: 1px solid var(--line);
  border-radius: 3px;
  padding: 2px 6px;
}
.chip.ok { color: var(--good); border-color: var(--accent-dim); }
.chip.solid { background: var(--panel-2); }

/* ---------------------------------------------------------------- buttons */

.btn {
  font: 500 13.5px var(--sans);
  border-radius: var(--radius);
  border: 1px solid var(--line);
  background: var(--panel-2);
  color: var(--text);
  padding: 7px 14px;
  cursor: pointer;
}
.btn:hover { border-color: var(--text-faint); }
.btn:disabled { opacity: 0.45; cursor: default; }
.btn-primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); font-weight: 600; }
.btn-primary:hover { filter: brightness(1.08); }
.btn-ghost { background: transparent; color: var(--text-dim); }
.btn-danger { color: var(--warn); }
.btn-row { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; }

/* ---------------------------------------------------------------- quiz */

.quiz-head {
  display: flex; align-items: baseline; gap: 14px; flex-wrap: wrap;
  border-bottom: 1px solid var(--line); padding-bottom: 12px; margin-bottom: 20px;
}
.quiz-count { font: 600 12px var(--mono); color: var(--text-faint); }
.quiz-score { font: 600 12px var(--mono); color: var(--accent); margin-left: auto; }

.qtext { font-size: 15.5px; margin-bottom: 16px; }
.qtext pre { margin-top: 10px; }

.choices { display: flex; flex-direction: column; gap: 8px; margin-bottom: 18px; }
.choice {
  display: flex; gap: 11px; align-items: flex-start;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 11px 14px;
  cursor: pointer;
  text-align: left;
  font: 14px/1.5 var(--sans);
  color: var(--text);
  width: 100%;
}
.choice:hover:not(:disabled) { border-color: var(--text-faint); }
.choice:disabled { cursor: default; }
.choice .key {
  font: 600 12px var(--mono);
  color: var(--text-faint);
  border: 1px solid var(--line);
  border-radius: 3px;
  padding: 1px 6px;
  flex: 0 0 auto;
  margin-top: 1px;
}
.choice .val { font-family: var(--mono); font-size: 13.5px; }
.choice.right { border-color: var(--accent); background: color-mix(in srgb, var(--accent) 9%, var(--panel)); }
.choice.right .key { color: var(--accent); border-color: var(--accent-dim); }
.choice.wrong { border-color: var(--bad); background: color-mix(in srgb, var(--bad) 9%, var(--panel)); }
.choice.wrong .key { color: var(--bad); border-color: var(--bad); }

.explain {
  background: var(--panel);
  border: 1px solid var(--line);
  border-left: 3px solid var(--accent);
  border-radius: var(--radius);
  padding: 14px 16px;
  margin-bottom: 18px;
}
.explain h2 { margin: 0 0 6px; color: var(--accent); font-size: 12px; letter-spacing: 0.06em; text-transform: uppercase; }
.explain.wrong { border-left-color: var(--bad); }
.explain.wrong h4 { color: var(--bad); }
.explain p:last-child { margin-bottom: 0; }

.verdict { font: 600 12px var(--mono); letter-spacing: 0.06em; text-transform: uppercase; }
.verdict.ok { color: var(--good); }
.verdict.no { color: var(--bad); }

/* ---------------------------------------------------------------- mock exam */

.exam-bar {
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 10px 14px;
  margin-bottom: 20px;
  position: sticky;
  top: calc(var(--topbar-h) + 8px);
  z-index: 20;
}
.clock {
  font: 600 18px var(--mono);
  letter-spacing: 0.02em;
  color: var(--text);
  font-variant-numeric: tabular-nums;
}
.clock.low { color: var(--bad); }
.clock-label {
  font: 600 10px var(--sans);
  letter-spacing: 0.09em;
  text-transform: uppercase;
  color: var(--text-faint);
  display: block;
  margin-bottom: 1px;
}

.palette { display: flex; gap: 6px; margin-left: auto; }
.palette button {
  width: 30px; height: 30px;
  border-radius: var(--radius);
  border: 1px solid var(--line);
  background: transparent;
  color: var(--text-dim);
  font: 600 12px var(--mono);
  cursor: pointer;
}
.palette button:hover { border-color: var(--text-faint); color: var(--text); }
.palette button.done { border-color: var(--accent-dim); color: var(--accent); }
.palette button.on { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }

.review { border-top: 1px solid var(--line); padding-top: 20px; margin-top: 26px; }
.review:first-of-type { border-top: 0; padding-top: 0; margin-top: 0; }
.review-head {
  display: flex; align-items: baseline; gap: 12px; margin-bottom: 10px;
  font: 600 12px var(--mono);
}
.review-head .n { color: var(--text-faint); }
.scoreline {
  font: 600 34px var(--mono);
  letter-spacing: -0.5px;
  margin: 6px 0 2px;
}
.scoreline.ok { color: var(--good); }
.scoreline.mid { color: var(--warn); }
.scoreline.no { color: var(--bad); }

/* ---------------------------------------------------------------- custom input */

.custom-input {
  flex: 0 0 auto;
  border-top: 1px solid var(--line);
  background: var(--panel);
  padding: 12px 14px;
}
.custom-input[hidden] { display: none; }
.custom-input label {
  display: block;
  font: 600 10.5px var(--sans);
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--text-faint);
  margin-bottom: 6px;
}
.custom-input textarea {
  width: 100%;
  min-height: 68px;
  resize: vertical;
  background: var(--code-bg);
  color: var(--text);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 8px 10px;
  font: 12.5px/1.5 var(--mono);
}
.custom-input textarea:focus { outline: none; border-color: var(--accent-dim); }
.custom-input .row { display: flex; gap: 8px; align-items: center; margin-top: 8px; }
.custom-input .hint { color: var(--text-faint); font-size: 12px; }

/* ---------------------------------------------------------------- problem workspace */

.ws { display: flex; height: calc(100vh - var(--topbar-h)); }
.ws-left {
  flex: 0 0 44%;
  max-width: 640px;
  overflow-y: auto;
  padding: 24px 28px 60px;
  border-right: 1px solid var(--line);
}
.ws-right { flex: 1; min-width: 0; display: flex; flex-direction: column; background: var(--panel); }

.ws-bar {
  display: flex; align-items: center; gap: 10px;
  padding: 8px 12px;
  border-bottom: 1px solid var(--line);
  background: var(--panel);
  flex: 0 0 auto;
}
.ws-bar select {
  background: var(--panel-2);
  color: var(--text);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 5px 8px;
  font: 13px var(--sans);
}
.ws-bar .spacer { margin-left: auto; }

.editor-host { flex: 1 1 auto; min-height: 0; overflow: hidden; }
.CodeMirror { height: 100%; font-family: var(--mono); font-size: 13px; line-height: 1.55; }
html[data-theme="light"] .CodeMirror { background: #fff; color: #1b2330; }
html[data-theme="light"] .CodeMirror-gutters { background: #f0f3f7; border-right: 1px solid var(--line); }
html[data-theme="light"] .CodeMirror-cursor { border-left: 1px solid #1b2330; }
html[data-theme="light"] .CodeMirror-activeline-background { background: #eef2f7; }

.results {
  flex: 0 0 auto;
  display: flex;
  flex-direction: column;
  min-height: 0;
  max-height: 50%;
  border-top: 1px solid var(--line);
  background: var(--code-bg);
}
.results[hidden] { display: none; }
.results.collapsed { max-height: none; }
.results.collapsed .console { display: none; }

.results-bar {
  flex: 0 0 auto;
  display: flex;
  align-items: center;
  gap: 12px;
  width: 100%;
  padding: 8px 14px;
  background: var(--panel);
  border: 0;
  border-bottom: 1px solid var(--line-soft);
  color: var(--text-dim);
  font: 12.5px var(--mono);
  text-align: left;
  cursor: pointer;
}
.results-bar:hover { color: var(--text); }
.results.collapsed .results-bar { border-bottom: 0; }

.results-summary { display: flex; align-items: baseline; gap: 12px; min-width: 0; }
.results-summary .st { width: auto; font-weight: 600; }
.results-summary .st.ok { color: var(--good); }
.results-summary .st.no { color: var(--bad); }

.chev { margin-left: auto; display: flex; color: var(--text-faint); }
.chev svg { transition: transform 120ms ease; }
.results.collapsed .chev svg { transform: rotate(180deg); }
.results-bar:hover .chev { color: var(--text); }

.console {
  flex: 1 1 auto;
  min-height: 0;
  overflow-y: auto;
  padding: 12px 14px;
  font-family: var(--mono);
  font-size: 12.5px;
  line-height: 1.55;
}
.console:empty { display: none; }
.console .muted { color: var(--text-faint); }

.res-line { display: flex; gap: 10px; align-items: baseline; padding: 3px 0; }
.res-line .n { color: var(--text-faint); width: 62px; flex: 0 0 auto; }
.res-line .st { width: 62px; flex: 0 0 auto; font-weight: 600; }
.res-line .st.ok { color: var(--good); }
.res-line .st.no { color: var(--bad); }
.res-line .st.hid { color: var(--text-faint); }
.res-detail { padding: 2px 0 8px 72px; color: var(--text-dim); white-space: pre-wrap; word-break: break-word; }
.res-detail b { color: var(--text); font-weight: 500; }

.io-table { width: 100%; border-collapse: collapse; font-family: var(--mono); font-size: 12.5px; margin-bottom: 14px; }
.io-table th {
  text-align: left; padding: 6px 10px; border-bottom: 1px solid var(--line);
  color: var(--text-faint); font: 600 10.5px var(--sans); letter-spacing: 0.08em; text-transform: uppercase;
}
.io-table td { padding: 6px 10px; border-bottom: 1px solid var(--line-soft); vertical-align: top; white-space: pre-wrap; word-break: break-all; }
.io-table td.idx { color: var(--text-faint); width: 34px; }
.io-table tr.locked td { color: var(--text-faint); font-style: italic; }

.spec { background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); padding: 12px 15px; margin-bottom: 14px; }
.spec h4 { margin: 0 0 5px; color: var(--accent); font-size: 11px; letter-spacing: 0.09em; text-transform: uppercase; }
.spec p { margin: 0; font-size: 14px; }

h3.sec {
  font: 600 11px var(--sans);
  letter-spacing: 0.11em;
  text-transform: uppercase;
  color: var(--accent);
  border-bottom: 1px solid var(--line);
  padding-bottom: 5px;
  margin: 28px 0 12px;
}

table.ex { border-collapse: collapse; width: 100%; margin: 0 0 16px; font-size: 13.5px; }
table.ex th {
  text-align: left;
  vertical-align: top;
  width: 96px;
  padding: 9px 12px 9px 0;
  border-top: 1px solid var(--line);
  font: 600 10.5px var(--sans);
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--text-faint);
}
table.ex td {
  padding: 9px 0;
  border-top: 1px solid var(--line);
  font-family: var(--mono);
  font-size: 12.5px;
  line-height: 1.65;
}
table.ex td pre { margin: 8px 0; }

ol.data { margin: 0 0 16px; padding-left: 26px; }
ol.data li { margin: 0 0 8px; }
ol.data li::marker { color: var(--text-faint); font-family: var(--mono); font-size: 12px; }
ol.data pre { margin: 0; padding: 7px 11px; }
ol.data .ans {
  font-family: var(--mono);
  font-size: 12.5px;
  color: var(--accent);
  padding: 4px 0 0 11px;
  word-break: break-all;
}
ol.data .locked { color: var(--text-faint); font-style: italic; }
ol.data li.sealed {
  border: 1px dashed var(--line);
  border-radius: var(--radius);
  padding: 7px 11px;
  font-family: var(--mono);
  font-size: 12.5px;
}

details.reveal { border: 1px solid var(--line); border-radius: var(--radius); margin: 16px 0; background: var(--panel); }
details.reveal > summary { cursor: pointer; padding: 10px 14px; font-weight: 600; font-size: 14px; }
details.reveal > div { padding: 0 14px 14px; }

.note { color: var(--text-dim); font-size: 13px; }
.banner {
  border: 1px solid var(--line);
  border-left: 3px solid var(--warn);
  background: var(--panel);
  border-radius: var(--radius);
  padding: 11px 14px;
  margin-bottom: 16px;
  font-size: 13.5px;
  color: var(--text-dim);
}

.empty { color: var(--text-faint); font-size: 14px; padding: 30px 0; }

.footer {
  border-top: 1px solid var(--line);
  background: var(--panel);
  padding: 12px 32px;
  display: flex;
  gap: 14px;
  align-items: baseline;
  flex-wrap: wrap;
  font-size: 12px;
  color: var(--text-faint);
}
.footer.hidden { display: none; }
.linkish {
  background: none;
  border: 0;
  color: var(--text-dim);
  font: 12px var(--sans);
  text-decoration: underline;
  cursor: pointer;
  padding: 0;
}
.linkish:hover { color: var(--text); }

@media (max-width: 1000px) {
  .ws { flex-direction: column; height: auto; }
  .ws-left { flex: none; max-width: none; border-right: 0; border-bottom: 1px solid var(--line); }
  .ws-right { height: 78vh; }
}
/* --------------------------------------------------------------- narrow screens

   The sidebar goes, so #topicbar takes over as the way between categories, and the top bar
   becomes two rows: identity and settings above, the four sections below. Everything stays
   reachable without a menu to open, which is one less thing to get wrong on a touch screen. */

.topicbar { display: none; }

.ws-needs-desk { display: none; padding: 26px 16px 40px; border-top: 1px solid var(--line); }
.ws-needs-desk h2 { margin: 0 0 10px; }
.ws-needs-desk p { color: var(--text-dim); max-width: 56ch; }

@media (max-width: 780px) {
  :root { --topbar-h: 96px; }

  .topbar {
    height: auto;
    min-height: var(--topbar-h);
    flex-wrap: wrap;
    gap: 0 12px;
    padding: 8px 12px 0;
  }
  .brand { order: 1; }
  .brand-name { display: none; }
  .topbar-right { order: 2; margin-left: auto; }

  /* Second row, full width, and scrollable sideways if the four labels ever outgrow it
     rather than wrapping into a taller bar. */
  .tabs {
    order: 3;
    width: 100%;
    margin-top: 8px;
    gap: 0;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
    scrollbar-width: none;
  }
  .tabs::-webkit-scrollbar { display: none; }
  .tabs a {
    flex: 1 0 auto;
    text-align: center;
    padding: 10px 12px;
    border-radius: 0;
    border-bottom: 2px solid transparent;
    white-space: nowrap;
  }
  .tabs a.on { background: transparent; border-bottom-color: var(--accent); }

  .sidebar { display: none; }
  .main { padding: 20px 16px 60px; }

  .topicbar:not(.empty) {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 10px 16px;
    background: var(--panel);
    border-bottom: 1px solid var(--line);
  }
  .topicbar label {
    font: 600 11px var(--sans);
    letter-spacing: 0.09em;
    text-transform: uppercase;
    color: var(--text-faint);
  }
  .topicbar select {
    flex: 1;
    min-width: 0;
    background: var(--panel-2);
    color: var(--text);
    border: 1px solid var(--line);
    border-radius: var(--radius);
    padding: 9px 10px;
    font: 14px var(--sans);
  }

  /* Writing code needs a keyboard and room to see the problem beside it. The statement and
     the test data still read well here, so only the editor half is swapped out. */
  .ws-right { display: none; }
  .ws-needs-desk { display: block; }
  .ws-left { border-bottom: 0; }
}

/* Small phones. The four section labels overflow their strip by about 57px at 320, and the
   strip scrolls, but a section you cannot see is a section you will not press. Tighten until
   all four fit and keep the scroll as the backstop for longer labels. */
@media (max-width: 400px) {
  .topbar { padding: 8px 10px 0; }
  .tabs a { padding: 10px 7px; font-size: 12.5px; }
  .topicbar:not(.empty) { padding: 10px 12px; }
  .main { padding: 18px 12px 56px; }
  .pager { flex-direction: column; }
  .pager a.nxt { text-align: left; align-items: flex-start; }
}


/* Previous and next category, which is the only way to walk them in order without a sidebar. */
.pager { display: flex; gap: 12px; margin: 34px 0 0; border-top: 1px solid var(--line); padding-top: 18px; }
.pager a {
  flex: 1 1 0; min-width: 0;
  display: flex; flex-direction: column; gap: 3px;
  padding: 12px 14px;
  border: 1px solid var(--line); border-radius: var(--radius);
  color: var(--text); font-weight: 600;
}
.pager a:hover { border-color: var(--accent-dim); text-decoration: none; }
.pager a.nxt { text-align: right; align-items: flex-end; }
.pager span { font: 600 11px var(--sans); letter-spacing: 0.09em; text-transform: uppercase; color: var(--text-faint); }

/* Shown only when localStorage refuses a write, which is the one time the app has to
   interrupt: the editor saves on every keystroke and silence would look like success. */
.storage-warning {
  position: fixed; left: 16px; right: 16px; bottom: 16px; z-index: 90;
  display: flex; gap: 14px; align-items: center;
  padding: 12px 16px; border-radius: 8px;
  background: #b4232a; color: #fff;
  font-size: 14px; line-height: 1.45;
  box-shadow: 0 6px 24px rgba(0, 0, 0, .35);
}
.storage-warning button {
  margin-left: auto; flex: none; cursor: pointer;
  padding: 5px 12px; border-radius: 6px;
  border: 1px solid rgba(255, 255, 255, .55);
  background: transparent; color: #fff; font: inherit;
}

/* The not found page, both the static one and the view the router shows for a dead hash. */
.lost { padding-top: 40px; }
.lost h1 { margin-bottom: 14px; }
.lost p { color: var(--text-dim); max-width: 60ch; }
.lost .btn-row { margin-top: 26px; }

/* Worked steps inside a generated explanation. It is a <pre>, so the frame and the
   horizontal scroll come from there; only the rhythm differs. */
.steps { margin: 12px 0; }
.steps code { line-height: 1.65; }

/* Question bank / Endless switch. */
.modes { display: flex; align-items: center; gap: 8px; flex-wrap: wrap; margin: 0 0 18px; }
.modes button {
  cursor: pointer; padding: 5px 12px; border-radius: 999px; font: inherit; font-size: 13px;
  border: 1px solid var(--line); background: transparent; color: var(--text-dim);
}
.modes button:hover { color: var(--text); }
.modes button.on {
  background: var(--accent); border-color: var(--accent); color: var(--accent-ink);
  font-weight: 600;
}
.modes .note { flex: 1 1 260px; min-width: 0; }