/* /css/exercise.css: exercises, fields, options, bins, ordering, feedback, hints, solutions, checkpoint (§6.5).
   Correct / incorrect are always icon + word + color, never color alone. */

.exercise {
  position: relative;
  margin: var(--s-6) 0;
  padding: var(--s-5);
  background: var(--surface);
  border: 1px solid var(--rule);
  border-radius: var(--r-l);
  box-shadow: var(--shadow);
  --flow: .9em;
}
.exercise > * + * { margin-top: var(--flow); }
@media (max-width: 699px) { .exercise { padding: var(--s-4); } }
.ex-prompt { font-size: 1rem; }
.ex-prompt > * + * { margin-top: .7em; }

/* Tier marks ◇ ◆ ★ (§3.0) */
.exercise[data-level] > .ex-prompt::before {
  display: block;
  margin-bottom: .45em;
  font-family: var(--font-sans);
  font-size: .72rem;
  font-weight: 700;
  letter-spacing: .07em;
  text-transform: uppercase;
  color: var(--ink-muted);
}
.exercise[data-level="1"] > .ex-prompt::before { content: "\25C7\2002 Back of the envelope"; content: "\25C7\2002 Back of the envelope" / "Level 1, back of the envelope:"; }
.exercise[data-level="2"] > .ex-prompt::before { content: "\25C6\2002 Pencil and paper"; content: "\25C6\2002 Pencil and paper" / "Level 2, pencil and paper:"; }
.exercise[data-level="3"] > .ex-prompt::before { content: "\2605\2002 Head-scratcher"; content: "\2605\2002 Head-scratcher" / "Level 3, head-scratcher:"; color: var(--c-light-text); }

.ex-prev, .ex-key {
  display: inline-flex;
  align-items: center;
  gap: .35em;
  font-family: var(--font-sans);
  font-size: .72rem;
  font-weight: 650;
  color: var(--c-ok);
  margin: 0 0 var(--s-2);
}
.ex-key { color: var(--c-inv); padding: .1em .6em; border: 1px solid color-mix(in srgb, var(--c-inv) 40%, var(--rule)); border-radius: 999px; background: var(--tint-inv); }
.ex-key.ex-unscored { color: var(--ink-muted); border-color: var(--rule); background: var(--surface-2); }
.cp-unscored { font-family: var(--font-sans); font-size: .85rem; color: var(--ink-muted); }
.ex-card-fb { display: block; flex-basis: 100%; margin-top: var(--s-1); font-family: var(--font-sans); font-size: .85rem; color: var(--c-bad); }
.ex-confidence.needs-answer .ui-toggle { outline: 2px solid var(--c-light); outline-offset: 2px; }
.ex-instructions { font-family: var(--font-sans); font-size: .78rem; color: var(--ink-muted); }
.ex-help { font-family: var(--font-sans); font-size: .76rem; color: var(--ink-muted); }

/* ---------------------------------------------------------------- numeric fields */

.ex-field.is-live {
  display: inline-grid;
  grid-template-columns: auto auto auto;
  grid-template-areas: "label input unit" ". preview preview";
  align-items: center;
  column-gap: var(--s-2);
  row-gap: 2px;
  margin: .3em var(--s-5) .3em 0;
  font-family: var(--font-sans);
  vertical-align: middle;
}
.ex-field.is-live .ex-field-label { grid-area: label; }
.ex-field-label { font-family: var(--font-sans); font-size: .85rem; font-weight: 600; color: var(--ink); }
.ex-field-label .katex { font-size: 1.05em; }
.ex-field.is-live .ex-input-wrap { grid-area: input; }
.ex-field.is-live .ex-unit { }
.ex-field.is-live .ex-preview { }
.ex-input-wrap { position: relative; display: inline-block; }
.ex-unit { grid-area: unit; font-size: .82rem; color: var(--ink-muted); }
.ex-preview { grid-area: preview; font-size: .74rem; color: var(--ink-muted); min-height: 1em; font-variant-numeric: tabular-nums; }
.ex-input {
  width: 9.5em;
  min-height: 44px;
  padding: .35em 2em .35em .7em;
  border: 1.5px solid var(--rule);
  border-radius: var(--r-s);
  background: var(--bg);
  color: var(--ink);
  font-family: var(--font-sans);
  font-size: 1rem;
  font-variant-numeric: tabular-nums;
  transition: border-color var(--t-ui) var(--ease), box-shadow var(--t-ui) var(--ease);
}
.ex-input:hover { border-color: var(--ink-muted); }
.ex-input:focus-visible { outline: 3px solid var(--focus); outline-offset: 2px; border-color: var(--ink-muted); }
.ex-input:disabled { opacity: .75; }
.ex-mark { position: absolute; right: .6em; top: 50%; transform: translateY(-50%); font-family: var(--font-sans); font-weight: 800; pointer-events: none; }
.ex-field.is-correct .ex-input { border-color: var(--c-ok); background: var(--tint-ok); }
.ex-field.is-correct .ex-mark { color: var(--c-ok); }
.ex-field.is-incorrect .ex-input { border-color: var(--c-bad); background: var(--tint-bad); }
.ex-field.is-incorrect .ex-mark { color: var(--c-bad); }

/* ---------------------------------------------------------------- feedback */

.ex-actions { display: flex; flex-wrap: wrap; align-items: center; gap: var(--s-2); }
.ex-feedback {
  display: flex;
  align-items: flex-start;
  gap: .55em;
  font-family: var(--font-sans);
  font-size: .88rem;
  line-height: 1.5;
}
.ex-feedback:empty { display: none; }
.ex-feedback.is-correct, .ex-feedback.is-incorrect, .ex-feedback.is-info {
  padding: var(--s-3) var(--s-4);
  border-radius: var(--r-m);
}
.ex-feedback.is-correct { color: var(--ink); background: var(--tint-ok); border: 1px solid color-mix(in srgb, var(--c-ok) 35%, transparent); }
.ex-feedback.is-correct strong, .ex-feedback.is-correct .fb-icon { color: var(--c-ok); }
.ex-feedback.is-incorrect { color: var(--ink); background: var(--tint-bad); border: 1px solid color-mix(in srgb, var(--c-bad) 30%, transparent); }
.ex-feedback.is-incorrect strong, .ex-feedback.is-incorrect .fb-icon { color: var(--c-bad); }
.ex-feedback.is-info { color: var(--ink); background: var(--surface-2); }
.ex-feedback.is-info .fb-icon { color: var(--ink-muted); }
.fb-icon { width: 1.3em; height: 1.3em; margin-top: .05em; stroke-width: 2.3; }
.ex-feedback.pulse .fb-icon { animation: ok-pulse 300ms var(--ease); }
@keyframes ok-pulse { 0% { transform: scale(.6); } 60% { transform: scale(1.18); } 100% { transform: scale(1); } }
.ex-wrongs > p, .ex-wrong {
  margin-top: var(--s-2);
  padding: var(--s-2) var(--s-4);
  border-left: 3px solid var(--c-bad);
  font-family: var(--font-sans);
  font-size: .86rem;
  background: var(--surface-2);
  border-radius: 0 var(--r-s) var(--r-s) 0;
}

/* ---------------------------------------------------------------- choice / multi */

.ex-fieldset { border: 0; padding: 0; min-width: 0; margin-inline: 0; margin-bottom: 0; }
.ex-option-list { display: grid; gap: var(--s-2); }
.ex-option {
  display: flex;
  align-items: flex-start;
  gap: .7em;
  padding: var(--s-3) var(--s-4);
  border: 1.5px solid var(--rule);
  border-radius: var(--r-m);
  background: var(--bg);
  cursor: pointer;
  min-height: 44px;
  transition: border-color var(--t-ui) var(--ease), background-color var(--t-ui) var(--ease);
}
.ex-option:hover { border-color: var(--ink-muted); }
.ex-option:has(input:checked) { border-color: var(--c-lab); background: var(--tint-lab); }
.ex-option:has(input:focus-visible) { outline: 3px solid var(--focus); outline-offset: 2px; }
.ex-option input { width: 1.15em; height: 1.15em; margin: .25em 0 0; flex: none; }
.ex-option input:focus-visible { outline: none; }
.ex-option-body { flex: 1; }
.ex-option-mark { order: 3; font-family: var(--font-sans); font-weight: 800; }
.ex-option.is-correct { border-color: var(--c-ok); background: var(--tint-ok); }
.ex-option.is-correct .ex-option-mark::after { content: "\2713 right"; color: var(--c-ok); font-size: .78rem; }
.ex-option.is-incorrect { border-color: var(--c-bad); background: var(--tint-bad); }
.ex-option.is-incorrect .ex-option-mark::after { content: "\2717 not this"; color: var(--c-bad); font-size: .78rem; }
.ex-option-fb { display: block; margin: var(--s-1) 0 0 calc(var(--s-4) + 1.85em); font-family: var(--font-sans); font-size: .82rem; color: var(--ink); }
.ex-option-input:disabled + * { opacity: .8; }

/* ---------------------------------------------------------------- sort */

.ex-cards.is-live { list-style: none; padding: 0; display: grid; grid-template-columns: minmax(0, 1fr); gap: var(--s-2); }
.ex-cards.is-live > li {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: var(--s-2) var(--s-4);
  margin: 0;
  padding: var(--s-2) var(--s-2) var(--s-2) var(--s-4);
  border: 1.5px solid var(--rule);
  border-radius: var(--r-m);
  background: var(--bg);
  border-left-width: 5px;
}
.ex-cards.is-live > li[data-chosen="0"] { border-left-color: var(--c-inv); }
.ex-cards.is-live > li[data-chosen="1"] { border-left-color: var(--c-light); }
.ex-cards.is-live > li[data-chosen="2"] { border-left-color: var(--c-you); }
.ex-cards.is-live > li[data-chosen="3"] { border-left-color: var(--c-lab); }
.ex-card-text { flex: 1 1 14em; min-width: 0; }
/* min-width 0: a long group name must shrink its select, not push the card past a 320px column */
.ex-bin-pick { display: inline-flex; align-items: center; gap: var(--s-2); min-width: 0; max-width: 100%; }
.ex-bin-pick .ex-bin-select { min-width: 0; flex: 0 1 auto; }
.ex-bin-select {
  min-height: 40px;
  padding: .3em .6em;
  border: 1.5px solid var(--rule);
  border-radius: var(--r-s);
  background: var(--surface);
  font-family: var(--font-sans);
  font-size: .82rem;
  max-width: 100%;
}
.ex-cards > li.is-correct { border-color: var(--c-ok); background: var(--tint-ok); }
.ex-cards > li.is-incorrect { border-color: var(--c-bad); background: var(--tint-bad); }
.ex-cards > li .ex-mark { position: static; transform: none; width: 1em; }
.ex-cards > li.is-correct .ex-mark { color: var(--c-ok); }
.ex-cards > li.is-incorrect .ex-mark { color: var(--c-bad); }

/* ---------------------------------------------------------------- order */

.ex-order.is-live { list-style: none; padding: 0; display: grid; gap: var(--s-2); counter-reset: ord; }
.ex-order.is-live > li {
  counter-increment: ord;
  display: flex;
  align-items: center;
  gap: var(--s-3);
  margin: 0;
  padding: var(--s-2) var(--s-2) var(--s-2) var(--s-3);
  border: 1.5px solid var(--rule);
  border-radius: var(--r-m);
  background: var(--bg);
  transition: border-color var(--t-ui) var(--ease);
}
.ex-order.is-live > li::before {
  content: counter(ord);
  width: 1.7em;
  height: 1.7em;
  flex: none;
  display: grid;
  place-items: center;
  border-radius: 50%;
  background: var(--surface-2);
  font-family: var(--font-sans);
  font-size: .76rem;
  font-weight: 700;
  color: var(--ink-muted);
}
.ex-order-grip { display: none; }
.ex-order-text { flex: 1; }
.ex-order-btns { display: inline-flex; gap: 2px; }
.ex-order-btns .btn { min-height: 36px; min-width: 36px; padding: 0; }
.ex-order > li.is-correct { border-color: var(--c-ok); background: var(--tint-ok); }
.ex-order > li.is-incorrect { border-color: var(--c-bad); background: var(--tint-bad); }

/* ---------------------------------------------------------------- diagram */

.ex-stage {
  min-height: 140px;
  border: 1.5px dashed var(--rule);
  border-radius: var(--r-m);
  background: var(--bg);
  padding: var(--s-3);
  font-family: var(--font-sans);
  font-size: .85rem;
}
.ex-stage.is-mounted { border-style: solid; background: var(--surface); }
.ex-stage:empty::before { content: "The interactive diagram loads here."; color: var(--ink-muted); font-size: .8rem; }
.ex-alt { font-family: var(--font-sans); font-size: .82rem; color: var(--ink-muted); }

/* ---------------------------------------------------------------- reveal and predict text */

.ex-reveal { display: grid; gap: var(--s-2); }
.ex-reveal-text {
  width: 100%;
  min-height: 7.5em;
  padding: var(--s-3) var(--s-4);
  border: 1.5px solid var(--rule);
  border-radius: var(--r-m);
  background: var(--bg);
  font-family: var(--font-serif);
  font-size: 1rem;
  line-height: 1.55;
  resize: vertical;
}
.ex-reveal-text:focus-visible { outline: 3px solid var(--focus); outline-offset: 2px; }
.ex-reveal-text:disabled { opacity: .8; }
details.ex-model {
  border: 1px solid color-mix(in srgb, var(--c-inv) 35%, var(--rule));
  background: var(--tint-inv);
  border-radius: var(--r-m);
  padding: 0 var(--s-4);
}
details.ex-model > summary {
  font-family: var(--font-sans);
  font-weight: 650;
  font-size: .85rem;
  padding: var(--s-3) 0;
  color: var(--c-inv);
  min-height: 44px;
  display: flex;
  align-items: center;
}
details.ex-model[open] { padding-bottom: var(--s-4); }
.ex-checklist { list-style: none; padding: 0; display: grid; gap: var(--s-2); }
.ex-checklist li { display: flex; gap: .6em; align-items: flex-start; margin: 0; font-family: var(--font-sans); font-size: .88rem; }
.ex-checklist li::before { content: "\25C6"; color: var(--c-inv); font-size: .7em; margin-top: .45em; }
.ex-checklist li:has(.ex-tick)::before { content: none; }
.ex-tick { width: 1.15em; height: 1.15em; margin-top: .2em; flex: none; accent-color: var(--c-inv); }

.ex-confidence { border: 0; padding: 0; margin-inline: 0; margin-bottom: 0; display: flex; flex-wrap: wrap; align-items: center; gap: var(--s-2) var(--s-3); }
.ex-confidence legend { float: left; margin-right: var(--s-3); font-family: var(--font-sans); font-size: .82rem; font-weight: 650; color: var(--ink); }
.exercise[data-type="predict"] { border-color: color-mix(in srgb, var(--c-light) 45%, var(--rule)); }
.exercise[data-type="predict"].is-locked { background: color-mix(in srgb, var(--tint-light) 60%, var(--surface)); }
.exercise.is-locked .ex-option { cursor: default; }

/* ---------------------------------------------------------------- hints and solutions */

details.hint, details.solution {
  border-radius: var(--r-m);
  padding: 0 var(--s-4);
  background: var(--surface-2);
  border-left: 3px solid var(--c-light);
}
details.solution { border-left-color: var(--c-inv); }
details.hint > summary, details.solution > summary {
  list-style: none;
  display: flex;
  align-items: center;
  gap: .5em;
  min-height: 40px;
  font-family: var(--font-sans);
  font-size: .82rem;
  font-weight: 650;
  color: var(--ink);
}
details.hint > summary::before, details.solution > summary::before {
  content: "";
  width: .5em;
  height: .5em;
  border-right: 2px solid currentColor;
  border-bottom: 2px solid currentColor;
  transform: rotate(-45deg);
  transition: transform var(--t-ui) var(--ease);
}
details.hint[open] > summary::before, details.solution[open] > summary::before { transform: rotate(45deg); }
details.hint[open], details.solution[open] { padding-bottom: var(--s-4); }
details.hint + details.hint, details.hint + details.solution, .ex-solution-anyway + details.solution { margin-top: var(--s-2); }
.ex-solution-anyway { font-size: .78rem; color: var(--ink-muted); min-height: 36px; }

/* ---------------------------------------------------------------- checkpoint */

.checkpoint {
  margin: var(--s-6) 0;
  padding: var(--s-5);
  border: 1.5px solid var(--ink);
  border-radius: var(--r-l);
  background:
    linear-gradient(var(--surface-2), var(--surface-2)) padding-box;
  position: relative;
}
.checkpoint::before {
  content: "Checkpoint";
  position: absolute;
  top: -.75em;
  left: var(--s-5);
  padding: 0 .6em;
  background: var(--bg);
  font-family: var(--font-sans);
  font-size: .72rem;
  font-weight: 700;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--ink);
}
.checkpoint > .exercise:first-of-type { margin-top: var(--s-3); }
@media (max-width: 699px) { .checkpoint { padding: var(--s-3); } }
.cp-prev { display: flex; align-items: center; gap: .4em; font-family: var(--font-sans); font-size: .8rem; color: var(--c-ok); font-weight: 600; }
.cp-footer { margin-top: var(--s-5); display: grid; gap: var(--s-3); }
.cp-note { font-family: var(--font-sans); font-size: .8rem; color: var(--ink-muted); }
.cp-summary { font-family: var(--font-sans); font-size: .9rem; outline: none; }
.cp-summary:empty { display: none; }
.cp-summary > * + * { margin-top: .5em; }
.cp-summary.is-passed, .cp-summary.is-missed { padding: var(--s-4) var(--s-5); border-radius: var(--r-m); background: var(--surface); border: 1px solid var(--rule); }
.cp-summary.is-passed { border-color: var(--c-ok); box-shadow: inset 4px 0 0 var(--c-ok); }
.cp-summary.is-missed { border-color: var(--c-light); box-shadow: inset 4px 0 0 var(--c-light); }
.cp-score { display: flex; align-items: center; gap: .5em; font-size: 1rem; }
.cp-score .icon { width: 1.4em; height: 1.4em; stroke-width: 2.3; }
.is-passed .cp-score .icon { color: var(--c-ok); }
.cp-reasons, .cp-review { padding-left: 1.3em; font-size: .85rem; }
.cp-spacing { color: var(--ink-muted); font-size: .82rem; font-style: italic; }
.exercise.in-checkpoint { box-shadow: none; }

/* Narrow columns: stack label above a full-width input so a long label plus a unit never overflows 320px
   (WCAG 1.4.10; docs/requests/twins.md item 4). */
@media (max-width: 479px) {
  .ex-field.is-live {
    display: grid;
    grid-template-columns: minmax(0, 1fr) auto;
    grid-template-areas: "label label" "input unit" "preview preview";
    margin-inline-end: 0;
    max-width: 100%;
  }
  .ex-field.is-live .ex-input-wrap { display: block; min-width: 0; }
  .ex-field.is-live .ex-input { width: 100%; max-width: 14em; }
}

/* ---------------------------------------------------------------- reserve the live height (CLS, §8.2)
   Before exercise.js runs, a numeric field is an empty span and there is no Check row; enhancing a warm-up in the
   first viewport then pushed everything below it down by about 150px. With scripting on, reserve that room up front
   (the same pattern widgets.css uses for labs). Checkpoint items get no Check row, so they reserve only the field. */
@media (scripting: enabled) {
  .exercise:not(.is-live) .ex-field { display: inline-block; min-height: 4.8rem; vertical-align: middle; }
  .exercise:is([data-type="numeric"], [data-type="choice"], [data-type="sort"], [data-type="order"]):not(.is-live):not(.checkpoint *) {
    padding-bottom: calc(var(--s-5) + 2.75rem + var(--flow));
  }
}
@media (scripting: enabled) and (max-width: 699px) {
  .exercise:is([data-type="numeric"], [data-type="choice"], [data-type="sort"], [data-type="order"]):not(.is-live):not(.checkpoint *) {
    padding-bottom: calc(var(--s-4) + 2.75rem + var(--flow));
  }
}