/* ───────────────────────────────────────────────────────────────────────────
   How a First Stage Works — PADUM DIVING theme.

   Tokens are the production theme's, by way of gradientfactor.padum-diving.com
   (padum.css v1.4.0): deep-ocean page gradient, white glass cards, Oswald over
   IBM Plex Sans, coral actions, 6/10/16/pill radii. The rail + stage shell is
   deliberately the same one the Gradient Factors app uses — two teaching tools
   from the same shop should not feel like two products.

   DIAGRAM COLOURS carry the same meanings as in Gradient Factors, so a reader
   moving between the tools reads a colour the same way in both:

     · ambient = brand sun #FAC669. In GF this is the reference every chart is
       read against; here ambient water pressure is literally the thing the
       first stage balances itself against. Same role, same colour.
     · HP gas  = coral #D2564E — tank pressure, the sharp end.
     · IP gas  = sky #95D1EB — what you actually breathe off.

   Coral and sun are only ~ΔE 9 apart under deuteranopia, so nothing rests on
   hue alone: the two gas paths also differ in dash pattern, and every band in
   the force chart carries a text label.
   ─────────────────────────────────────────────────────────────────────────── */

:root {
  color-scheme: dark;

  /* ── brand palette ── */
  --padum-ocean:  #244D78;
  --padum-sea:    #346B8F;
  --padum-lagoon: #539DB8;
  --padum-sky:    #95D1EB;
  --padum-coral:  #D2564E;
  --padum-coral-hover: #BC463F;
  --padum-sand:   #EEA570;
  --padum-sun:    #FAC669;

  --surface-page-top:  #14375A;
  --surface-page-mid:  #0E2740;
  --surface-page-deep: #0A1E33;
  --surface-card: linear-gradient(180deg, rgba(255,255,255,.10) 0%, rgba(255,255,255,.045) 100%);

  --surface-1: #0A1E33;
  --plane:     #0E2740;
  --ink:       #FFFFFF;
  --ink-2:     #D3D6DA;
  --ink-muted: #A2AAB2;
  --grid:      #233448;
  --axis:      #405060;
  --hairline:  rgba(255,255,255,.14);

  /* ── diagram roles ── */
  --c-hp:      #D2564E;
  --c-ip:      #95D1EB;
  --c-ambient: #FAC669;
  --c-spring:  #539DB8;
  --c-metal:   #405060;
  --c-seal:    #E4F5FD;

  --st-good:    #0ca30c;
  --st-warning: #FAC669;
  --st-bad:     #D2564E;

  /* ── type ── */
  --font-display: "Oswald", "DIN Condensed", "Arial Narrow", sans-serif;
  --font-ui: "IBM Plex Sans", "IBM Plex Sans Thai", system-ui, sans-serif;
  --font-mono: "IBM Plex Mono", ui-monospace, SFMono-Regular, Menlo, monospace;
  --track-display: .06em;
  --track-eyebrow: .22em;
  --leading-body: 1.55;

  /* ── shape & motion ── */
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-pill: 999px;
  --shadow-card: 0 22px 46px rgba(4,14,26,.5), inset 0 1px 0 rgba(255,255,255,.16);
  --ring-focus: 0 0 0 3px rgba(83,157,184,.45);
  --blur-glass: saturate(130%) blur(14px);
  --ease-swell: cubic-bezier(.22,.61,.36,1);
  --dur-fast: 120ms;
  --dur-base: 200ms;

  --gap: 16px;
  --rail-w: 320px;
}

/* ─────────────────────────────── base ─────────────────────────────── */

*, *::before, *::after { box-sizing: border-box; }

body {
  margin: 0;
  font-family: var(--font-ui);
  font-size: 14px;
  line-height: var(--leading-body);
  color: var(--ink-2);
  background: var(--surface-page-deep);
  background-image: linear-gradient(180deg,
    var(--surface-page-top) 0%,
    var(--surface-page-mid) 45%,
    var(--surface-page-deep) 100%);
  background-attachment: fixed;
  -webkit-font-smoothing: antialiased;
}

h1, h2, h3 { margin: 0; font-weight: 500; }
button, input, select { font: inherit; color: inherit; }
a { color: var(--padum-sky); text-decoration: none; }
a:hover { color: var(--ink); }
strong { color: var(--ink); font-weight: 600; }

/* ── language switching ──
   Both languages ship in the markup and CSS hides one, so a switch is a single
   attribute flip: no re-render, no flash, and the page still reads with JS off
   (it just shows both). */
html[data-lang="en"] [data-lang="th"] { display: none; }
html[data-lang="th"] [data-lang="en"] { display: none; }

/* ─────────────────────────── masthead ─────────────────────────── */

.masthead {
  display: flex;
  flex-wrap: wrap;
  align-items: flex-end;
  justify-content: space-between;
  gap: 20px;
  padding: 26px 28px 20px;
  border-bottom: 1px solid var(--hairline);
}

.masthead__id { display: flex; align-items: flex-start; gap: 18px; }

.brand {
  width: 62px;
  height: 62px;
  flex-shrink: 0;
  display: block;
  /* The mark is a flat illustration with white keylines, so it sits on the
     dark ground unaided — no plate, no ring. */
}

.masthead .rule {
  display: block;
  width: 62px;
  height: 3px;
  border-radius: var(--radius-pill);
  background: var(--padum-coral);
  margin-bottom: 12px;
}

.masthead h1 {
  font-family: var(--font-display);
  font-weight: 600;
  font-size: clamp(30px, 4.2vw, 46px);
  line-height: 1.02;
  letter-spacing: var(--track-display);
  text-transform: uppercase;
  color: var(--ink);
}
.masthead h1 em { font-style: normal; color: var(--padum-coral); }

.masthead__sub {
  margin: 10px 0 0;
  max-width: 62ch;
  color: var(--ink-muted);
  font-size: 13.5px;
}

.masthead__meta { display: flex; flex-direction: column; align-items: flex-end; gap: 10px; }
.chrome { display: flex; align-items: center; gap: 8px; flex-wrap: wrap; }

.disclaimer {
  margin: 0;
  font-size: 10px;
  font-weight: 500;
  letter-spacing: var(--track-eyebrow);
  text-transform: uppercase;
  color: var(--ink-muted);
  text-align: right;
  max-width: 34ch;
  line-height: 1.7;
}
.disclaimer strong { color: var(--padum-coral); font-weight: 600; }

/* ─────────────────────────── buttons ─────────────────────────── */

.ghost-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 7px;
  background: transparent;
  border: 1px solid rgba(255,255,255,.22);
  border-radius: var(--radius-pill);
  color: var(--ink-2);
  padding: 7px 14px;
  font-size: 10px;
  font-weight: 500;
  letter-spacing: .12em;
  text-transform: uppercase;
  cursor: pointer;
  transition: border-color var(--dur-base) var(--ease-swell),
              color var(--dur-base) var(--ease-swell),
              background var(--dur-base) var(--ease-swell);
}
.ghost-btn:hover { border-color: var(--padum-coral); color: var(--ink); background: rgba(210,86,78,.14); }
.ghost-btn:active { transform: scale(.98); }
.ghost-btn:focus-visible { outline: none; box-shadow: var(--ring-focus); }
.ghost-btn--sm { padding: 5px 11px; font-size: 9.5px; }

.ghost-btn[aria-pressed="true"] {
  border-color: var(--padum-coral);
  background: var(--padum-coral);
  color: #fff;
}
.ghost-btn[aria-pressed="true"]:hover { background: var(--padum-coral-hover); border-color: var(--padum-coral-hover); }

.lang { display: flex; gap: 4px; }
.lang .ghost-btn { padding: 7px 12px; }

/* A pair of mutually exclusive options, sized to split the rail evenly. */
.switch { display: grid; grid-template-columns: 1fr 1fr; gap: 6px; }
.switch .ghost-btn { width: 100%; }

/* ─────────────────────────── layout ─────────────────────────── */

.shell {
  display: grid;
  grid-template-columns: var(--rail-w) minmax(0, 1fr);
  gap: var(--gap);
  padding: var(--gap) 28px 28px;
  align-items: start;
}

.rail {
  display: flex;
  flex-direction: column;
  gap: var(--gap);
  position: sticky;
  top: var(--gap);
  max-height: calc(100vh - 2 * var(--gap));
  overflow-y: auto;
  padding-right: 4px;
  scrollbar-width: thin;
  scrollbar-color: var(--axis) transparent;
}

.stage { display: flex; flex-direction: column; gap: var(--gap); min-width: 0; }

/* ─────────────────────── panel (glass card) ─────────────────────── */

.panel {
  background: var(--surface-card);
  backdrop-filter: var(--blur-glass);
  -webkit-backdrop-filter: var(--blur-glass);
  border: 1px solid var(--hairline);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-card);
  padding: 18px 20px 20px;
}

.panel__title {
  display: flex;
  align-items: center;
  gap: 10px;
  margin-bottom: 12px;
  font-family: var(--font-display);
  font-size: 12px;
  font-weight: 500;
  letter-spacing: var(--track-eyebrow);
  text-transform: uppercase;
  color: var(--ink);
}

.panel__num {
  font-family: var(--font-mono);
  font-size: 10px;
  font-variant-numeric: tabular-nums;
  letter-spacing: .04em;
  color: var(--ink);
  background: var(--padum-sea);
  padding: 2px 6px;
  border-radius: var(--radius-sm);
}

.hint { margin: 10px 0 0; font-size: 11.5px; color: var(--ink-muted); line-height: 1.45; }

/* ─────────────────────────── controls ─────────────────────────── */

.control { display: flex; flex-direction: column; gap: 7px; }
.control + .control { margin-top: 14px; }

.control__head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 10px;
}

.control__label {
  font-size: 9.5px;
  font-weight: 500;
  letter-spacing: var(--track-eyebrow);
  text-transform: uppercase;
  color: var(--ink-muted);
}

.control__value {
  font-family: var(--font-mono);
  font-size: 13px;
  font-variant-numeric: tabular-nums;
  color: var(--ink);
}
.control__value .unit { font-size: 10.5px; color: var(--ink-muted); margin-left: 3px; }

input[type="range"] {
  -webkit-appearance: none;
  appearance: none;
  width: 100%;
  height: 22px;
  background: transparent;
  cursor: pointer;
  margin: 0;
}
input[type="range"]::-webkit-slider-runnable-track {
  height: 4px;
  border-radius: var(--radius-pill);
  background: var(--grid);
}
input[type="range"]::-moz-range-track {
  height: 4px;
  border-radius: var(--radius-pill);
  background: var(--grid);
}
input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none;
  appearance: none;
  width: 16px;
  height: 16px;
  margin-top: -6px;
  border-radius: var(--radius-pill);
  background: var(--padum-coral);
  border: 2px solid #fff;
  box-shadow: 0 2px 6px rgba(4,14,26,.6);
}
input[type="range"]::-moz-range-thumb {
  width: 16px;
  height: 16px;
  border-radius: var(--radius-pill);
  background: var(--padum-coral);
  border: 2px solid #fff;
  box-shadow: 0 2px 6px rgba(4,14,26,.6);
}
input[type="range"]:focus-visible { outline: none; box-shadow: var(--ring-focus); border-radius: var(--radius-pill); }

.scale {
  display: flex;
  justify-content: space-between;
  font-family: var(--font-mono);
  font-size: 9.5px;
  color: var(--ink-muted);
}

/* ─────────────────────── "try this" prompts ─────────────────────── */

.tries { display: flex; flex-direction: column; gap: 8px; }

.try {
  display: flex;
  flex-direction: column;
  gap: 2px;
  text-align: left;
  width: 100%;
  background: rgba(255,255,255,.05);
  border: 1px solid rgba(255,255,255,.14);
  border-radius: var(--radius-md);
  padding: 9px 11px;
  cursor: pointer;
  transition: border-color var(--dur-base) var(--ease-swell),
              background var(--dur-base) var(--ease-swell);
}
.try:hover { border-color: var(--padum-lagoon); background: rgba(83,157,184,.16); }
.try:focus-visible { outline: none; box-shadow: var(--ring-focus); }
.try__name {
  font-family: var(--font-display);
  font-size: 12px;
  font-weight: 500;
  letter-spacing: var(--track-display);
  text-transform: uppercase;
  color: var(--ink);
}
.try__note { font-size: 10.5px; color: var(--ink-muted); line-height: 1.4; }

/* ─────────────────────────── readouts ─────────────────────────── */

.readouts {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  gap: 1px;
  background: var(--hairline);
  border: 1px solid var(--hairline);
  border-radius: var(--radius-lg);
  overflow: hidden;
  box-shadow: var(--shadow-card);
}

.stat {
  background: rgba(20, 55, 90, .82);
  backdrop-filter: var(--blur-glass);
  -webkit-backdrop-filter: var(--blur-glass);
  padding: 13px 16px 14px;
  display: flex;
  flex-direction: column;
  gap: 3px;
}

.stat__label {
  font-size: 9.5px;
  font-weight: 500;
  letter-spacing: var(--track-eyebrow);
  text-transform: uppercase;
  color: var(--ink-muted);
  display: flex;
  align-items: center;
  gap: 6px;
}

.stat__value {
  font-family: var(--font-display);
  font-weight: 500;
  font-size: 32px;
  line-height: 1.02;
  letter-spacing: var(--track-display);
  color: var(--ink);
  font-variant-numeric: tabular-nums;
}
.stat__value .unit {
  font-family: var(--font-mono);
  font-size: 11px;
  font-weight: 400;
  letter-spacing: 0;
  color: var(--ink-muted);
  margin-left: 4px;
}
.stat__note { font-size: 11px; color: var(--ink-muted); line-height: 1.4; }

/* The one readout that is the point of the whole page: it must visibly NOT
   move while the absolute pressures climb. */
.stat--pinned .stat__value { color: var(--padum-sun); }
.stat--pinned .stat__label .swatch { background: var(--padum-sun); }

.swatch { width: 8px; height: 8px; border-radius: 2px; flex-shrink: 0; }
.swatch--hp { background: var(--c-hp); }
.swatch--ip { background: var(--c-ip); }
.swatch--ambient { background: var(--c-ambient); }

/* ─────────────────────────── cutaway ─────────────────────────── */

.stagecard {
  background: var(--surface-card);
  backdrop-filter: var(--blur-glass);
  -webkit-backdrop-filter: var(--blur-glass);
  border: 1px solid var(--hairline);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-card);
  padding: 16px 18px 18px;
  display: flex;
  flex-direction: column;
  gap: 12px;
  min-width: 0;
}

.well {
  background: var(--surface-1);
  border: 1px solid var(--hairline);
  border-radius: var(--radius-md);
  padding: 8px;
}
.well svg { display: block; width: 100%; height: auto; }

.cut-row {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(220px, 280px);
  gap: var(--gap);
  align-items: start;
}

.parts { display: flex; flex-wrap: wrap; gap: 6px; }

.part-btn {
  background: rgba(255,255,255,.05);
  border: 1px solid var(--hairline);
  border-radius: var(--radius-pill);
  color: var(--ink-2);
  padding: 5px 11px;
  font-size: 11px;
  cursor: pointer;
  transition: border-color var(--dur-base) var(--ease-swell),
              background var(--dur-base) var(--ease-swell),
              color var(--dur-base) var(--ease-swell);
}
.part-btn:hover { border-color: var(--padum-lagoon); background: rgba(83,157,184,.16); color: var(--ink); }
.part-btn:focus-visible { outline: none; box-shadow: var(--ring-focus); }
.part-btn[aria-pressed="true"] {
  border-color: var(--padum-lagoon);
  background: rgba(83,157,184,.24);
  color: var(--ink);
}

.readout-box {
  display: flex;
  flex-direction: column;
  gap: 4px;
  min-height: 96px;
  border-radius: var(--radius-md);
  border: 1px solid var(--hairline);
  background: rgba(20,55,90,.82);
  padding: 13px 15px;
}
.readout-box--empty {
  border-style: dashed;
  background: transparent;
  justify-content: center;
  color: var(--ink-muted);
  font-size: 12.5px;
}
.readout-box__name {
  font-family: var(--font-display);
  font-size: 13px;
  font-weight: 500;
  letter-spacing: var(--track-display);
  text-transform: uppercase;
  color: var(--ink);
}
.readout-box__desc { font-size: 12.5px; color: var(--ink-2); }

/* SVG hit targets. The highlight lives on the shape so the target and its
   feedback cannot drift apart. */
.part { cursor: pointer; }
.part:focus-visible { outline: none; }
.part-shape { transition: stroke var(--dur-fast) var(--ease-swell), stroke-width var(--dur-fast) var(--ease-swell); }
.part.is-active .part-shape,
.part:focus-visible .part-shape { stroke: var(--padum-sky); stroke-width: 2.5; }

.flow { animation: flow 1.4s linear infinite; }
@keyframes flow { to { stroke-dashoffset: -28; } }
/* Set on the flow GROUP: animation-play-state on a parent does not reach the
   lines inside, so pausing is a class the children's rule can see. */
.flow-paused .flow { animation-play-state: paused; }

/* ─────────────────────── force balance chart ─────────────────────── */

.forces { display: flex; flex-direction: column; gap: 12px; }

.force-row { display: grid; grid-template-columns: 92px minmax(0, 1fr); gap: 12px; align-items: center; }

.force-row__label {
  font-size: 9.5px;
  font-weight: 500;
  letter-spacing: var(--track-eyebrow);
  text-transform: uppercase;
  color: var(--ink-muted);
  text-align: right;
}

.bar {
  position: relative;
  display: flex;
  height: 34px;
  border-radius: var(--radius-sm);
  background: var(--plane);
  border: 1px solid var(--hairline);
  overflow: hidden;
}

.bar__seg {
  display: flex;
  align-items: center;
  justify-content: center;
  min-width: 0;
  font-family: var(--font-mono);
  font-size: 10.5px;
  color: #0A1E33;
  font-weight: 500;
  white-space: nowrap;
  overflow: hidden;
  /* Not transitioned: the breathing loop repaints these every frame, and a
     200 ms ease on top of a 60 fps update reads as lag, not smoothness. */
}
.bar__seg--spring  { background: var(--c-spring); }
.bar__seg--ambient { background: var(--c-ambient); }
.bar__seg--ip      { background: var(--c-ip); }

.bar__gap {
  flex-shrink: 0;
  background: repeating-linear-gradient(
    45deg, rgba(210,86,78,.30) 0 6px, rgba(210,86,78,.10) 6px 12px);
  border-left: 2px solid var(--c-hp);
  display: flex;
  align-items: center;
  justify-content: center;
}

.force-note {
  display: flex;
  align-items: center;
  gap: 10px;
  border-radius: var(--radius-md);
  border: 1px solid var(--hairline);
  background: rgba(255,255,255,.05);
  padding: 10px 13px;
  font-size: 12.5px;
  color: var(--ink-2);
  min-height: 44px;
}
.force-note .dot { width: 9px; height: 9px; border-radius: var(--radius-pill); flex-shrink: 0; }
.force-note--open .dot   { background: var(--c-hp); }
.force-note--closed .dot { background: var(--st-good); }

/* ─────────────────────────── notes ─────────────────────────── */

.notes { columns: 280px 3; column-gap: 26px; }
.note { break-inside: avoid; margin-bottom: 20px; }
.note h3 {
  font-family: var(--font-display);
  font-size: 12px;
  font-weight: 500;
  letter-spacing: var(--track-eyebrow);
  text-transform: uppercase;
  color: var(--ink);
  margin-bottom: 7px;
}
.note p { margin: 0 0 9px; font-size: 12.5px; color: var(--ink-muted); }
.note p:last-child { margin-bottom: 0; }
.note strong { color: var(--ink-2); font-weight: 600; }
.note em { color: var(--ink-2); font-style: normal; color: var(--padum-sky); }
.note code {
  font-family: var(--font-mono);
  font-size: 11.5px;
  background: rgba(255,255,255,.10);
  border-radius: 4px;
  padding: 1px 5px;
  color: var(--ink);
}
.note--warn { border-left: 3px solid var(--padum-coral); padding-left: 13px; }

/* ─────────────────────────── footer ─────────────────────────── */

.foot {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
  padding: 16px 28px 26px;
  border-top: 1px solid var(--hairline);
  font-size: 10px;
  font-weight: 500;
  letter-spacing: var(--track-eyebrow);
  text-transform: uppercase;
  color: var(--ink-muted);
}
.foot__id { display: flex; align-items: center; gap: 10px; }
.foot__mark { width: 26px; height: 26px; display: block; opacity: .85; }
.foot strong { color: var(--padum-coral); font-weight: 600; }

/* ─────────────────────────── narrow ─────────────────────────── */

@media (max-width: 1100px) {
  .shell { grid-template-columns: 1fr; }
  .rail { position: static; max-height: none; }
  .cut-row { grid-template-columns: 1fr; }
}

@media (max-width: 720px) {
  .masthead { padding: 22px 20px 16px; }
  .masthead__id { gap: 14px; }
  .brand { width: 48px; height: 48px; }
  .masthead .rule { width: 48px; }
  .masthead__meta { align-items: flex-start; width: 100%; }
  .disclaimer { text-align: left; max-width: none; }
  .shell { padding: var(--gap) 16px 24px; }
  .panel, .stagecard { padding: 16px; }
  .force-row { grid-template-columns: 1fr; gap: 5px; }
  .force-row__label { text-align: left; }
  .foot { padding: 16px 20px 24px; flex-direction: column; align-items: flex-start; gap: 8px; }
  /* 44px minimum hit target on touch. */
  .ghost-btn { min-height: 44px; }
  .part-btn { min-height: 36px; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: .001ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: .001ms !important;
  }
}