/*
 * The scroll-driven assembly story.
 *
 * Three rules hold this layout together, and every one of them is a response to
 * something that breaks in Instagram's in-app browser rather than to taste:
 *
 *  - The stage is `position: sticky` inside a tall track. No fixed positioning,
 *    no scroll hijacking, no transformed ancestors -- a `transform`, `filter` or
 *    `will-change` anywhere above a sticky element silently turns it into a
 *    containing block and the stage scrolls away.
 *  - Heights are `svh` with a `vh` fallback. The in-app browser's chrome grows
 *    and shrinks as you scroll; `100vh` there is the *large* viewport, so a
 *    100vh stage is taller than the visible area and the caption sits under
 *    Instagram's own toolbar.
 *  - Nothing animates on a timer. Everything is a pure function of scroll
 *    position, so a fling, a rubber-band and a jump to an anchor all land on a
 *    coherent frame rather than on whatever an in-flight transition thought.
 */

:root {
    --ink: #1b1f1e;
    --muted: #6a7370;
    --faint: #9aa5a1;
    --rule: #e4e8e6;
    --paper: #ffffff;
    --accent: #2f6f52;
}

* { box-sizing: border-box; }

html {
    /* The story is scrubbed, never scrolled to. Smooth scrolling would put the
       browser's own animation between the wheel and the frame. */
    scroll-behavior: auto;
    -webkit-text-size-adjust: 100%;
}

body {
    margin: 0;
    background: var(--paper);
    color: var(--ink);
    font-family: 'Red Hat Display', system-ui, -apple-system, 'Segoe UI', Roboto, sans-serif;
    font-size: 17px;
    line-height: 1.5;
    /* Guards the sticky stage against an ancestor that establishes a containing
       block. Nothing above .fa-track may grow a transform. */
    overflow-x: hidden;
}

/* ---------------------------------------------------------------- furniture */

.fa-intro,
.fa-outro {
    max-width: 720px;
    margin: 0 auto;
    padding: 18vh 24px;
}

.fa-intro h1 {
    font-size: clamp(34px, 7vw, 60px);
    line-height: 1.05;
    letter-spacing: -0.02em;
    font-weight: 700;
    margin: 0 0 18px;
}

.fa-intro p,
.fa-outro p {
    color: var(--muted);
    font-size: clamp(17px, 2.4vw, 20px);
    margin: 0 0 16px;
}

.fa-kicker {
    font-size: 12px;
    letter-spacing: 0.14em;
    text-transform: uppercase;
    color: var(--faint);
    font-weight: 700;
    margin: 0 0 12px;
}

/*
 * The scroll hint: the word, and a small chevron under it pointing the way.
 *
 * Written as `.fa-intro .fa-hint` because `.fa-intro p` above outranks a bare
 * `.fa-hint`, which is how the first version ended up in body type with a
 * stray vertical rule beside it. Nothing here animates unless the reader has
 * not asked for less motion, and even then it is a slow nudge, not a bounce.
 */
.fa-intro .fa-hint {
    margin: 40px 0 0;
    color: var(--faint);
    font-size: 12px;
    font-weight: 700;
    letter-spacing: 0.14em;
    text-transform: uppercase;
    line-height: 1;
    display: inline-flex;
    flex-direction: column;
    align-items: center;
    gap: 9px;
}

.fa-intro .fa-hint::after {
    content: "";
    display: block;
    width: 8px;
    height: 8px;
    border-right: 1.5px solid currentColor;
    border-bottom: 1.5px solid currentColor;
    transform: rotate(45deg);
    margin-top: -2px;
}

@media (prefers-reduced-motion: no-preference) {
    .fa-intro .fa-hint::after {
        animation: fa-nudge 2.4s ease-in-out infinite;
    }
    @keyframes fa-nudge {
        0%, 100% { transform: translateY(0) rotate(45deg); }
        50% { transform: translateY(4px) rotate(45deg); }
    }
}

.fa-cta {
    display: inline-block;
    margin-top: 8px;
    padding: 14px 26px;
    border-radius: 999px;
    background: var(--ink);
    color: #fff;
    text-decoration: none;
    font-weight: 600;
}

/* -------------------------------------------------------- track and stage */

/*
 * The track is the scroll distance the story is spread over; the stage is the
 * one viewport-sized frame that stays put while it passes. Progress is
 * (scrolled past the top of the track) / (track height - stage height), so the
 * story starts exactly when the stage locks and ends exactly when it unlocks.
 */
.fa-track {
    position: relative;
    height: var(--fa-track-height, 560vh);
}

.fa-stage {
    position: sticky;
    top: 0;
    height: 100vh;
    height: 100svh;
    overflow: hidden;
    background: var(--paper);
}

.fa-canvas {
    display: block;
    width: 100%;
    height: 100%;
    /* The canvas never receives the gesture: scrolling over it must scroll the
       page, on every platform, without a touch handler being involved. */
    touch-action: pan-y;
}

.fa-overlay {
    position: absolute;
    inset: 0;
    pointer-events: none;
}

.fa-pins {
    position: absolute;
    inset: 0;
    width: 100%;
    height: 100%;
    overflow: visible;
}

/* ------------------------------------------------------------- annotations */

.fa-caption {
    position: absolute;
    left: 32px;
    bottom: 40px;
    width: min(400px, calc(100% - 64px));
    padding: 22px 24px;
    background: rgba(255, 255, 255, 0.86);
    -webkit-backdrop-filter: blur(8px);
    backdrop-filter: blur(8px);
    border: 1px solid var(--rule);
    border-radius: 14px;
    opacity: 0;
    /* Composited, and driven straight from scroll -- no transition, so a fast
       fling cannot leave a card half-faded behind the frame it belongs to. */
    transform: translate3d(0, 12px, 0);
    will-change: opacity, transform;
}

.fa-caption h2 {
    font-size: clamp(20px, 3.2vw, 26px);
    line-height: 1.18;
    letter-spacing: -0.01em;
    margin: 0 0 8px;
    font-weight: 700;
}

.fa-caption p {
    margin: 0;
    color: var(--muted);
    font-size: 15.5px;
    line-height: 1.5;
}

.fa-pin-label {
    position: absolute;
    padding: 5px 10px;
    border-radius: 8px;
    background: var(--ink);
    color: #fff;
    font-size: 12.5px;
    font-weight: 600;
    letter-spacing: 0.01em;
    white-space: nowrap;
    transform: translate3d(-50%, -50%, 0);
    opacity: 0;
    will-change: opacity, transform;
}

/* ------------------------------------------------------------ progress rail */

.fa-rail {
    position: absolute;
    right: 22px;
    top: 50%;
    transform: translateY(-50%);
    display: flex;
    flex-direction: column;
    gap: 9px;
}

.fa-rail span {
    width: 5px;
    height: 5px;
    border-radius: 50%;
    background: var(--rule);
}

.fa-rail span.is-on { background: var(--ink); }

/* ------------------------------------------------------------------ mobile */

@media (max-width: 760px) {
    /*
     * On a phone the caption is not an overlay, it is the bottom of the screen.
     *
     * A 360px card floating over a portrait viewport covers the lower third of
     * the model, and the lower third is where the shelf's feet, its lowest
     * joint and most of its width are. So the canvas gives up that band
     * instead: the model gets the top, the words get the bottom, and neither is
     * ever on top of the other.
     *
     * The band is measured at load from the tallest caption in the story (see
     * scroll-story.js) rather than guessed, because the copy changes and a
     * guessed height is either wasted space or a clipped last line.
     */
    .fa-canvas {
        height: calc(100% - var(--fa-caption-band, 200px));
    }

    .fa-caption {
        left: 16px;
        right: 16px;
        width: auto;
        /* Clears Instagram's bottom toolbar, which overlaps the small viewport
           on iOS without ever being reported to the page. */
        bottom: max(20px, env(safe-area-inset-bottom, 0px) + 14px);
        padding: 18px 18px;
        background: var(--paper);
        border-color: transparent;
        -webkit-backdrop-filter: none;
        backdrop-filter: none;
    }

    /*
     * The rail moves off the model's right edge and lies along the bottom of
     * the canvas band instead. On a phone the right edge is where a pin label
     * ends up once it has been flipped inward, and two small things fighting
     * over the same 20px reads as a bug.
     */
    .fa-rail {
        top: auto;
        right: auto;
        left: 50%;
        bottom: calc(var(--fa-caption-band, 200px) + 12px);
        transform: translateX(-50%);
        flex-direction: row;
        gap: 7px;
    }
    .fa-intro, .fa-outro { padding: 14vh 20px; }
}

/* ----------------------------------------------------- the stepped fallback */

/*
 * What a device gets when it should not be asked to animate: reduced motion,
 * Save-Data, a 2G connection, or no WebGL at all. Same story, told as stills.
 * The stills are produced by the same renderer at load (see scroll-story.js),
 * so they cannot drift from the animation the way a folder of exported PNGs
 * would.
 */
.fa-steps {
    max-width: 780px;
    margin: 0 auto;
    padding: 0 24px 12vh;
}

.fa-step {
    padding: 40px 0;
    border-top: 1px solid var(--rule);
}

.fa-step:first-child { border-top: 0; }

.fa-step img,
.fa-step .fa-step-blank {
    display: block;
    width: 100%;
    aspect-ratio: 4 / 3;
    background: #f6f8f7;
    border-radius: 12px;
    margin-bottom: 20px;
}

.fa-step h2 {
    font-size: clamp(21px, 3.6vw, 27px);
    margin: 0 0 8px;
    letter-spacing: -0.01em;
}

.fa-step p {
    margin: 0;
    color: var(--muted);
}

/* ------------------------------------------------------------------ badge */

/* Bench only: says which tier is running and why, because "it looks fine on my
   laptop" is exactly the failure this page has to be tested against. */
.fa-badge {
    position: fixed;
    left: 12px;
    top: 12px;
    z-index: 50;
    font: 600 11px/1.5 ui-monospace, SFMono-Regular, Menlo, monospace;
    background: rgba(27, 31, 30, 0.86);
    color: #fff;
    padding: 6px 10px;
    border-radius: 7px;
    letter-spacing: 0.02em;
    white-space: pre;
}

.fa-badge a { color: #9ad8bb; text-decoration: none; }