party-player/public/player/motion.js

// Frame motion. Everything here writes to #frameMover's transform and nothing
// else, so additional animation layers can be nested inside without conflict.
//
// An animation is `start(context) => stop()`. `context` carries the element to
// move and a capture handle (possibly null). `stop()` must undo everything the
// animation set up — timers, listeners, DOM — because the next animation takes
// over the same element immediately.

import {
  FRAME_SCALE,
  BOUNCE_SPEED,
  CORNER_INTERVAL_SECONDS,
  CORNER_MARGIN,
  DRAG_FRAME_SCALE,
  DRAG_MIN_SCALE,
  DRAG_MAX_SCALE,
  TRAIL_SPEED,
  TRAIL_TURN_RATE,
  TRAIL_SPACING,
  TRAIL_LENGTH,
  TRAIL_FADE_SECONDS,
  WASH_CYCLE_SECONDS,
  WASH_SPREAD,
  WASH_STRENGTH,
  AUTO_ANIMATION,
} from '/config.js';

const ASPECT = 9 / 16;

/**
 * Frame size for a viewport: `scale` of the width, but never taller than
 * `scale` of the height, so a wide screen does not get a letterbox strip.
 */
export function frameSize(viewportWidth, viewportHeight, scale = FRAME_SCALE) {
  let width = viewportWidth * scale;
  let height = width * ASPECT;

  const maxHeight = viewportHeight * scale;
  if (height > maxHeight) {
    height = maxHeight;
    width = height / ASPECT;
  }

  return { width, height };
}

/** Size the frame to the current viewport and report the size back. */
function applyFrameSize(element) {
  const size = frameSize(window.innerWidth, window.innerHeight);
  element.style.width = `${size.width}px`;
  element.style.height = `${size.height}px`;
  return size;
}

function place(element, x, y) {
  element.style.transform = `translate3d(${x.toFixed(1)}px, ${y.toFixed(1)}px, 0)`;
}

// --- picker -------------------------------------------------------------

/**
 * Shuffle bag over 0..count-1: every animation comes up exactly once per pass,
 * so selection stays even, and a swap at the bag boundary keeps the same one
 * from landing twice in a row. Pure apart from the injected RNG.
 */
export function createPicker(count, random = Math.random) {
  let bag = [];
  let last = -1;

  function refill() {
    bag = Array.from({ length: count }, (_, i) => i);

    // Fisher–Yates.
    for (let i = bag.length - 1; i > 0; i--) {
      const j = Math.floor(random() * (i + 1));
      [bag[i], bag[j]] = [bag[j], bag[i]];
    }

    // next() pops off the end, so bag[length-1] is served first. A repeat can
    // only happen right here, at the seam between two bags.
    if (bag.length > 1 && bag[bag.length - 1] === last) {
      const j = Math.floor(random() * (bag.length - 1));
      [bag[bag.length - 1], bag[j]] = [bag[j], bag[bag.length - 1]];
    }
  }

  return {
    next() {
      if (count <= 1) return 0;
      if (bag.length === 0) refill();
      last = bag.pop();
      return last;
    },
  };
}

// --- bounce -------------------------------------------------------------

/**
 * Diagonal drift with reflection off the viewport edges. Pure state + step, so
 * the physics can be exercised without a browser and without waiting on rAF.
 */
export function createBouncer({ x = 0, y = 0 } = {}) {
  const bouncer = {
    x,
    y,
    // A shallow diagonal reads better than 45°, and never retraces its path.
    vx: BOUNCE_SPEED,
    vy: BOUNCE_SPEED * 0.62,

    step(dt, bounds) {
      const maxX = Math.max(0, bounds.width);
      const maxY = Math.max(0, bounds.height);

      bouncer.x += bouncer.vx * dt;
      bouncer.y += bouncer.vy * dt;

      // Reflect, mirroring the overshoot back inside the viewport.
      if (bouncer.x <= 0) {
        bouncer.x = Math.min(-bouncer.x, maxX);
        bouncer.vx = Math.abs(bouncer.vx);
      } else if (bouncer.x >= maxX) {
        bouncer.x = Math.max(maxX - (bouncer.x - maxX), 0);
        bouncer.vx = -Math.abs(bouncer.vx);
      }

      if (bouncer.y <= 0) {
        bouncer.y = Math.min(-bouncer.y, maxY);
        bouncer.vy = Math.abs(bouncer.vy);
      } else if (bouncer.y >= maxY) {
        bouncer.y = Math.max(maxY - (bouncer.y - maxY), 0);
        bouncer.vy = -Math.abs(bouncer.vy);
      }

      return bouncer;
    },
  };

  return bouncer;
}

export function startBounce({ element }) {
  let size = applyFrameSize(element);
  const bouncer = createBouncer({
    x: (window.innerWidth - size.width) / 2,
    y: (window.innerHeight - size.height) / 2,
  });

  function bounds() {
    return {
      width: window.innerWidth - size.width,
      height: window.innerHeight - size.height,
    };
  }

  function measure() {
    size = applyFrameSize(element);

    const limit = bounds();
    bouncer.x = Math.min(bouncer.x, Math.max(0, limit.width));
    bouncer.y = Math.min(bouncer.y, Math.max(0, limit.height));
  }

  window.addEventListener('resize', measure);

  let last = performance.now();
  let handle = 0;
  let stopped = false;

  function frame(now) {
    if (stopped) return;

    // Clamped so a stalled tab does not teleport the frame on resume.
    const dt = Math.min((now - last) / 1000, 0.05);
    last = now;

    bouncer.step(dt, bounds());
    place(element, bouncer.x, bouncer.y);
    handle = requestAnimationFrame(frame);
  }

  handle = requestAnimationFrame(frame);

  return function stop() {
    // The flag matters as much as the cancel: a frame already queued would
    // otherwise fire once more and fight whatever animation comes next.
    stopped = true;
    cancelAnimationFrame(handle);
    window.removeEventListener('resize', measure);
  };
}

// --- corners ------------------------------------------------------------

// Clockwise from the top left.
const CORNERS = 4;

/** Top-left offset for a corner, inset by `margin` from the viewport edge. */
export function cornerPosition(index, size, viewport, margin) {
  const right = index === 1 || index === 2;
  const bottom = index === 2 || index === 3;

  return {
    x: right ? Math.max(margin, viewport.width - size.width - margin) : margin,
    y: bottom ? Math.max(margin, viewport.height - size.height - margin) : margin,
  };
}

// How many stills each corner holds, and how far apart they sit as a fraction
// of the frame size. Deep enough that a corner reads as a pile, shallow enough
// that the stack stays in its own quarter of the screen.
const CORNER_STACK = 3;
const CORNER_STACK_STEP = 0.09;

/**
 * Top-left offset for the `depth`-th still of a corner: the same spot as the
 * corner itself, nudged inward (toward the middle of the screen) once per
 * depth. Successive stills therefore never land on top of each other — each
 * one leaves the edges of the ones below it showing.
 */
export function cornerStackPosition(index, depth, size, viewport, margin) {
  const at = cornerPosition(index, size, viewport, margin);
  const right = index === 1 || index === 2;
  const bottom = index === 2 || index === 3;
  const step = depth * CORNER_STACK_STEP;

  return {
    x: at.x + (right ? -1 : 1) * step * size.width,
    y: at.y + (bottom ? -1 : 1) * step * size.height,
  };
}

/**
 * The video parks in one random corner and stays there; every few seconds a
 * screenshot of it lands in the next of the three *other* corners, cycling
 * clockwise. Because the video's own corner is never a slot, a still can never
 * cover the video — and since the crop only ever covers the video's rect, a
 * still never contains another still either.
 *
 * Each corner stacks CORNER_STACK stills at staggered offsets, and the newest
 * shot always moves to the top of the layer, so a full corner still reads as
 * something new arriving rather than as an unchanged picture.
 */
export function startCorners(context) {
  const { element } = context;
  const layer = document.getElementById('stills');
  const home = Math.floor(Math.random() * CORNERS);

  // The three non-home corners, in clockwise order starting after the video.
  const slots = [1, 2, 3].map((step) => (home + step) % CORNERS);

  // Slot-major: consecutive shots move to the next corner, and the depth only
  // advances once every corner at the current depth has been filled.
  const cells = [];
  for (let depth = 0; depth < CORNER_STACK; depth += 1) {
    for (const corner of slots) {
      const img = document.createElement('img');
      layer.appendChild(img);
      cells.push({ img, corner, depth });
    }
  }

  let next = 0;
  let size;

  function layout() {
    size = applyFrameSize(element);
    const viewport = { width: window.innerWidth, height: window.innerHeight };
    const margin = Math.min(viewport.width, viewport.height) * CORNER_MARGIN;

    const spot = cornerPosition(home, size, viewport, margin);
    place(element, spot.x, spot.y);

    for (const cell of cells) {
      const at = cornerStackPosition(cell.corner, cell.depth, size, viewport, margin);
      cell.img.style.width = `${size.width}px`;
      cell.img.style.height = `${size.height}px`;
      place(cell.img, at.x, at.y);
    }
  }

  function shoot() {
    // Read late: the capture handle may still be waiting on the operator to
    // answer the screen-share prompt when this animation starts.
    if (!context.capture) return;

    const rect = element.getBoundingClientRect();
    const shot = context.capture.grab(rect);
    if (!shot) return;

    const cell = cells[next];
    cell.img.src = shot;
    cell.img.classList.add('visible');
    // Re-append so the freshest still paints over the rest of its pile.
    layer.appendChild(cell.img);
    next = (next + 1) % cells.length;
  }

  layout();
  window.addEventListener('resize', layout);
  const timer = setInterval(shoot, CORNER_INTERVAL_SECONDS * 1000);

  return function stop() {
    clearInterval(timer);
    window.removeEventListener('resize', layout);
    for (const cell of cells) cell.img.remove();
  };
}

// --- trail --------------------------------------------------------------

// The frame glides along a curving path and drops a photograph of itself every
// so often. Each still stays where it was taken and fades, so the live video
// reads as the head of a comet whose tail is its own recent past.

/**
 * Steering toward a waypoint at constant speed, with a cap on how fast the
 * heading may change. The cap is the whole point: an instant turn would lay the
 * stills out in straight dotted lines, and it is the curve that makes a tail
 * read as one. Pure apart from the injected RNG, so the path can be exercised
 * without a browser.
 */
export function createGlider({ x = 0, y = 0 } = {}, random = Math.random) {
  // How close counts as arrived. Anything tighter than the turn radius
  // (speed / turn rate) risks the glider circling a waypoint forever, unable to
  // steer hard enough to land on it.
  const reach = TRAIL_SPEED / TRAIL_TURN_RATE;

  function normalise(angle) {
    return Math.atan2(Math.sin(angle), Math.cos(angle));
  }

  const glider = {
    x,
    y,
    heading: random() * Math.PI * 2,
    target: null,

    retarget(bounds) {
      glider.target = {
        x: random() * Math.max(0, bounds.width),
        y: random() * Math.max(0, bounds.height),
      };
      return glider.target;
    },

    step(dt, bounds) {
      const maxX = Math.max(0, bounds.width);
      const maxY = Math.max(0, bounds.height);

      if (!glider.target) glider.retarget(bounds);
      if (Math.hypot(glider.target.x - glider.x, glider.target.y - glider.y) < reach) {
        glider.retarget(bounds);
      }

      // Shortest way round to the bearing of the waypoint, then take at most
      // this step's share of the turn rate.
      const bearing = Math.atan2(glider.target.y - glider.y, glider.target.x - glider.x);
      const turn = normalise(bearing - glider.heading);
      glider.heading = normalise(
        glider.heading + clamp(turn, -TRAIL_TURN_RATE * dt, TRAIL_TURN_RATE * dt),
      );

      glider.x += Math.cos(glider.heading) * TRAIL_SPEED * dt;
      glider.y += Math.sin(glider.heading) * TRAIL_SPEED * dt;

      // Reflect off the edges rather than clamping and sliding along them. The
      // waypoint is always inside the bounds, so steering picks up from there
      // and pulls the glider back inward instead of straight into the wall.
      if (glider.x < 0 || glider.x > maxX) {
        glider.x = clamp(glider.x, 0, maxX);
        glider.heading = normalise(Math.PI - glider.heading);
      }
      if (glider.y < 0 || glider.y > maxY) {
        glider.y = clamp(glider.y, 0, maxY);
        glider.heading = normalise(-glider.heading);
      }

      return glider;
    },
  };

  return glider;
}

export function startTrail(context) {
  const { element } = context;
  const layer = document.getElementById('stills');

  let size = applyFrameSize(element);
  const glider = createGlider({
    x: (window.innerWidth - size.width) / 2,
    y: (window.innerHeight - size.height) / 2,
  });

  // A ring buffer: the tail is a fixed number of images, and the oldest one is
  // what the next drop reuses. `rect` doubles as the "currently showing" flag.
  const cells = [];
  for (let i = 0; i < TRAIL_LENGTH; i += 1) {
    const img = document.createElement('img');
    img.classList.add('trail');
    layer.appendChild(img);
    cells.push({ img, born: 0, rect: null });
  }

  let next = 0;
  let travelled = 0;

  function bounds() {
    return {
      width: window.innerWidth - size.width,
      height: window.innerHeight - size.height,
    };
  }

  function measure() {
    size = applyFrameSize(element);

    const limit = bounds();
    glider.x = Math.min(glider.x, Math.max(0, limit.width));
    glider.y = Math.min(glider.y, Math.max(0, limit.height));
  }

  /** True when the frame overlaps nothing that is still showing in the tail. */
  function clearOfTail(rect) {
    for (const cell of cells) {
      const other = cell.rect;
      if (!other) continue;
      if (
        rect.left < other.right &&
        rect.right > other.left &&
        rect.top < other.bottom &&
        rect.bottom > other.top
      ) {
        return false;
      }
    }

    return true;
  }

  /** Photograph the frame where it stands. Reports whether a still landed. */
  function drop(now) {
    // Read late: the capture handle may still be waiting on the operator to
    // answer the screen-share prompt when this animation starts.
    if (!context.capture) return false;

    const rect = element.getBoundingClientRect();

    // The capture is a crop of the tab, so an older still lying under the frame
    // would be photographed into the new one. Skip this drop instead; the
    // travelled distance is not spent, so it happens as soon as the frame is
    // back in the clear.
    if (!clearOfTail(rect)) return false;

    const shot = context.capture.grab(rect);
    if (!shot) return false;

    const cell = cells[next];
    cell.img.src = shot;
    cell.img.style.width = `${rect.width}px`;
    cell.img.style.height = `${rect.height}px`;
    place(cell.img, rect.left, rect.top);
    cell.born = now;
    cell.rect = { left: rect.left, top: rect.top, right: rect.right, bottom: rect.bottom };
    // Re-append so the freshest still paints over the rest of the tail.
    layer.appendChild(cell.img);

    next = (next + 1) % cells.length;
    return true;
  }

  function fade(now) {
    for (const cell of cells) {
      if (!cell.rect) continue;

      const life = 1 - (now - cell.born) / 1000 / TRAIL_FADE_SECONDS;
      if (life <= 0) {
        cell.img.style.opacity = '0';
        // Gone, so it stops holding up drops that pass over its old spot.
        cell.rect = null;
        continue;
      }

      // Squared: a still holds its brightness for a moment and then goes
      // quickly, which thins the tail out instead of trailing a long grey smear.
      cell.img.style.opacity = (life * life).toFixed(3);
    }
  }

  place(element, glider.x, glider.y);
  window.addEventListener('resize', measure);

  let last = performance.now();
  let handle = 0;
  let stopped = false;

  function frame(now) {
    if (stopped) return;

    const dt = Math.min((now - last) / 1000, 0.05);
    last = now;

    const wasX = glider.x;
    const wasY = glider.y;
    glider.step(dt, bounds());
    place(element, glider.x, glider.y);

    // Spacing by distance rather than by a timer keeps the stills evenly spread
    // however the frame is turning.
    travelled += Math.hypot(glider.x - wasX, glider.y - wasY);
    if (travelled >= size.width * TRAIL_SPACING && drop(now)) travelled = 0;

    fade(now);
    handle = requestAnimationFrame(frame);
  }

  handle = requestAnimationFrame(frame);

  return function stop() {
    stopped = true;
    cancelAnimationFrame(handle);
    window.removeEventListener('resize', measure);
    for (const cell of cells) cell.img.remove();
  };
}

// --- drag and resize ----------------------------------------------------

// Someone shoving a video window around a desktop: a pointer travels to the
// frame, grabs it, drags it somewhere else, lets go for a moment, then takes
// hold of one corner and pulls the frame to a new size. The pointer is drawn,
// so every move on screen has a visible cause.

const ARROW = `<svg viewBox="0 0 24 24" width="27" height="27" aria-hidden="true">
  <path d="M5 2.5 L5 19.6 L9.5 15.4 L12.3 21.5 L15.2 20.1 L12.4 14.2 L18.6 14 Z"
        fill="#fff" stroke="#14060f" stroke-width="1.3" stroke-linejoin="round" />
</svg>`;

// Where on the frame the pointer takes hold, as a fraction of its height —
// the top strip, where a window's title bar would be.
const GRAB_BAND = [0.06, 0.3];

// Seconds the pointer rests between grabs, and how far it drifts while resting.
const PAUSE = [0.5, 1.7];
const IDLE_DRIFT = 2.5;

function clamp(value, min, max) {
  return Math.min(Math.max(value, min), max);
}

function between(min, max) {
  return min + Math.random() * (max - min);
}

/** Smoothstep: no jerk at either end, which is what an arm does. */
function smooth(t) {
  return t * t * (3 - 2 * t);
}

/**
 * Progress along a hand-driven move: a gentle launch, then a slight overshoot
 * that pulls back onto the target. `overshoot` 0 is a plain smooth ramp; the
 * bigger it gets, the further past the target the move swings first.
 */
export function settle(t, overshoot = 0.6) {
  const back = smooth(clamp(t, 0, 1)) - 1;
  return 1 + (overshoot + 1) * back ** 3 + overshoot * back ** 2;
}

/**
 * Pointer position at `t` (0..1) on its way from `from` to `to`. `curve` bows
 * the path sideways — a hand does not move in a straight line — and its sign
 * decides which way the arc leans.
 */
export function pointerAt(from, to, t, { curve = 0, overshoot = 0.6 } = {}) {
  const dx = to.x - from.x;
  const dy = to.y - from.y;
  const p = settle(t, overshoot);
  const bow = Math.sin(Math.PI * clamp(t, 0, 1)) * curve;

  return {
    x: from.x + dx * p - dy * bow,
    y: from.y + dy * p + dx * bow,
  };
}

/**
 * The rect you get by dragging `corner` (0 = top left, clockwise) to `pointer`
 * while the opposite corner stays pinned, the way a window resize works.
 *
 * The aspect ratio is locked — a squashed video reads as broken rather than as
 * someone resizing — so the axis the pointer pulled furthest wins, and the
 * width is clamped into `min`..`max`.
 */
export function resizeRect(rect, corner, pointer, { min, max }) {
  const anchor = anchorOf(rect, corner);
  const pulled = Math.max(
    Math.abs(pointer.x - anchor.x),
    Math.abs(pointer.y - anchor.y) / ASPECT,
  );

  return rectFromWidth(rect, corner, clamp(pulled, min, max));
}

/** The corner that stays put while `corner` is dragged. */
function anchorOf(rect, corner) {
  const right = corner === 1 || corner === 2;
  const bottom = corner === 2 || corner === 3;

  return {
    x: right ? rect.x : rect.x + rect.width,
    y: bottom ? rect.y : rect.y + rect.height,
  };
}

/** Grow `rect` to `width` about its anchor, keeping the aspect ratio. */
function rectFromWidth(rect, corner, width) {
  const anchor = anchorOf(rect, corner);
  const right = corner === 1 || corner === 2;
  const bottom = corner === 2 || corner === 3;
  const height = width * ASPECT;

  return {
    x: right ? anchor.x : anchor.x - width,
    y: bottom ? anchor.y : anchor.y - height,
    width,
    height,
  };
}

/** Screen position of one corner of a rect, same indexing as resizeRect. */
function cornerOf(rect, corner) {
  const right = corner === 1 || corner === 2;
  const bottom = corner === 2 || corner === 3;

  return {
    x: right ? rect.x + rect.width : rect.x,
    y: bottom ? rect.y + rect.height : rect.y,
  };
}

function distance(a, b) {
  return Math.hypot(b.x - a.x, b.y - a.y);
}

/** Longer trips take longer, but not proportionally — Fitts, roughly. */
function reachTime(px) {
  return clamp(0.28 + px / 1500, 0.28, 1.2);
}

/** Dragging is slower than moving an empty hand, and much slower than a flick. */
function dragTime(px) {
  return clamp(0.55 + px / 700, 0.55, 2.4);
}

export function startDragResize({ element }) {
  const pointer = document.createElement('div');
  pointer.id = 'pointer';
  pointer.innerHTML = ARROW;
  document.body.appendChild(pointer);

  let base = frameSize(window.innerWidth, window.innerHeight, DRAG_FRAME_SCALE);
  let rect = {
    x: (window.innerWidth - base.width) / 2,
    y: (window.innerHeight - base.height) / 2,
    width: base.width,
    height: base.height,
  };

  // The pointer starts off screen and slides in for its first grab.
  let at = { x: -40, y: window.innerHeight * 0.75 };

  function margin() {
    return Math.min(window.innerWidth, window.innerHeight) * CORNER_MARGIN;
  }

  /** Widths the frame may take: never wider than the screen allows. */
  function widthLimits() {
    const fit = Math.min(
      window.innerWidth - 2 * margin(),
      (window.innerHeight - 2 * margin()) / ASPECT,
    );
    const min = Math.min(base.width * DRAG_MIN_SCALE, fit);
    return { min, max: clamp(base.width * DRAG_MAX_SCALE, min, fit) };
  }

  function draw() {
    element.style.width = `${rect.width.toFixed(1)}px`;
    element.style.height = `${rect.height.toFixed(1)}px`;
    place(element, rect.x, rect.y);
    place(pointer, at.x, at.y);
  }

  // --- the script ---
  //
  // One cycle is a fixed sequence of beats; each beat gets its endpoints when
  // it starts, so a viewport resize part-way through never strands the pointer
  // at coordinates that no longer exist.

  let steps = [];
  let index = 0;
  let elapsed = 0;

  // Filled in by the press beats and read by the drag beats that follow.
  let held = { x: 0, y: 0 };
  let corner = 0;
  let anchored = rect;

  function travel(to, options) {
    const from = at;
    const target = to();
    return {
      duration: options.time(distance(from, target)),
      update(p) {
        at = pointerAt(from, target, p, options);
        options.moved?.();
      },
    };
  }

  /** Grab: the pointer lands on the frame's title-bar strip. */
  function reachFrame() {
    return travel(
      () => ({
        x: rect.x + between(0.2, 0.8) * rect.width,
        y: rect.y + between(GRAB_BAND[0], GRAB_BAND[1]) * rect.height,
      }),
      // An empty hand shoots past the target and pulls back onto it.
      { time: reachTime, curve: between(-0.07, 0.07), overshoot: 1.6 },
    );
  }

  function reachCorner() {
    corner = Math.floor(Math.random() * CORNERS);
    return travel(() => cornerOf(rect, corner), {
      time: reachTime,
      curve: between(-0.05, 0.05),
      overshoot: 1.4,
    });
  }

  function press() {
    return {
      duration: between(0.1, 0.22),
      begin() {
        pointer.classList.add('down');
        element.classList.add('dragging');
        held = { x: at.x - rect.x, y: at.y - rect.y };
        anchored = { ...rect };
      },
      update() {},
    };
  }

  function release() {
    const from = at;
    return {
      duration: between(PAUSE[0], PAUSE[1]),
      begin() {
        pointer.classList.remove('down');
        element.classList.remove('dragging');
      },
      // A resting hand is never quite still.
      update(p) {
        at = {
          x: from.x + Math.sin(p * 7) * IDLE_DRIFT,
          y: from.y + Math.sin(p * 5 + 1) * IDLE_DRIFT * 0.7,
        };
      },
    };
  }

  /** Drag: the frame hangs off the pointer at the offset it was grabbed by. */
  function dragFrame() {
    const gap = margin();
    return travel(
      () => {
        const limitX = Math.max(gap, window.innerWidth - rect.width - gap);
        const limitY = Math.max(gap, window.innerHeight - rect.height - gap);
        return {
          x: between(gap, limitX) + held.x,
          y: between(gap, limitY) + held.y,
        };
      },
      {
        time: dragTime,
        curve: between(-0.04, 0.04),
        // A loaded hand overshoots less than an empty one.
        overshoot: 0.8,
        moved() {
          rect.x = at.x - held.x;
          rect.y = at.y - held.y;
        },
      },
    );
  }

  /** Resize: the held corner follows the pointer, the opposite one stays put. */
  function dragCorner() {
    const limits = widthLimits();
    const gap = margin();

    return travel(
      () => {
        // How far the corner can travel before it leaves the screen — the same
        // wall a real hand would run the window into. The anchor never moves,
        // so the room is measured from there.
        const anchor = anchorOf(anchored, corner);
        const right = corner === 1 || corner === 2;
        const bottom = corner === 2 || corner === 3;
        const room = Math.min(
          right ? window.innerWidth - gap - anchor.x : anchor.x - gap,
          (bottom ? window.innerHeight - gap - anchor.y : anchor.y - gap) / ASPECT,
        );

        const width = clamp(
          between(limits.min, limits.max),
          limits.min,
          Math.max(limits.min, room),
        );

        return cornerOf(rectFromWidth(anchored, corner, width), corner);
      },
      {
        time: dragTime,
        curve: between(-0.03, 0.03),
        // Least of all when pulling against a corner: the size lands and stays.
        overshoot: 0.5,
        moved() {
          rect = resizeRect(anchored, corner, at, limits);
        },
      },
    );
  }

  // Beats are built lazily: each one reads the frame where the previous one
  // left it.
  const CYCLE = [reachFrame, press, dragFrame, release, reachCorner, press, dragCorner, release];

  function step() {
    const build = CYCLE[index % CYCLE.length];
    index += 1;
    const current = build();
    current.begin?.();
    return current;
  }

  let current = step();

  function measure() {
    base = frameSize(window.innerWidth, window.innerHeight, DRAG_FRAME_SCALE);

    const gap = margin();
    const limits = widthLimits();
    rect.width = clamp(rect.width, limits.min, limits.max);
    rect.height = rect.width * ASPECT;
    rect.x = clamp(rect.x, gap, Math.max(gap, window.innerWidth - rect.width - gap));
    rect.y = clamp(rect.y, gap, Math.max(gap, window.innerHeight - rect.height - gap));

    // Whatever the pointer was aiming at is stale now; start the next beat
    // from where things actually are.
    pointer.classList.remove('down');
    element.classList.remove('dragging');
    elapsed = 0;
    current = step();
  }

  window.addEventListener('resize', measure);

  let last = performance.now();
  let handle = 0;
  let stopped = false;

  function frame(now) {
    if (stopped) return;

    const dt = Math.min((now - last) / 1000, 0.05);
    last = now;
    elapsed += dt;

    const p = current.duration > 0 ? Math.min(elapsed / current.duration, 1) : 1;
    current.update(p);
    draw();

    if (p >= 1) {
      elapsed = 0;
      current = step();
    }

    handle = requestAnimationFrame(frame);
  }

  draw();
  handle = requestAnimationFrame(frame);

  return function stop() {
    stopped = true;
    cancelAnimationFrame(handle);
    window.removeEventListener('resize', measure);
    element.classList.remove('dragging');
    pointer.remove();
    // The next animation sizes the frame itself, but leaving a stretched frame
    // up for even one paint would flash.
    applyFrameSize(element);
  };
}

// --- colour wash --------------------------------------------------------

// The frame parks in the middle and the room changes colour around it: a wash
// creeps round the colour wheel behind the poster, and the frame's glow travels
// with it, so the whole screen shifts as one. It is the quiet one — everything
// else in the registry moves the frame, and after a few of those a still frame
// with a slow colour behind it is a relief rather than a dead spot.

/**
 * The two ends of the wash gradient at `elapsed` seconds, as hues in degrees.
 * One trip round the wheel takes WASH_CYCLE_SECONDS, and the second hue trails
 * the first by WASH_SPREAD so the gradient always has somewhere to go. Pure, so
 * the cycle can be exercised without a browser.
 */
export function washHues(elapsed) {
  const turn = ((elapsed / WASH_CYCLE_SECONDS) * 360) % 360;
  return [turn, (turn + WASH_SPREAD) % 360];
}

/** A wash stop as a colour: saturated and mid-bright, before the blend mode. */
function washStop(hue, alpha = 1) {
  return `hsl(${hue.toFixed(1)} 85% 55% / ${alpha})`;
}

// The frame's glow is the wash colour held back to the same weight the fixed
// pink had, so a hue with more punch to it does not brighten the whole frame.
const GLOW_ALPHA = 0.55;

export function startWash({ element }) {
  const wash = document.createElement('div');
  wash.id = 'wash';
  wash.style.opacity = String(WASH_STRENGTH);
  // Behind the stills and the frame, in front of the poster it tints.
  document.body.appendChild(wash);
  element.classList.add('washing');

  function centre() {
    const size = applyFrameSize(element);
    place(element, (window.innerWidth - size.width) / 2, (window.innerHeight - size.height) / 2);
  }

  centre();
  window.addEventListener('resize', centre);

  const started = performance.now();
  let handle = 0;
  let stopped = false;

  function frame(now) {
    if (stopped) return;

    const [from, to] = washHues((now - started) / 1000);
    wash.style.background = `linear-gradient(135deg, ${washStop(from)}, ${washStop(to)})`;
    // The frame's glow rides the leading hue, so it belongs to the wash instead
    // of sitting on top of it in a fixed pink.
    element.style.setProperty('--wash', washStop(from, GLOW_ALPHA));

    handle = requestAnimationFrame(frame);
  }

  handle = requestAnimationFrame(frame);

  return function stop() {
    stopped = true;
    cancelAnimationFrame(handle);
    window.removeEventListener('resize', centre);
    wash.remove();
    element.classList.remove('washing');
    element.style.removeProperty('--wash');
  };
}

// --- registry -----------------------------------------------------------

// Keyed by the names in config.js, which is what travels over the socket.
export const ANIMATIONS = {
  bounce: startBounce,
  corners: startCorners,
  drag: startDragResize,
  trail: startTrail,
  wash: startWash,
};

const NAMES = Object.keys(ANIMATIONS);

/**
 * Run one animation at a time. In auto mode next() swaps to a freshly picked
 * one on every track change; pinned to a name (from the queue page, for
 * testing) it stays put until the mode changes again.
 */
export function startAnimationCycle(context, mode = AUTO_ANIMATION) {
  const picker = createPicker(NAMES.length);
  let stop = null;
  let current = null;

  function run(name) {
    stop?.();
    current = name;
    stop = ANIMATIONS[name](context);
    return current;
  }

  function next() {
    // Pinned: the track changed, but the operator asked for this one.
    if (mode !== AUTO_ANIMATION) return current;
    return run(NAMES[picker.next()]);
  }

  function setMode(value) {
    mode = ANIMATIONS[value] ? value : AUTO_ANIMATION;
    // Always restarts, so switching back to auto takes effect immediately
    // rather than waiting out the current track.
    return mode === AUTO_ANIMATION ? run(NAMES[picker.next()]) : run(mode);
  }

  setMode(mode);

  return {
    next,
    setMode,
    get mode() {
      return mode;
    },
    get name() {
      return current;
    },
  };
}