Palette resolve

/**
 * palette-resolve
 *
 * A colour field whose origin is the page's own tokens: lightness
 * starts at the ground's and is pushed by a lift, hue is the accent's
 * plus a spread, chroma is a dial, all in OKLCH. One number falls from
 * 1 to 0 over the run and scales lift and chroma together, so the last
 * frame is the ground token to the byte, and then the canvas leaves.
 * The page ends in its own colours with nothing left running.
 *
 * Copy this file. It imports nothing.
 *
 *     import { mount } from './palette-resolve';
 *     mount(document.body);
 *
 * It reads `--ground`, `--accent` and `--muted`, the weakest text
 * colour on the page, and by default caps its lift so that text keeps
 * 4.5 to 1 against the field. It renders at a quarter of the pixels:
 * a field this soft has no detail to lose.
 */

export interface Options {
  /** How long the run takes, in ms. Falls back to --load-duration. */
  duration: number;
  /** Canvas pixels per CSS pixel. A quarter: the field has no detail, and the browser's upscale smooths it. */
  maxPixelRatio: number;
  /** Ceiling on canvas pixels in total. */
  maxPixels: number;
  /** Play only when the reader arrived from outside the site. */
  arrivalOnly: boolean;
  /** How it leaves: 0 drains evenly, 1 a warped front sweeps it off, 2 a band crosses and leaves, 3 the hues close to the accent's and then drain. */
  envelope: number;
  /** Colourfulness, as OKLCH chroma. 0.014 is the most every hue can reach on a near-white ground; above 0.09 reads as a toy. */
  chroma: number;
  /** How far lightness moves from the ground's, in OKLab L. Darker on a light ground, lighter on a dark one. 0 cannot move text contrast. */
  lift: number;
  /** How far the hue strays from the accent's, in turns either way. 0 is one hue; 0.5 is the whole wheel. */
  spread: number;
  /** Size of one blob, in CSS px. Never a share of the element. */
  scale: number;
  /** How far the field is folded over itself. 0 is round blobs; 1.5 is ribbons. */
  warp: number;
  /** Layers of detail, 1 to 3. More is wasted on a field this soft. */
  octaves: number;
  /** Which way the front or band travels, in degrees. 90 is bottom to top. */
  direction: number;
  /** The front's softness, as a share of the element. */
  softness: number;
  /** The band's width, as a share of the element. */
  band: number;
  /** How fast the blobs slide while it plays, in cells a second. 0 holds the shapes still under the moving envelope. */
  drift: number;
  /** One step of noise on the output, against terraces. Always on a dark ground. */
  dither: boolean;
  /** Cap the lift so the weakest text (--muted) keeps 4.5 to 1 against the field. */
  guard: boolean;
  /** Where the canvas sits: 0 behind the content, 1 over it with multiply (light grounds), 2 over it with screen (dark grounds). */
  layer: number;
  /** Which field. The same number is the same shapes. */
  seed: number;
}

export const DEFAULTS: Options = {
  duration: 1100,
  maxPixelRatio: 0.25,
  maxPixels: 2_500_000,
  arrivalOnly: true,
  envelope: 0,
  chroma: 0.06,
  lift: 0.05,
  spread: 0.18,
  scale: 350,
  warp: 0.9,
  octaves: 2,
  direction: 90,
  softness: 0.12,
  band: 0.28,
  drift: 0,
  dither: true,
  guard: true,
  layer: 0,
  seed: 3,
};

export interface Handle {
  update(options: Partial<Options>): void;
  destroy(): void;
}

// ---- the effect -------------------------------------------------------------

// The colour science runs once per frame, not once per pixel: the
// tokens are uniforms, so their conversion is the same answer for every
// pixel, and the full-screen triangle's vertex shader does it three
// times a frame and hands the fragment shader `flat` values. L is the
// sRGB transfer to linear light, B linear to OKLab (the matrices are
// the published ones; a cube root between them), Y relative luminance.
//
// vG is the ground in OKLab. vA holds the accent's hue, the largest
// lift the weakest text allows, and whether the ground is dark. The
// guard: on a light ground the field may not go darker than the
// luminance that keeps 4.5 to 1 against the muted token,
// (Yt + 0.05) * 4.5 - 0.05; on a dark ground not lighter than
// (Yt + 0.05) / 4.5 - 0.05. A grey of OKLab lightness l has luminance
// l cubed, so the cap in L is the cube root of that luminance against
// the ground's L. With the guard off the cap is 1, which is no cap.
const VERTEX = `#version 300 es
precision highp float;
uniform vec3 uGround,uAccent,uMuted;
uniform vec4 uMore;
flat out vec3 vG;flat out vec4 vA;
vec3 L(vec3 c){return mix(c/12.92,pow((c+.055)/1.055,vec3(2.4)),step(.04045,c));}
vec3 B(vec3 c){vec3 l=mat3(.4122215,.2119035,.0883025,.5363325,.6806996,.2817188,.0514460,.1073970,.6299787)*c;
l=sign(l)*pow(abs(l),vec3(1./3.));return mat3(.2104543,1.9779985,.0259040,.7936178,-2.4285922,.7827718,-.0040720,.4505937,-.8086758)*l;}
float Y(vec3 c){return dot(c,vec3(.2126,.7152,.0722));}
void main(){
gl_Position=vec4(vec2(gl_VertexID<<1&2,gl_VertexID&2)*2.-1.,0,1);
vec3 g=L(uGround),a=B(L(uAccent));vG=B(g);
float dk=step(vG.x,.5),yt=Y(L(uMuted));
float lim=dk>.5?pow(max((yt+.05)/4.5-.05,0.),1./3.)-vG.x:vG.x-pow(max((yt+.05)*4.5-.05,0.),1./3.);
vA=vec4(atan(a.z,a.y),uMore.x>.5?max(lim,0.):1.,dk,0.);
}`;

// H is an integer hash, N value noise with the quintic fade (its
// lattice cannot be seen when one cell spans a third of the element),
// F octaves of it normalised. P is OKLab to linear light, E linear to
// sRGB. Q fits a colour to the gamut by sliding it toward the grey of
// its own lightness, in linear light, just far enough that every
// channel fits: a plain clamp would shift the hue.
//
// main: q is the point in the field, drifted and warped. f and s are
// two samples of it, for hue and for lightness. d is the pixel's place
// along the direction of travel. The envelope gives k, 1 to 0, and sp,
// the share of the spread that is still open: drain is the same k
// everywhere; front is a soft step at a bent edge that crosses the
// element with margins wide enough that k is 1 everywhere at the start
// and 0 everywhere at the end; band is a Gaussian on the same edge;
// converge closes the spread first, then drains. The colour is the
// ground's Lab plus the lift and the sweep scaled by k, so at k = 0 it
// is the ground exactly, which survives the round trip to the byte.
// Over the content the output is rebased so its neutral is white
// (multiply) or black (screen). The dither is one step of interleaved
// gradient noise on the encoded value, last, scaled away as k goes to
// zero: a value a hair under a step, pushed down half a step, rounds
// to the step below, and the last frame must be the ground to the byte.
const FRAGMENT = `#version 300 es
precision highp float;precision highp int;
uniform vec2 uResolution;
uniform float uProgress,uTime,uPixelRatio;
uniform vec3 uGround;
uniform vec4 uField,uTone,uEnv,uMore;
uniform int uTerrain;
flat in vec3 vG;flat in vec4 vA;
out vec4 o;
float H(ivec2 c){uint h=uint(c.x)*0x9E3779B1u+uint(c.y)*0x85EBCA6Bu+uint(uTerrain)*0xC2B2AE35u;
h^=h>>15;h*=0x2C1B3C6Du;h^=h>>12;h*=0x297A2D39u;h^=h>>15;return float(h>>8)/16777216.;}
float N(vec2 p){vec2 i=floor(p),f=p-i,u=f*f*f*(f*(f*6.-15.)+10.);ivec2 c=ivec2(i);
return mix(mix(H(c),H(c+ivec2(1,0)),u.x),mix(H(c+ivec2(0,1)),H(c+ivec2(1,1)),u.x),u.y)*2.-1.;}
float F(vec2 p){float a=.5,s=0.,n=0.;for(int i=0;i<3;i++){if(i>=int(uField.y))break;
s+=a*N(p);n+=a;p=mat2(.8,.6,-.6,.8)*p*2.;a*=.5;}return s/n;}
vec3 P(vec3 c){vec3 l=mat3(1.,1.,1.,.3963378,-.1055613,-.0894842,.2158038,-.0638542,-1.2914855)*c;l=l*l*l;
return mat3(4.0767416,-1.2684380,-.0041961,-3.3077115,2.6097574,-.7034186,.2309699,-.3413194,1.7076147)*l;}
vec3 E(vec3 c){c=clamp(c,0.,1.);return mix(c*12.92,1.055*pow(c,vec3(1./2.4))-.055,step(.0031308,c));}
vec3 Q(vec3 l,float L){float g=L*L*L;vec3 d=l-g,k=mix((1.-g)/max(d,vec3(1e-6)),-g/min(d,vec3(-1e-6)),step(d,vec3(0.)));
return g+d*min(1.,min(k.x,min(k.y,k.z)));}
void main(){
vec2 r=uResolution,fc=gl_FragCoord.xy,q=fc/(uPixelRatio*uField.x)+uTime*uField.w;
if(uField.z>0.)q+=uField.z*vec2(F(q+vec2(3.1,7.7)),F(q+vec2(-5.3,1.9)));
float t=uProgress,e=t*t*(3.-2.*t),f=F(q),s=F(q+17.3),k,sp=1.;
float d=dot(fc/r-.5,vec2(cos(uEnv.y),sin(uEnv.y)))+.5;int m=int(uEnv.x+.5);
if(m==0)k=1.-e;
else if(m==1){float w=uEnv.z,g=mix(-3.*w,1.+3.*w,e)+w*f*.8;k=smoothstep(g-w,g+w,d);}
else if(m==2){float w=uEnv.w,g=mix(-3.*w,1.+3.*w,e)+w*f*.8,z=(d-g)/w;k=exp(-4.*z*z);}
else{sp=1.-smoothstep(0.,.5,t);k=1.-smoothstep(.5,1.,t);}
float lf=min(uTone.y,vA.y)*(vA.z>.5?1.:-1.)*k*(.6+.4*s),C=uTone.x*k*(.7+.3*f),h=vA.x+uTone.z*sp*f*6.2831853;
vec3 lab=vG+vec3(lf,C*cos(h),C*sin(h)),c=E(Q(P(lab),lab.x));
if(uMore.y>1.5)c=clamp((c-uGround)/max(1.-uGround,vec3(1e-4)),0.,1.);
else if(uMore.y>.5)c=clamp(c/max(uGround,vec3(1e-4)),0.,1.);
if(uTone.w>.5)c+=(fract(52.9829189*fract(dot(fc,vec2(.06711056,.00583715))))-.5)/255.*min(1.,k*4.);
o=vec4(c,1.);
}`;

/**
 * CSS tokens the shader gets as vec3 uniforms, with what stands in for
 * a missing one. The muted fallback is the grey that just passes 4.5
 * to 1 on white, so a page that has not named its weakest text gets
 * almost no lift rather than an unreadable one.
 */
export const TOKENS: readonly (readonly [uniform: string, property: string, fallback: string])[] = [
  ['uGround', '--ground', '#fff'],
  ['uAccent', '--accent', '#00f'],
  ['uMuted', '--muted', '#767676'],
];

type Quad = [number, number, number, number];
const clamp = (value: number, low: number, high: number): number => Math.min(high, Math.max(low, value));

/** The options as the shader reads them, made safe. The panel can send any number. */
export function dials(o: Options): { field: Quad; tone: Quad; env: Quad; more: Quad; terrain: number } {
  return {
    field: [clamp(o.scale, 60, 2000), clamp(Math.round(o.octaves), 1, 3), clamp(o.warp, 0, 2), clamp(o.drift, 0, 0.5)],
    tone: [clamp(o.chroma, 0, 0.2), clamp(o.lift, 0, 0.2), clamp(o.spread, 0, 0.5), o.dither ? 1 : 0],
    env: [clamp(Math.round(o.envelope), 0, 3), ((Number.isFinite(o.direction) ? o.direction : 90) * Math.PI) / 180, clamp(o.softness, 0.01, 0.5), clamp(o.band, 0.05, 0.8)],
    more: [o.guard ? 1 : 0, clamp(Math.round(o.layer), 0, 2), 0, 0],
    terrain: Math.abs(Math.round(o.seed)) % 65536,
  };
}

const uploadDials = (gl: WebGL2RenderingContext, at: (name: string) => WebGLUniformLocation | null, o: Options): void => {
  const d = dials(o);
  gl.uniform4f(at('uField'), ...d.field);
  gl.uniform4f(at('uTone'), ...d.tone);
  gl.uniform4f(at('uEnv'), ...d.env);
  gl.uniform4f(at('uMore'), ...d.more);
  gl.uniform1i(at('uTerrain'), d.terrain);
};

/** Its end is the page in its own tokens; a frozen tint would be a background with a contrast obligation that never ends. */
export const keepsStill = (_options: Options): boolean => false;
/** Drain, front and converge start from a full tint, which must not appear in a single frame. A band starts on plain ground. */
export const fadeInMs = (options: Options): number => (Math.round(options.envelope) === 2 ? 0 : 100);
/** Behind the content unless laid over it with a blend mode. */
export const layerUnder = (options: Options): boolean => Math.round(options.layer) === 0;
/** Over the content it multiplies on a light ground or screens on a dark one; the shader rebases its neutral to match. */
export const canvasStyle = (options: Options): string => {
  const layer = Math.round(options.layer);
  return layer === 1 ? 'mix-blend-mode:multiply' : layer === 2 ? 'mix-blend-mode:screen' : '';
};
const geometry = (gl: WebGL2RenderingContext, _options: Options): [mode: number, count: number] => [gl.TRIANGLES, 3];
/** No depth buffer: one pass, nothing in front of anything. */
const DEPTH = false;
/** Nothing to set up before the first frame. */
const prepare: Prepare = () => {};
/** Starts after the first paint, wherever the element is. */
const startsOnView = (_options: Options): boolean => false;

/**
 * Named versions of the effect: sets of options that belong together.
 * The first is the defaults. `mount(element, PRESETS.Front)`.
 */
export const PRESETS: Record<string, Partial<Options>> = {
  Resolve: {},
  Front: { envelope: 1 },
  Ribbon: { envelope: 2, lift: 0.03, band: 0.22 },
  Converge: { envelope: 3, spread: 0.4 },
  Unguarded: { guard: false, lift: 0.08, chroma: 0.08 },
};
// ---- HARNESS one-shot: begin ------------------------------------------------
//
// Identical in every one-shot shader recipe. `npm run check` fails if a
// copy differs, so change it in one recipe and copy it to the others.
//
// Above this line a recipe declares, by these names: Options (with
// duration, maxPixelRatio, maxPixels and arrivalOnly), DEFAULTS, Handle,
// VERTEX, FRAGMENT, TOKENS, uploadDials, keepsStill, fadeInMs,
// layerUnder, canvasStyle, geometry (the draw mode and vertex count),
// DEPTH (whether the context has a depth buffer and tests with it),
// prepare (the recipe's own setup after the program is in use, which
// may return a promise the start waits on) and startsOnView (whether
// nothing is made until the element enters the viewport).
//
// What it promises: it never throws, never hides or moves content, and
// changes nothing on the page until its first frame is ready. It never
// asks the GPU a blocking question where a non-blocking one exists.
// When the effect ends, or on destroy, nothing of it remains.

/** The recipe's setup, given the context, the uniform locations, the host and the options; a promise delays the start. */
type Prepare = (gl: WebGL2RenderingContext, at: (name: string) => WebGLUniformLocation | null, element: HTMLElement, options: Options) => void | Promise<void>;

const NOOP: Handle = { update() {}, destroy() {} };

/** One mount to an element. A second replaces the first. */
const mounted = new WeakMap<HTMLElement, Handle>();

/** The buffer size for a box measured in device pixels, under both ceilings. */
export function bufferSize(
  deviceWidth: number,
  deviceHeight: number,
  pixelRatio: number,
  maxPixelRatio: number,
  maxPixels: number,
): [width: number, height: number] {
  const area = deviceWidth * deviceHeight;
  if (!(area > 0)) return [1, 1];
  const scale = Math.min(1, maxPixelRatio / Math.max(pixelRatio, 0.01), Math.sqrt(maxPixels / area));
  return [Math.max(1, Math.round(deviceWidth * scale)), Math.max(1, Math.round(deviceHeight * scale))];
}

/** True when this page load came from outside the site, and is not a reload or a history move. */
export function isArrival(type: string | undefined, referrer: string, origin: string): boolean {
  if (type && type !== 'navigate') return false;
  if (!referrer) return true;
  try {
    return new URL(referrer).origin !== origin;
  } catch {
    return true;
  }
}

/** `color(srgb r g b)` as the browser serialises it, to three numbers in 0..1. */
export function parseSrgb(value: string): [number, number, number] | null {
  const match = /color\(srgb\s+([-\d.e]+)\s+([-\d.e]+)\s+([-\d.e]+)/.exec(value);
  if (!match) return null;
  const channel = (text: string | undefined): number => Math.min(1, Math.max(0, Number(text) || 0));
  return [channel(match[1]), channel(match[2]), channel(match[3])];
}

/** A CSS time, in ms. `null` for anything that is not one. */
export function parseTime(value: string): number | null {
  const match = /^\s*(\d*\.?\d+)(ms|s)\s*$/.exec(value);
  if (!match) return null;
  return match[2] === 's' ? Number(match[1]) * 1000 : Number(match[1]);
}

/**
 * Options over a base, keeping only what is the right type. A control
 * panel, a message from another frame and a careless caller can all
 * send anything, and none of them may reach the GPU unchecked.
 */
export function mergeOptions(base: Options, next: Partial<Options>): Options {
  const merged: Record<string, unknown> = { ...base };
  for (const [key, standard] of Object.entries(DEFAULTS)) {
    const value: unknown = (next as Record<string, unknown>)[key];
    if (typeof value === typeof standard && (typeof value !== 'number' || Number.isFinite(value))) merged[key] = value;
  }
  return merged as unknown as Options;
}

export function mount(element: HTMLElement | null, options: Partial<Options> = {}): Handle {
  try {
    return start(element, options);
  } catch {
    return NOOP;
  }
}

function start(element: HTMLElement | null, explicit: Partial<Options>): Handle {
  if (!element || typeof WebGL2RenderingContext === 'undefined' || typeof ResizeObserver === 'undefined') return NOOP;
  // The one place motion is decided, so a host can override matchMedia.
  const asks = (query: string): boolean => typeof matchMedia === 'function' && matchMedia(query).matches;
  if (asks('(prefers-reduced-motion: reduce)') || asks('(forced-colors: active)')) return NOOP;

  mounted.get(element)?.destroy();

  const fromToken = parseTime(getComputedStyle(element).getPropertyValue('--load-duration'));
  let options = mergeOptions(fromToken === null ? DEFAULTS : { ...DEFAULTS, duration: fromToken }, explicit);
  if (options.arrivalOnly) {
    const entry = performance.getEntriesByType?.('navigation')[0] as PerformanceNavigationTiming | undefined;
    if (!isArrival(entry?.type, document.referrer, location.origin)) return NOOP;
  }

  const page = element === document.body || element === document.documentElement;
  const under = layerUnder(options);
  const canvas = document.createElement('canvas');
  canvas.setAttribute('aria-hidden', 'true');
  canvas.style.cssText = `position:${page ? 'fixed' : 'absolute'};inset:0;width:100%;height:100%;pointer-events:none;z-index:${under ? -1 : 2147483647};${canvasStyle(options)}`;

  let gl: WebGL2RenderingContext | null = null;
  let program: WebGLProgram | null = null;
  let observer: ResizeObserver | null = null;
  let clock: Animation | null = null;
  let still: HTMLCanvasElement | null = null;
  let seen: IntersectionObserver | null = null;
  let hostStyle: string | null | undefined;
  let frame = 0;
  let timer = 0;
  let slow = 0;
  let last = 0;
  let seconds = 0;
  let done = false;
  const locations = new Map<string, WebGLUniformLocation | null>();
  const at = (name: string): WebGLUniformLocation | null => {
    if (!locations.has(name)) locations.set(name, gl && program ? gl.getUniformLocation(program, name) : null);
    return locations.get(name) ?? null;
  };

  /** Ends the effect. With `keep`, the last frame stays as a plain 2D canvas. */
  const finish = (keep: boolean): void => {
    if (done) return;
    done = true;
    cancelAnimationFrame(frame);
    clearTimeout(timer);
    observer?.disconnect();
    seen?.disconnect();
    clock?.cancel();
    document.removeEventListener('visibilitychange', wake);
    removeEventListener('pagehide', stop);
    removeEventListener('pageswap', stop);
    canvas.removeEventListener('webglcontextlost', stop);
    if (keep && gl && canvas.isConnected) {
      // Copied in the same task as the last draw: after the next
      // composite the drawing buffer is gone.
      still = document.createElement('canvas');
      still.width = canvas.width;
      still.height = canvas.height;
      still.setAttribute('aria-hidden', 'true');
      still.style.cssText = canvas.style.cssText;
      still.dataset.state = 'rest';
      still.getContext('2d')?.drawImage(canvas, 0, 0);
      canvas.replaceWith(still);
    } else {
      // Removed first, then released: a lost canvas left in the page
      // shows a broken-image mark.
      canvas.remove();
    }
    if (gl) {
      gl.deleteProgram(program);
      gl.getExtension('WEBGL_lose_context')?.loseContext();
    }
    canvas.width = canvas.height = 0;
    gl = program = null;
    if (!still) restoreHost();
  };
  const stop = (): void => finish(false);

  const restoreHost = (): void => {
    if (hostStyle === undefined) return;
    // Read it before removing it. In Chrome, a style set through
    // `.style` and removed before anyone has read the attribute leaves
    // `style=""` behind, and "as it was" would be nearly true.
    element.getAttribute('style');
    if (hostStyle === null) element.removeAttribute('style');
    else element.setAttribute('style', hostStyle);
    hostStyle = undefined;
  };

  const tokens = (): void => {
    if (!gl) return;
    const probe = canvas.style;
    for (const [uniform, property, fallback] of TOKENS) {
      // Any colour syntax in, one serialisation out: mixing a colour
      // with itself in sRGB makes the browser do the conversion.
      probe.color = `color-mix(in srgb, var(${property}, ${fallback}) 100%, #000 0%)`;
      const rgb = parseSrgb(getComputedStyle(canvas).color) ?? [0, 0, 0];
      gl.uniform3f(at(uniform), rgb[0], rgb[1], rgb[2]);
    }
    probe.color = '';
  };

  const draw = (now: number): void => {
    frame = 0;
    if (done || !gl || !clock) return;
    const progress = clock.effect?.getComputedTiming().progress ?? 1;
    const gap = last ? now - last : 0;
    last = now;
    seconds += Math.min(gap, 100) / 1000;
    // Four frames in a row slower than 15 a second: this is not a GPU worth keeping.
    slow = gap > 66 ? slow + 1 : 0;
    gl.uniform1f(at('uProgress'), progress);
    gl.uniform1f(at('uTime'), seconds);
    uploadDials(gl, at, options);
    // Cleared first: a fragment that discards, above a horizon, must not
    // show the last frame through the hole.
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
    const [mode, count] = geometry(gl, options);
    gl.drawArrays(mode, 0, count);
    if (progress >= 1) finish(keepsStill(options));
    else if (slow > 3) finish(false);
    else frame = requestAnimationFrame(draw);
  };

  const resize = (entries: ResizeObserverEntry[]): void => {
    const entry = entries[0];
    if (done || !gl || !entry) return;
    const ratio = devicePixelRatio || 1;
    const exact = entry.devicePixelContentBoxSize?.[0];
    const box = entry.contentBoxSize?.[0];
    const cssWidth = box ? box.inlineSize : entry.contentRect.width;
    const cssHeight = box ? box.blockSize : entry.contentRect.height;
    // The device-pixel box is for snapping, not for scale: it is trusted
    // only where it agrees with devicePixelRatio to within a pixel.
    // Under DevTools device emulation it reports CSS pixels.
    const snapped = exact && Math.abs(exact.inlineSize - cssWidth * ratio) < 2 && Math.abs(exact.blockSize - cssHeight * ratio) < 2;
    const [width, height] = bufferSize(
      snapped ? exact.inlineSize : cssWidth * ratio,
      snapped ? exact.blockSize : cssHeight * ratio,
      ratio,
      options.maxPixelRatio,
      options.maxPixels,
    );
    if (canvas.width !== width || canvas.height !== height) {
      canvas.width = width;
      canvas.height = height;
      gl.viewport(0, 0, width, height);
    }
    gl.uniform2f(at('uResolution'), width, height);
    gl.uniform1f(at('uPixelRatio'), width / Math.max(1, cssWidth));
    if (!clock) {
      // The clock is a Web Animation with nothing in it: the browser
      // keeps the time and applies the easing token.
      const easing = getComputedStyle(element).getPropertyValue('--load-ease').trim() || 'linear';
      const timing = { duration: Math.max(options.duration, 1), fill: 'forwards' as const };
      try {
        clock = canvas.animate(null, { ...timing, easing });
      } catch {
        clock = canvas.animate(null, timing);
      }
      const fade = fadeInMs(options);
      if (fade > 0) canvas.animate({ opacity: [0, 1] }, { duration: fade });
    }
    // Sizing clears the buffer, so draw before this frame is shown.
    cancelAnimationFrame(frame);
    draw(performance.now());
  };

  const attach = (): void => {
    if (done || !gl) return;
    if (under || !page) {
      // The host becomes the canvas's box, and behind the content a
      // stacking context too, so the canvas sits above its background
      // and below everything in it.
      hostStyle = element.getAttribute('style');
      if (getComputedStyle(element).position === 'static') element.style.position = 'relative';
    }
    if (under) {
      element.style.isolation = 'isolate';
      element.prepend(canvas);
    } else {
      element.append(canvas);
    }
    canvas.dataset.state = 'running';
    // In the document first: an element outside it has no computed style.
    tokens();
    observer = new ResizeObserver(resize);
    try {
      observer.observe(canvas, { box: 'device-pixel-content-box' });
    } catch {
      observer.observe(canvas);
    }
  };

  const ready = (): void => {
    if (done || !gl || !program) return;
    if (!gl.getProgramParameter(program, gl.LINK_STATUS)) return stop();
    gl.useProgram(program);
    if (DEPTH) gl.enable(gl.DEPTH_TEST);
    gl.uniform1f(at('uSeed'), Math.random());
    // The recipe's own setup, which may wait: for a font, for a picture.
    // A throw, sync or async, ends the effect with nothing shown.
    const context = gl;
    Promise.resolve()
      .then(() => prepare(context, at, element, options))
      .then(attach, stop);
  };

  const build = (): void => {
    if (done || gl) return;
    if (document.visibilityState !== 'visible') return;
    document.removeEventListener('visibilitychange', wake);
    gl = canvas.getContext('webgl2', {
      alpha: true,
      antialias: false,
      depth: DEPTH,
      stencil: false,
      powerPreference: 'low-power',
      // A software renderer would burn a weak machine's cores for decoration.
      failIfMajorPerformanceCaveat: true,
    });
    if (!gl) return stop();
    canvas.addEventListener('webglcontextlost', stop);
    program = gl.createProgram();
    for (const [type, source] of [[gl.VERTEX_SHADER, VERTEX], [gl.FRAGMENT_SHADER, FRAGMENT]] as const) {
      const shader = gl.createShader(type);
      if (!shader) return stop();
      gl.shaderSource(shader, source);
      gl.compileShader(shader);
      gl.attachShader(program, shader);
      gl.deleteShader(shader);
    }
    gl.linkProgram(program);
    const parallel = gl.getExtension('KHR_parallel_shader_compile');
    let waited = 0;
    const poll = (): void => {
      frame = 0;
      if (done || !gl || !program) return;
      waited += 1;
      // With the extension, ask without blocking. Without it, give the
      // driver two frames, then ask and take the wait. Asking early
      // moved first paint by more than a second in testing.
      if (parallel ? gl.getProgramParameter(program, parallel.COMPLETION_STATUS_KHR) : waited > 2) ready();
      else frame = requestAnimationFrame(poll);
    };
    frame = requestAnimationFrame(poll);
  };
  function wake(): void {
    build();
  }

  const handle: Handle = {
    update(next) {
      options = mergeOptions(options, next);
    },
    destroy() {
      stop();
      still?.remove();
      still = null;
      restoreHost();
      if (mounted.get(element) === handle) mounted.delete(element);
    },
  };
  mounted.set(element, handle);

  addEventListener('pagehide', stop);
  addEventListener('pageswap', stop);
  document.addEventListener('visibilitychange', wake);
  // After a paint, never before one: the page is on screen first, and
  // creating a context is a blocking call of tens of milliseconds.
  const begin = (): void => {
    frame = requestAnimationFrame(() => {
      frame = 0;
      timer = window.setTimeout(build, 0);
    });
  };
  if (startsOnView(options) && typeof IntersectionObserver === 'function') {
    // Nothing is made until the element is in view: no context held
    // below the fold, and the recipe prepares against the layout and
    // fonts of the moment it starts.
    seen = new IntersectionObserver((entries) => {
      if (done || !entries.some((entry) => entry.isIntersecting)) return;
      seen?.disconnect();
      seen = null;
      begin();
    });
    seen.observe(element);
  } else {
    begin();
  }
  return handle;
}

// ---- HARNESS one-shot: end --------------------------------------------------