Feedback trails

/**
 * feedback-trails
 *
 * The pointer leaves a trail that streams, spirals or drifts and fades
 * to nothing. Each frame is last frame read through a small transform,
 * faded, and the pointer's dot laid over it: the Feedback TOP, in two
 * textures and a swap.
 *
 * Copy this file. It imports nothing.
 *
 *     import { mount } from './feedback-trails';
 *     mount(document.querySelector('.hero'));
 *
 * It runs only while driven. With nothing happening it is at rest: no
 * frames, nothing drawn, and the pointer wakes it. A budget is a second
 * way to rest; only destroy releases it.
 */

export interface Options {
  /** Ceiling on canvas pixels per CSS pixel. */
  maxPixelRatio: number;
  /** Ceiling on canvas pixels in total. */
  maxPixels: number;
  /** Running time in ms since the last touch before it rests. 0 rests only when the field is empty. */
  budget: number;
  /** Stops the frames. Nothing is lost; unpausing carries on. */
  paused: boolean;
  /** When the budget ends it, leave the last frame as a plain picture. */
  keepStill: boolean;
  /** Long side of the simulation in texels. The cost dial: a phone runs the same size as a laptop. */
  simSize: number;
  /** Time in seconds for a trail to fade by half. */
  halfLife: number;
  /** Scale of the image per second. Under 1 it streams outward, over 1 it falls into a tunnel, 1 is a hard trail that only fades. */
  zoom: number;
  /** Turn of the image, in radians a second. Spirals. */
  spin: number;
  /** Drift of the image, as a share of the width a second. Wind. */
  drift: number;
  /** Radius of the dot the pointer lays down, as a share of the height. */
  radius: number;
  /** How the dot joins the trail: 0 maximum, 1 over, 2 add. Add glows and clips. */
  composite: number;
  /** Which token colours the trail: 0 accent, 1 ink. */
  tint: number;
  /** Seconds after the last input before it goes to rest. */
  tail: number;
  /** With no pointer yet, draws a short path by itself for four seconds after mounting. */
  idlePath: boolean;
}

export const DEFAULTS: Options = {
  maxPixelRatio: 1.5,
  maxPixels: 2_500_000,
  budget: 0,
  paused: false,
  keepStill: false,
  simSize: 512,
  halfLife: 0.4,
  zoom: 0.7,
  spin: 0.35,
  drift: 0,
  radius: 0.035,
  composite: 0,
  tint: 0,
  tail: 3,
  idlePath: false,
};

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

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

// STEP reads the previous frame at a transformed coordinate, fades it,
// subtracts a floor so that 8-bit values reach exactly zero, and lays the
// dot over it. SHOW colours the field in a token, premultiplied over
// transparent. Both bodies get the harness's head: version, precision,
// `v` in 0..1 and `o` out.
const PROGRAMS: Record<string, string> = {
  step: `uniform sampler2D s;uniform vec4 m;uniform vec3 inj;uniform vec2 shift,asp;uniform float decay,rad,mode;
void main(){vec2 q=mat2(m.xy,m.zw)*(v-.5)+.5+shift;
vec4 p=max(texture(s,q)*decay-1./255.,0.);
float e=inj.z*smoothstep(rad,rad*.5,length((v-inj.xy)*asp));
o=mode<.5?max(p,vec4(e)):mode<1.5?mix(p,vec4(1),e):min(p+e,1.);}`,
  show: `uniform sampler2D s;uniform vec3 col;void main(){float x=texture(s,v).x;o=vec4(col*x,x);}`,
};

/** CSS tokens read into `kit.tokens` by these uniform names, with what stands in for a missing one. */
const TOKENS: readonly (readonly [uniform: string, property: string, fallback: string])[] = [
  ['uInk', '--ink', '#000'],
  ['uAccent', '--accent', '#00f'],
];

/** Trails live in 8 bits. */
const NEEDS_FLOAT = false;

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

/** The fade per frame for a half-life in seconds and a frame of `dt` seconds. */
export function decay(halfLife: number, dt: number): number {
  if (!(halfLife > 0)) return 0;
  return halfLife === Infinity ? 1 : Math.pow(0.5, dt / halfLife);
}

/** The 2 by 2 lookup transform for one frame: a zoom per second and a spin in radians a second. */
export function matrix(zoom: number, spin: number, dt: number): [number, number, number, number] {
  const z = Math.pow(clamp(zoom, 0.05, 20), dt);
  const c = Math.cos(spin * dt) * z;
  const s = Math.sin(spin * dt) * z;
  return [c, s, -s, c];
}

/** Where the idle path is at `t` seconds: a Lissajous figure inside the box. */
const path = (t: number): [number, number] => [0.5 + 0.3 * Math.cos(t * 1.7), 0.5 + 0.3 * Math.sin(t * 2.3)];

const makeSim = (kit: Kit, first: Options): Sim => {
  let options = first;
  const size = (): [number, number] => {
    const long = clamp(Math.round(options.simSize), 64, 2048);
    return kit.aspect >= 1 ? [long, Math.max(2, Math.round(long / kit.aspect))] : [Math.max(2, Math.round(long * kit.aspect)), long];
  };
  let [w, h] = size();
  let field = kit.pair(w, h, 0, true, false);
  let quiet = 0;
  let sinceInput = 0;
  return {
    step(dt, o, input) {
      options = o;
      const self = o.idlePath && !input.seen && input.time < 4;
      const at = self ? path(input.time) : [input.x, input.y];
      const on = self || input.active ? 1 : 0;
      if (on) {
        quiet = 0;
        sinceInput = 0;
      } else {
        quiet += dt;
        sinceInput += 1;
      }
      // Nothing coming in, and the floor has had time to take every texel to zero: rest.
      if (quiet > clamp(o.tail, 0, 60) && sinceInput > 255) return false;
      kit.pass(
        kit.program('step'),
        {
          s: { tex: field.r.tex },
          m: matrix(o.zoom, o.spin, dt),
          shift: [-clamp(o.drift, -1, 1) * dt, 0],
          asp: [kit.aspect, 1],
          inj: [at[0] ?? 0.5, at[1] ?? 0.5, on],
          decay: decay(clamp(o.halfLife, 0.02, 30), dt),
          rad: clamp(o.radius, 0.004, 0.5),
          mode: clamp(Math.round(o.composite), 0, 2),
        },
        field.w,
      );
      field.swap();
      return true;
    },
    show(o, tokens) {
      kit.pass(kit.program('show'), { s: { tex: field.r.tex }, col: tokens[Math.round(o.tint) === 1 ? 'uInk' : 'uAccent'] ?? [0, 0, 1] }, null);
    },
    listens: () => true,
    update(previous, next) {
      options = next;
      if (previous.simSize === next.simSize) return;
      [w, h] = size();
      kit.drop(field);
      field = kit.pair(w, h, 0, true, false);
    },
  };
};

/**
 * Named versions of the effect: sets of options that belong together.
 * The first is the defaults. `mount(element, PRESETS.Wind)`.
 */
export const PRESETS: Record<string, Partial<Options>> = {
  Spiral: {},
  Comet: { zoom: 1, spin: 0, halfLife: 0.25 },
  Wind: { zoom: 0.92, spin: 0, drift: 0.25, halfLife: 0.8 },
  Tunnel: { zoom: 1.6, spin: 0.1, halfLife: 0.5 },
};

// ---- HARNESS stateful: begin ------------------------------------------------
//
// Identical in every stateful 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
// maxPixelRatio, maxPixels, budget, paused, keepStill and simSize),
// DEFAULTS, Handle, PROGRAMS (fragment bodies, or { vertex, fragment }
// for a program with its own vertex shader), TOKENS, NEEDS_FLOAT and
// makeSim. The kit draws passes and points, and makes RGBA8, RGBA16F
// and RGBA32F targets (kinds 0, 1 and 2; the last is never filtered).
//
// What it promises: it never throws, never hides or moves content, and
// changes nothing on the page until its first frame is ready. It runs no
// frame while off screen, hidden, paused or at rest. When the budget ends
// it, or on destroy, nothing of it remains but a kept still.

const VERTEX = `#version 300 es
out vec2 v;void main(){vec2 p=vec2(gl_VertexID<<1&2,gl_VertexID&2);v=p;gl_Position=vec4(p*2.-1.,0,1);}`;
const HEAD = `#version 300 es
precision highp float;precision highp sampler2D;in vec2 v;out vec4 o;`;

/** A program is a fragment body under the shared head, or a pair with its own vertex shader. */
type Programs = Record<string, string | { vertex: string; fragment: string }>;

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

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

export interface Target {
  tex: WebGLTexture;
  fbo: WebGLFramebuffer;
  w: number;
  h: number;
  /** Whether the framebuffer is complete: the real feature test for the format. */
  ok: boolean;
}
export interface Pair {
  /** Read this one. */
  r: Target;
  /** Write that one, then swap. */
  w: Target;
  swap(): void;
}
export type Uniform = number | readonly number[] | { tex: WebGLTexture };
export interface Program {
  p: WebGLProgram;
  u: Map<string, WebGLUniformLocation | null>;
}
export type Rgb = [number, number, number];
/** What the harness hands a recipe to build and run its simulation with. */
export interface Kit {
  gl: WebGL2RenderingContext;
  /** Whether an RGBA16F target is complete here. */
  float: boolean;
  /** Width over height of the host when the simulation is built. */
  aspect: number;
  /** The element the recipe is mounted on, for what it may read from the page. */
  host: HTMLElement | null;
  /** The token colours, by uniform name, read when the canvas is attached. */
  tokens: Record<string, Rgb>;
  program(name: string): Program;
  /** kind 0 is RGBA8, 1 is RGBA16F, 2 is RGBA32F (never filtered). */
  target(w: number, h: number, kind: 0 | 1 | 2, linear: boolean, repeat: boolean): Target;
  pair(w: number, h: number, kind: 0 | 1 | 2, linear: boolean, repeat: boolean): Pair;
  /** An RGBA8 texture from a canvas or an image, flipped so y is up. */
  image(source: TexImageSource, linear: boolean): WebGLTexture;
  /** One full-target pass. `dst` null draws to the canvas. Textures are bound in the order given. */
  pass(program: Program, uniforms: Record<string, Uniform>, dst: Target | null): void;
  /** A POINTS draw of `count` vertices, added into `dst` or the canvas, blending on for this draw only. */
  points(program: Program, uniforms: Record<string, Uniform>, count: number, dst: Target | null): void;
  /** Deletes a target, a pair or a texture before its time. */
  drop(made: Target | Pair | WebGLTexture): void;
}
export interface Input {
  /** Pointer within the host, 0 to 1, y up. */
  x: number;
  y: number;
  /** True while the pointer moved over the host within the last 100 ms. */
  active: boolean;
  /** Whether a pointer has ever been over the host. */
  seen: boolean;
  /** Seconds since the last pointer event. Infinity if none, or it left. */
  idle: number;
  /** 1 while running, falling to 0 over the last half second of the budget. Scale the work a frame by it. */
  ease: number;
  /** Seconds of running time. */
  time: number;
  /** True once frames have come late: do about half the work a frame. */
  late: boolean;
}
export interface Sim {
  /** Runs the simulation forward by `dt` seconds. False when there is nothing left to draw: the harness then rests until input or an update. */
  step(dt: number, options: Options, input: Input): boolean;
  /** Draws the state to the canvas. */
  show(options: Options, tokens: Record<string, Rgb>): void;
  /** The host's size changed, in CSS pixels. */
  resize?(width: number, height: number): void;
  /** Options changed while mounted. */
  update?(previous: Options, next: Options): void;
  /** True while the pointer can bring it back: the budget then ends in rest, not release, and a touch restarts the clock. */
  listens?(options: Options): boolean;
}

/** 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))];
}

/** `color(srgb r g b)` as the browser serialises it, to three numbers in 0..1. */
export function parseSrgb(value: string): Rgb | 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' ||
    typeof IntersectionObserver === '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();
  let options = mergeOptions(DEFAULTS, explicit);

  const page = element === document.body || element === document.documentElement;
  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:2147483647`;

  let gl: WebGL2RenderingContext | null = null;
  let kit: Kit | null = null;
  let sim: Sim | null = null;
  let observer: ResizeObserver | null = null;
  let seer: IntersectionObserver | null = null;
  let still: HTMLCanvasElement | null = null;
  let hostStyle: string | null | undefined;
  const programs = new Map<string, Program>();
  const made: (() => void)[] = [];
  let frame = 0;
  let timer = 0;
  let slow = 0;
  let last = 0;
  let elapsed = 0;
  let built = false;
  let done = false;
  let visible = document.visibilityState === 'visible';
  let onScreen = true;
  let pointerAt = -Infinity;
  const input: Input = { x: 0.5, y: 0.5, active: false, seen: false, idle: Infinity, ease: 1, time: 0, late: false };

  /** Ends the effect for good. 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();
    seer?.disconnect();
    document.removeEventListener('visibilitychange', wake);
    removeEventListener('pagehide', stop);
    removeEventListener('pageswap', stop);
    canvas.removeEventListener('webglcontextlost', stop);
    for (const type of ['pointermove', 'pointerdown', 'pointerleave'] as const) element.removeEventListener(type, pointer);
    if (keep && gl && built && 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) {
      for (const free of made) free();
      for (const program of programs.values()) gl.deleteProgram(program.p);
      gl.getExtension('WEBGL_lose_context')?.loseContext();
    }
    canvas.width = canvas.height = 0;
    gl = kit = sim = 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 = (): Record<string, Rgb> => {
    const out: Record<string, Rgb> = {};
    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%)`;
      out[uniform] = parseSrgb(getComputedStyle(canvas).color) ?? [0, 0, 0];
    }
    probe.color = '';
    return out;
  };

  const running = (): boolean => !done && built && visible && onScreen && !options.paused;
  const schedule = (): void => {
    if (frame === 0 && running()) frame = requestAnimationFrame(draw);
  };
  function draw(now: number): void {
    frame = 0;
    if (!running() || !gl || !kit || !sim) return;
    const gap = last ? Math.min(now - last, 100) : 0;
    last = now;
    const dt = gap / 1000;
    elapsed += gap;
    input.time += dt;
    // Four frames in a row slower than 15 a second: do less from now on.
    slow = gap > 66 ? slow + 1 : 0;
    if (slow > 3) input.late = true;
    const left = options.budget > 0 ? options.budget - elapsed : Infinity;
    input.ease = Math.max(0, Math.min(1, left / 500));
    input.idle = (now - pointerAt) / 1000;
    input.active = input.idle < 0.1;
    const more = sim.step(dt, options, input);
    sim.show(options, kit.tokens);
    const listens = sim.listens?.(options) ?? false;
    if (left <= 0 && !listens) return finish(options.keepStill);
    if (more && left > 0) schedule();
    else {
      // At rest: nothing to draw until something comes in. A recipe that
      // listens for the pointer rests at its budget too, so the pointer
      // can bring it back.
      canvas.dataset.state = 'rest';
      last = 0;
    }
  }
  /**
   * Something came in: a pointer, an update, the page back on screen.
   * The frame clock starts afresh only from rest: reset on every event,
   * a pointer that never stops (a mouse at 125 Hz or more) gave every
   * frame a step of nothing, and the field froze under it.
   */
  const wakeLoop = (): void => {
    if (!built || done) return;
    canvas.dataset.state = 'running';
    if (frame === 0) last = 0;
    schedule();
  };

  const pointer = (event: PointerEvent): void => {
    if (event.type === 'pointerleave') {
      pointerAt = -Infinity;
      return;
    }
    const box = element.getBoundingClientRect();
    input.x = (event.clientX - box.left) / Math.max(1, box.width);
    input.y = 1 - (event.clientY - box.top) / Math.max(1, box.height);
    input.seen = true;
    pointerAt = performance.now();
    // A touch is a fresh start for the clock of a recipe that answers it.
    if (sim?.listens?.(options)) elapsed = 0;
    wakeLoop();
  };

  const resize = (entries: ResizeObserverEntry[]): void => {
    const entry = entries[0];
    if (done || !gl || !sim || !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;
    }
    sim.resize?.(cssWidth, cssHeight);
    // Sizing clears the buffer, so draw before this frame is shown.
    if (kit) sim.show(options, kit.tokens);
    schedule();
  };

  const makeKit = (g: WebGL2RenderingContext): Kit => {
    const filter = (tex: WebGLTexture, linear: boolean, repeat: boolean): void => {
      g.bindTexture(g.TEXTURE_2D, tex);
      const f = linear ? g.LINEAR : g.NEAREST;
      const r = repeat ? g.REPEAT : g.CLAMP_TO_EDGE;
      g.texParameteri(g.TEXTURE_2D, g.TEXTURE_MIN_FILTER, f);
      g.texParameteri(g.TEXTURE_2D, g.TEXTURE_MAG_FILTER, f);
      g.texParameteri(g.TEXTURE_2D, g.TEXTURE_WRAP_S, r);
      g.texParameteri(g.TEXTURE_2D, g.TEXTURE_WRAP_T, r);
    };
    const drop: Kit['drop'] = (thing) => {
      if ('r' in thing) {
        drop(thing.r);
        drop(thing.w);
      } else if ('fbo' in thing) {
        g.deleteFramebuffer(thing.fbo);
        g.deleteTexture(thing.tex);
      } else {
        g.deleteTexture(thing);
      }
    };
    /** Destination, viewport, program and uniforms for a draw; textures take units in the order given. */
    const bind = (program: Program, uniforms: Record<string, Uniform>, dst: Target | null): void => {
      g.bindFramebuffer(g.FRAMEBUFFER, dst ? dst.fbo : null);
      g.viewport(0, 0, dst ? dst.w : canvas.width, dst ? dst.h : canvas.height);
      g.useProgram(program.p);
      let unit = 0;
      for (const [name, value] of Object.entries(uniforms)) {
        if (!program.u.has(name)) program.u.set(name, g.getUniformLocation(program.p, name));
        const at = program.u.get(name) ?? null;
        if (typeof value === 'number') g.uniform1f(at, value);
        else if ('tex' in value) {
          g.activeTexture(g.TEXTURE0 + unit);
          g.bindTexture(g.TEXTURE_2D, value.tex);
          g.uniform1i(at, unit);
          unit += 1;
        } else if (value.length === 2) g.uniform2fv(at, value);
        else if (value.length === 3) g.uniform3fv(at, value);
        else g.uniform4fv(at, value);
      }
    };
    const target: Kit['target'] = (w, h, kind, linear, repeat) => {
      const tex = g.createTexture();
      const fbo = g.createFramebuffer();
      g.bindTexture(g.TEXTURE_2D, tex);
      g.texStorage2D(g.TEXTURE_2D, 1, kind === 2 ? g.RGBA32F : kind === 1 ? g.RGBA16F : g.RGBA8, w, h);
      filter(tex, linear && kind !== 2, repeat);
      g.bindFramebuffer(g.FRAMEBUFFER, fbo);
      g.framebufferTexture2D(g.FRAMEBUFFER, g.COLOR_ATTACHMENT0, g.TEXTURE_2D, tex, 0);
      const ok = g.checkFramebufferStatus(g.FRAMEBUFFER) === g.FRAMEBUFFER_COMPLETE;
      if (ok) {
        g.clearColor(0, 0, 0, 0);
        g.clear(g.COLOR_BUFFER_BIT);
      }
      const out = { tex, fbo, w, h, ok };
      made.push(() => drop(out));
      return out;
    };
    // Float capability is the completeness of a real target, never the
    // extension list: WebKit has shipped releases that advertised what
    // the hardware did not have.
    g.getExtension('EXT_color_buffer_float');
    const probe = target(2, 2, 1, true, false);
    const float = probe.ok;
    drop(probe);
    return {
      gl: g,
      float,
      aspect: 1,
      host: null,
      tokens: {},
      program: (name) => {
        const program = programs.get(name);
        if (!program) throw new Error(name);
        return program;
      },
      target,
      pair: (w, h, kind, linear, repeat) => {
        const pair: Pair = {
          r: target(w, h, kind, linear, repeat),
          w: target(w, h, kind, linear, repeat),
          swap() {
            const t = pair.r;
            pair.r = pair.w;
            pair.w = t;
          },
        };
        return pair;
      },
      image: (source, linear) => {
        const tex = g.createTexture();
        g.bindTexture(g.TEXTURE_2D, tex);
        g.pixelStorei(g.UNPACK_FLIP_Y_WEBGL, true);
        g.texImage2D(g.TEXTURE_2D, 0, g.RGBA, g.RGBA, g.UNSIGNED_BYTE, source);
        g.pixelStorei(g.UNPACK_FLIP_Y_WEBGL, false);
        filter(tex, linear, false);
        made.push(() => g.deleteTexture(tex));
        return tex;
      },
      pass: (program, uniforms, dst) => {
        bind(program, uniforms, dst);
        g.drawArrays(g.TRIANGLES, 0, 3);
      },
      points: (program, uniforms, count, dst) => {
        bind(program, uniforms, dst);
        g.enable(g.BLEND);
        g.blendFunc(g.ONE, g.ONE);
        g.drawArrays(g.POINTS, 0, count);
        g.disable(g.BLEND);
      },
      drop,
    };
  };

  const ready = (): void => {
    if (done || !gl) return;
    for (const program of programs.values()) {
      if (!gl.getProgramParameter(program.p, gl.LINK_STATUS)) return stop();
    }
    kit = makeKit(gl);
    if (NEEDS_FLOAT && !kit.float) return stop();
    const box = element.getBoundingClientRect();
    kit.aspect = box.width / Math.max(1, box.height);
    kit.host = element;
    try {
      sim = makeSim(kit, options);
    } catch {
      return stop();
    }
    if (!page && getComputedStyle(element).position === 'static') {
      // An absolutely placed canvas covers the nearest positioned box:
      // the host, not the page.
      hostStyle = element.getAttribute('style');
      element.style.position = 'relative';
    }
    element.append(canvas);
    canvas.dataset.state = 'running';
    built = true;
    // In the document first: an element outside it has no computed style.
    kit.tokens = tokens();
    observer = new ResizeObserver(resize);
    try {
      observer.observe(canvas, { box: 'device-pixel-content-box' });
    } catch {
      observer.observe(canvas);
    }
    seer = new IntersectionObserver((entries) => {
      onScreen = entries.some((entry) => entry.isIntersecting);
      if (onScreen) wakeLoop();
      else last = 0;
    });
    seer.observe(element);
    for (const type of ['pointermove', 'pointerdown', 'pointerleave'] as const) element.addEventListener(type, pointer);
  };

  const build = (): void => {
    if (done || gl) return;
    if (document.visibilityState !== 'visible') return;
    gl = canvas.getContext('webgl2', {
      alpha: true,
      antialias: false,
      depth: false,
      stencil: false,
      powerPreference: 'low-power',
      // A software renderer would burn a weak machine's cores for decoration.
      failIfMajorPerformanceCaveat: true,
    });
    if (!gl) return stop();
    const g = gl;
    canvas.addEventListener('webglcontextlost', stop);
    for (const [name, body] of Object.entries(PROGRAMS as Programs)) {
      const p = g.createProgram();
      const [vert, frag] = typeof body === 'string' ? [VERTEX, HEAD + body] : [body.vertex, HEAD + body.fragment];
      for (const [type, source] of [[g.VERTEX_SHADER, vert], [g.FRAGMENT_SHADER, frag]] as const) {
        const shader = g.createShader(type);
        if (!shader) return stop();
        g.shaderSource(shader, source);
        g.compileShader(shader);
        g.attachShader(p, shader);
        g.deleteShader(shader);
      }
      g.linkProgram(p);
      programs.set(name, { p, u: new Map() });
    }
    const parallel = g.getExtension('KHR_parallel_shader_compile');
    let waited = 0;
    const poll = (): void => {
      frame = 0;
      if (done || !gl) return;
      waited += 1;
      // With the extension, ask without blocking, and wait for the
      // slowest. 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.
      const linked = parallel
        ? [...programs.values()].every((program) => g.getProgramParameter(program.p, parallel.COMPLETION_STATUS_KHR))
        : waited > 2;
      if (linked) ready();
      else frame = requestAnimationFrame(poll);
    };
    frame = requestAnimationFrame(poll);
  };
  function wake(): void {
    visible = document.visibilityState === 'visible';
    if (!gl) build();
    else if (visible) wakeLoop();
    else last = 0;
  }

  const handle: Handle = {
    update(next) {
      const previous = options;
      options = mergeOptions(options, next);
      try {
        sim?.update?.(previous, options);
      } catch {
        return stop();
      }
      if (options.paused) last = 0;
      else wakeLoop();
    },
    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.
  frame = requestAnimationFrame(() => {
    frame = 0;
    timer = window.setTimeout(build, 0);
  });
  return handle;
}

// ---- HARNESS stateful: end --------------------------------------------------