.bg {
  position: fixed; inset: 0; z-index: -1;
  background:
    radial-gradient(40% 50% at 20% 30%, #5b8cff55, transparent 60%),
    radial-gradient(45% 55% at 80% 20%, #b05bff55, transparent 60%),
    radial-gradient(50% 60% at 50% 80%, #2de1c255, transparent 60%),
    #0b0e1a;
  background-size: 200% 200%;
  animation: meshmove 24s ease-in-out infinite;
}
@keyframes meshmove {
  0%, 100% { background-position: 0% 0%, 100% 0%, 50% 100%; }
  50%      { background-position: 30% 20%, 70% 30%, 60% 70%; }
}
@media (prefers-reduced-motion: reduce) {
  .bg { animation: none; }
}

import * as THREE from 'three';

const canvas = document.querySelector('#bg');
const renderer = new THREE.WebGLRenderer({canvas, antialias: true});
renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2)); // cap for perf
const scene = new THREE.Scene();
const camera = new THREE.OrthographicCamera(-1, 1, 1, -1, 0, 1);

const uniforms = {
  uTime: {value: 0},
  uRes: {value: new THREE.Vector2()},
  uColorA: {value: new THREE.Color('#5b8cff')},
  uColorB: {value: new THREE.Color('#b05bff')},
};

const material = new THREE.ShaderMaterial({
  uniforms,
  vertexShader: `void main(){ gl_Position = vec4(position, 1.0); }`,
  fragmentShader: `
    precision highp float;
    uniform float uTime; uniform vec2 uRes;
    uniform vec3 uColorA, uColorB;

    // hash + value noise
    float hash(vec2 p){ return fract(sin(dot(p, vec2(127.1,311.7)))*43758.5453); }
    float noise(vec2 p){
      vec2 i = floor(p), f = fract(p);
      float a = hash(i), b = hash(i+vec2(1,0));
      float c = hash(i+vec2(0,1)), d = hash(i+vec2(1,1));
      vec2 u = f*f*(3.0-2.0*f);
      return mix(mix(a,b,u.x), mix(c,d,u.x), u.y);
    }
    float fbm(vec2 p){
      float v=0.0, a=0.5;
      for(int i=0;i<5;i++){ v += a*noise(p); p*=2.0; a*=0.5; }
      return v;
    }
    void main(){
      vec2 uv = gl_FragCoord.xy / uRes.xy;
      float n = fbm(uv*3.0 + vec2(uTime*0.05, uTime*0.03));
      vec3 col = mix(uColorA, uColorB, smoothstep(0.2, 0.8, n + uv.y*0.3));
      gl_FragColor = vec4(col, 1.0);
    }`,
});

const quad = new THREE.Mesh(new THREE.PlaneGeometry(2, 2), material);
scene.add(quad);

function resize(){
  renderer.setSize(window.innerWidth, window.innerHeight);
  uniforms.uRes.value.set(window.innerWidth, window.innerHeight);
}
window.addEventListener('resize', resize); resize();

const clock = new THREE.Clock();
let running = true;
function loop(){
  if (running){
    uniforms.uTime.value = clock.getElapsedTime();
    renderer.render(scene, camera);
  }
  requestAnimationFrame(loop);
}
loop();

const canvas = document.querySelector('#stars');
const ctx = canvas.getContext('2d');
let W, H, pts;
const COUNT = 80, LINK = 120;

function init(){
  W = canvas.width = innerWidth; H = canvas.height = innerHeight;
  pts = Array.from({length: COUNT}, () => ({
    x: Math.random()*W, y: Math.random()*H,
    vx: (Math.random()-0.5)*0.3, vy: (Math.random()-0.5)*0.3,
  }));
}
addEventListener('resize', init); init();

function frame(){
  ctx.clearRect(0, 0, W, H);
  for (const p of pts){
    p.x += p.vx; p.y += p.vy;
    if (p.x<0||p.x>W) p.vx*=-1;
    if (p.y<0||p.y>H) p.vy*=-1;
    ctx.fillStyle = '#9db4ff'; ctx.fillRect(p.x, p.y, 2, 2);
  }
  for (let i=0;i<COUNT;i++) for (let j=i+1;j<COUNT;j++){
    const dx=pts[i].x-pts[j].x, dy=pts[i].y-pts[j].y;
    const d=Math.hypot(dx, dy);
    if (d<LINK){
      ctx.strokeStyle = `rgba(157,180,255,${1-d/LINK})`;
      ctx.beginPath(); ctx.moveTo(pts[i].x,pts[i].y); ctx.lineTo(pts[j].x,pts[j].y); ctx.stroke();
    }
  }
  requestAnimationFrame(frame);
}
frame();

const PERIOD = 12; // seconds
const phase = (t % PERIOD) / PERIOD * Math.PI * 2;
const offset = Math.sin(phase) * amp;          // returns to start at t = PERIOD

const reduce = matchMedia('(prefers-reduced-motion: reduce)').matches;
if (reduce) {
  running = false;                 // render one static frame, then stop
  renderer.render(scene, camera);
}

// Pause when the tab is hidden
document.addEventListener('visibilitychange', () => {
  running = !document.hidden && !reduce;
});

// Pause when the canvas scrolls offscreen
new IntersectionObserver(([e]) => {
  running = e.isIntersecting && !document.hidden && !reduce;
}).observe(canvas);

<script>
  const t = new URLSearchParams(location.search).get("t");
  if (t !== null) {
    const T = parseFloat(t);
    // CSS mesh gradient:
    document.querySelectorAll(".bg").forEach(el => {
      el.style.animationDelay = (-T) + "s";
      el.style.animationPlayState = "paused";
    });
    // GLSL shader instead? → render exactly one frame at fixed time:
    //   uniforms.uTime.value = T; renderer.render(scene, camera); running = false;
    // Canvas particles? → seed deterministically, step the sim to T, draw once, stop the loop.
  }
  window.__ready = true;                                          // ready signal for headless wait
</script>

npx playwright screenshot --wait-for-timeout=600 "file://$PWD/bg.html?t=12" frame-mid.png