Mobile scroll perf: cut paint cost and re-render rate

Mobile scrolling was janky from a stack of expensive paint passes
running every frame. This pass:

- CSS: hide .grain (SVG feTurbulence is the single biggest cost),
  drop mix-blend-mode on .haze/.smoke-puff, drop backdrop-filter on
  .cmd-box, hide the blurred arm-glow, and remove blur filters from
  chip-glow / conveyor-pulse / scan-tear under <=760px.
- JSX: capture an IS_MOBILE flag once at boot. On mobile, halve
  smoke puffs / chimneys / sparks / chip-pulse dots / embers, and
  switch GSAP scrub from 0.6 (rAF interpolation) to true (direct
  scroll-tied) — which is cheaper and more responsive.
- HUD: gate setProgress() on a changed integer depth (0..100) so we
  re-render at most ~100 times per full scroll instead of every frame.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app.jsx

@@ -3,6 +3,12 @@
 
 const { useEffect, useRef, useState } = React;
 
+// Mobile heuristic — captured once at boot. Used to skip expensive
+// effects (extra particles, heavy scrub smoothing) on phones.
+const IS_MOBILE =
+  typeof window !== "undefined" &&
+  window.matchMedia("(max-width: 760px), (pointer: coarse) and (max-width: 900px)").matches;
+
 /* ───────────────── PixelLogo ─────────────────
    Bitmap glyphs rendered as a CSS grid of <span>s, so we control individual
    pixel colors / glow / animations. Original blocky design (no copyrighted mark). */
@@ -186,7 +192,7 @@ function PixelLogoStatic({ word, scale, gap, color }) {
    Each puff is a chunky, low-res-feeling translucent disc that drifts up + fades. */
 function SmokeLayer({ chimneys, scale = 1, intensity = 1 }) {
   // Each chimney emits N puffs on staggered phases.
-  const PUFFS_PER = 3;
+  const PUFFS_PER = IS_MOBILE ? 2 : 3;
   const puffs = [];
   chimneys.forEach((ch, ci) => {
     for (let p = 0; p < PUFFS_PER; p++) {
@@ -269,7 +275,8 @@ function SparkLayer() {
   const sparks = useRef(null);
   if (!sparks.current) {
     const arr = [];
-    for (let i = 0; i < 14; i++) {
+    const count = IS_MOBILE ? 7 : 14;
+    for (let i = 0; i < count; i++) {
       arr.push({
         a: -90 + (Math.random() - 0.5) * 160, // angle deg
         d: 18 + Math.random() * 36,           // distance
@@ -310,7 +317,7 @@ function SparkLayer() {
    Subtle pulsing orange dots over the PCB area. */
 function ChipPulses() {
   // hand-placed % over the chip image
-  const dots = [
+  const allDots = [
     { x: 76, y: 38, dur: 2.4, delay: 0 },
     { x: 82, y: 28, dur: 2.0, delay: 0.7 },
     { x: 88, y: 33, dur: 2.8, delay: 1.2 },
@@ -320,6 +327,7 @@ function ChipPulses() {
     { x: 70, y: 48, dur: 3.0, delay: 0.2 },
     { x: 95, y: 70, dur: 2.4, delay: 0.9 },
   ];
+  const dots = IS_MOBILE ? allDots.filter((_, i) => i % 2 === 0) : allDots;
   return (
     <div className="chip-pulses">
       {dots.map((d, i) => (
@@ -364,9 +372,9 @@ function HeroSection({ bgRef, contentRef }) {
             className="bg-image"
             style={{ backgroundImage: "url(assets/exterior-factory.png)" }}
           />
-          <SmokeLayer chimneys={chimneys} />
+          <SmokeLayer chimneys={IS_MOBILE ? chimneys.filter((_, i) => i % 2 === 0) : chimneys} />
           <EmberLayer
-            count={22}
+            count={IS_MOBILE ? 10 : 22}
             area={{ x: 32, y: 48, w: 65, h: 8 }}
           />
         </div>
@@ -508,6 +516,7 @@ function App() {
   const instSceneRef = useRef(null);
 
   const [progress, setProgress] = useState(0);
+  const lastDepth = useRef(-1);
 
   useEffect(() => {
     if (!window.gsap || !window.ScrollTrigger) return;
@@ -527,8 +536,18 @@ function App() {
           trigger: ".scroll-track",
           start: "top top",
           end: "bottom bottom",
-          scrub: 0.6,
-          onUpdate: (self) => setProgress(self.progress),
+          // On mobile, scrub:true (no smoothing lag) feels snappier and
+          // skips an extra rAF interpolation per frame.
+          scrub: IS_MOBILE ? true : 0.6,
+          onUpdate: (self) => {
+            // Re-render only when the displayed depth (0..100) changes —
+            // otherwise we re-render the whole HUD on every scroll frame.
+            const depth = Math.round(self.progress * 100);
+            if (depth !== lastDepth.current) {
+              lastDepth.current = depth;
+              setProgress(self.progress);
+            }
+          },
         },
       });