diff --git a/docs/overnight/compile-async-measure.mjs b/docs/overnight/compile-async-measure.mjs
new file mode 100644
index 0000000..a6e9a8a
--- /dev/null
+++ b/docs/overnight/compile-async-measure.mjs
@@ -0,0 +1,106 @@
+// usage: node measure.mjs <label> <runs> [--shot] [--nokhr] [--noscroll]
+import { chromium } from 'playwright';
+import { writeFileSync, mkdirSync, appendFileSync } from 'node:fs';
+
+const [label, runsArg] = process.argv.slice(2);
+const SHOT = process.argv.includes('--shot');
+const NOKHR = process.argv.includes('--nokhr');
+const NOSCROLL = process.argv.includes('--noscroll');
+const runs = Number(runsArg ?? 5);
+const out = process.env.OUT ?? 'compile-async-out'; // runs from the repo root; serve dist on 127.0.0.1:5181 first
+mkdirSync(out, { recursive: true });
+const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
+
+const init = `(() => {
+  const T = { links: [], lit: null, linksAtLit: null, longtasks: [], frames: [], shot: null };
+  window.__m = T;
+  for (const C of [WebGL2RenderingContext, WebGLRenderingContext]) {
+    const link = C.prototype.linkProgram;
+    C.prototype.linkProgram = function (p) { T.links.push(performance.now()); return link.call(this, p); };
+    if (${NOKHR}) {
+      const ge = C.prototype.getExtension;
+      C.prototype.getExtension = function (n) { return n === 'KHR_parallel_shader_compile' ? null : ge.call(this, n); };
+      const gs = C.prototype.getSupportedExtensions;
+      C.prototype.getSupportedExtensions = function () { return gs.call(this).filter((e) => e !== 'KHR_parallel_shader_compile'); };
+    }
+  }
+  new PerformanceObserver((l) => { for (const e of l.getEntries()) T.longtasks.push([e.startTime, e.duration]); })
+    .observe({ type: 'longtask', buffered: true });
+  const mo = new MutationObserver(() => {
+    if (T.lit !== null) return;
+    const c = document.querySelector('canvas.journey-canvas');
+    if (!c || c.dataset.lit === undefined) return;
+    T.lit = performance.now();
+    T.linksAtLit = T.links.length;
+    if (${SHOT}) T.shot = c.toDataURL('image/png');
+  });
+  mo.observe(document, { subtree: true, attributes: true, attributeFilter: ['data-lit'] });
+  let last = null;
+  const loop = (t) => { if (last !== null) T.frames.push([t, t - last]); last = t; requestAnimationFrame(loop); };
+  requestAnimationFrame(loop);
+})();`;
+
+const results = [];
+for (let i = 0; i < runs; i += 1) {
+  const browser = await chromium.launch({
+    channel: 'chromium',
+    headless: process.env.HEADED !== '1',
+    args: ['--use-gl=angle', '--use-angle=gl', '--ignore-gpu-blocklist', '--enable-gpu-rasterization',
+      '--enable-webgl', '--disable-gpu-sandbox', '--disable-gpu-shader-disk-cache'],
+  });
+  const context = await browser.newContext({ viewport: { width: 1440, height: 900 }, deviceScaleFactor: 1 });
+  const page = await context.newPage();
+  const errors = [];
+  page.on('console', (m) => { if (m.type() === 'error') errors.push(m.text()); });
+  await page.addInitScript(init);
+  await page.goto('http://127.0.0.1:5181/', { waitUntil: 'load' });
+  await page.waitForFunction(() => window.__m.lit !== null, null, { timeout: 20000 });
+  const env = await page.evaluate(() => {
+    const gl = document.createElement('canvas').getContext('webgl2');
+    const ext = gl.getExtension('WEBGL_debug_renderer_info');
+    return {
+      renderer: ext ? gl.getParameter(ext.UNMASKED_RENDERER_WEBGL) : gl.getParameter(gl.RENDERER),
+      khr: gl.getExtension('KHR_parallel_shader_compile') !== null,
+    };
+  });
+  await sleep(2500);
+  const scrollStart = await page.evaluate(() => performance.now());
+  if (!NOSCROLL) {
+    await page.mouse.move(720, 450);
+    const t0 = Date.now();
+    while (Date.now() - t0 < 10000) {
+      await page.mouse.wheel(0, 200);
+      await sleep(80);
+    }
+  } else {
+    await sleep(10000);
+  }
+  const scrollEnd = await page.evaluate(() => performance.now());
+  const m = await page.evaluate(() => window.__m);
+  const nav = await page.evaluate(() => performance.getEntriesByType('navigation')[0].toJSON());
+  const r = {
+    run: i,
+    renderer: env.renderer,
+    khr: env.khr,
+    litMs: Math.round(m.lit),
+    linksBeforeLit: m.linksAtLit,
+    linksAfterLit: m.links.length - m.linksAtLit,
+    linksAfterLitTimes: m.links.slice(m.linksAtLit).map((t) => Math.round(t - m.lit)),
+    frame0Task: (() => { const t = m.longtasks.find(([st, d]) => st <= m.lit && m.lit <= st + d + 5); return t ? Math.round(t[1]) : null; })(),
+    linksInFrame0Task: (() => { const t = m.longtasks.find(([st, d]) => st <= m.lit && m.lit <= st + d + 5); return t ? m.links.filter((x) => x >= t[0] && x <= m.lit).length : null; })(),
+    timeline: (() => { const t = m.longtasks.find(([st, d]) => st <= m.lit && m.lit <= st + d + 5); const f0 = t ? t[0] : m.lit; const pre = m.links.filter((x) => x < f0); return { firstLink: Math.round(m.links[0]), lastBuildLink: pre.length ? Math.round(pre[pre.length - 1]) : null, buildLinks: pre.length, frame0Start: Math.round(f0), lit: Math.round(m.lit) }; })(),
+    longestTask: Math.round(Math.max(0, ...m.longtasks.map((x) => x[1]))),
+    longTasks: m.longtasks.filter((x) => x[1] > 100).map((x) => [Math.round(x[0]), Math.round(x[1])]),
+    framesOver50Idle: m.frames.filter(([t, d]) => t > m.lit + 50 && t < scrollStart && d > 50).map(([t, d]) => [Math.round(t - m.lit), Math.round(d)]),
+    framesOver50Scroll: m.frames.filter(([t, d]) => t >= scrollStart && t <= scrollEnd && d > 50).map(([t, d]) => [Math.round(t - scrollStart), Math.round(d)]),
+    worstScrollFrame: Math.round(Math.max(0, ...m.frames.filter(([t]) => t >= scrollStart && t <= scrollEnd).map((x) => x[1]))),
+    scrollY: await page.evaluate(() => scrollY),
+    domContentLoaded: Math.round(nav.domContentLoadedEventEnd),
+    errors,
+  };
+  if (SHOT && m.shot) writeFileSync(`${out}/${label}-frame0-${Date.now()}.png`, Buffer.from(m.shot.split(',')[1], 'base64'));
+  console.log(JSON.stringify(r));
+  results.push(r);
+  await browser.close();
+}
+appendFileSync(`${out}/${label}.jsonl`, results.map((r) => JSON.stringify(r)).join('\n') + '\n');
diff --git a/docs/overnight/compile-async-results.md b/docs/overnight/compile-async-results.md
new file mode 100644
index 0000000..adb2848
--- /dev/null
+++ b/docs/overnight/compile-async-results.md
@@ -0,0 +1,152 @@
+# Issue 2, option A: `compileAsync` shader warm-up — measurements
+
+Branch `polish/compile-async`, against baseline `71465fa`.
+
+## What changed
+
+- `src/scene/world/shader-warmup.ts` (new): `warmUpShaders()` binds the
+  composer's scene target, reveals hidden subtrees (so `compile()` gathers the
+  same lights as the frame-0 warm render), calls `renderer.compileAsync()`,
+  restores target and visibility right after the synchronous `compile()` part,
+  then awaits the parallel link with a 2.5 s timeout (`ready` / `timeout` /
+  `failed`).
+- `src/scene/post-processing.ts`: `PostProcessor.sceneTarget()` returns the
+  EffectComposer `readBuffer` (the target RenderPass draws into). In three r185,
+  `WebGLPrograms.getParameters` keys tone mapping and output colour space on the
+  bound target. Any non-XR off-screen target gives the same key, so the
+  precompiled programs are the ones the composer uses.
+- `src/scene/world/environment.ts`: at the end of the async build, call
+  `resize()`, then `await warmUpShaders(...)`. The frame-0 warm render stays as
+  the safety net. Post-pass programs are not in the scene, so the warm render
+  still builds those. It also uploads textures and finishes whatever a
+  timed-out or extension-less warm-up left behind. Test builds dispatch
+  `portfolio-program-count` whenever `renderer.info.programs.length` changes.
+- Regression checks:
+  - `tests/browser/shader-warmup.spec.ts`: after frame 0 no program is ever
+    created (idle ignition, then 12 wheel steps). It also checks that the
+    warm-up ran and that frame 0 adds at most 12 programs on top of it (only
+    the post passes; a mis-keyed warm-up would add about 60). Passes on desktop-chromium and
+    mobile-chromium. In SwiftShader, desktop compiles 59 programs in the
+    warm-up and frame 0 adds 9 post-pass programs, for 68 total. Mobile:
+    53 + 0 = 53.
+  - `tests/unit/shader-warmup.test.ts` (fakes): target and visibility during
+    compile, both restored afterwards, the timeout, the failure paths, and the
+    timer cleanup.
+  - `tests/unit/world-models.test.ts`: `sceneTarget()`.
+
+## Method
+
+- Hardware: `ANGLE (Intel, Mesa Intel(R) Graphics (ARL), OpenGL ES 3.2)`.
+  Playwright Chromium 151.0.7922.34, **headed** on the desktop session. In a
+  headless trial the frame-0 task took 3.1 s and every scroll frame took
+  >50 ms, so headed is the representative setup.
+- Flags: `--use-gl=angle --use-angle=gl --ignore-gpu-blocklist
+  --enable-gpu-rasterization --enable-webgl --disable-gpu-sandbox
+  --disable-gpu-shader-disk-cache`. These are the hardware-GL flags from the
+  scratchpad `shots.mjs`; `r6.mjs` no longer existed. Every run launches a new
+  browser with a fresh temporary profile. Viewport 1440x900, DPR 1, production
+  preview build served by `python3 -m http.server` on 127.0.0.1:5181.
+- Runs were interleaved A/B/A/B. The harness is
+  `docs/overnight/compile-async-measure.mjs`. An init script wraps
+  `linkProgram` (count and timestamps, so it works on production builds),
+  watches `canvas[data-lit]` with a MutationObserver (it fires in the microtask
+  right after the frame-0 render, still inside that task), records
+  `PerformanceObserver('longtask')` and rAF deltas. After lit it idles 2.5 s,
+  then wheels 200 px every 80 ms for 10 s.
+- **Power and load:** `ADP0/online` read `1` before every run, and
+  `BAT0` reported "Not charging", 79 %. The coordinator reported battery power
+  earlier. Load average was **6-11** because four other agents shared the
+  machine. **Absolute times are inflated and noisy. Compare the A/B
+  differences, not the absolute numbers.**
+
+## Results (median [min-max])
+
+### KHR_parallel_shader_compile available (6 + 6 runs)
+
+| metric | baseline 71465fa | option A |
+|---|---|---|
+| time to first scene frame (`data-lit`, ms after navigation start) | 2788 [2733-3431] | 2667 [2422-2960] |
+| frame-0 task (long task that contains frame 0) | **1737 [1706-1771]** | **340 [323-405]** |
+| longest main-thread task | 1737 [1706-1771] | 458 [422-521] (page boot JS, not the scene) |
+| programs linked before frame 0 / inside frame 0 / after | 3 / 68 / 0 | 62 (async, during the build) / 9 (post passes) / 0 |
+| frame after frame 0 (presentation hitch) | 177 [154-229] | 98 [0-167] |
+| scroll frames >50 ms in 10 s (per run) | 13, 6, 5, 20, 72, 42 | 12, 6, 3, 29, 72, 28 |
+| worst scroll frame | 146 [71-229] | 158 [71-250] |
+| long tasks during scroll | none | none |
+
+Timeline from three more interleaved runs per variant, ms after navigation:
+
+| | first link | last link before frame 0 | frame 0 starts | first frame (lit) |
+|---|---|---|---|---|
+| baseline | 1497 / 834 / 842 | 3 links only | 1621 / 944 / 964 | 3144 / 2396 / 2453 |
+| option A | 752 / 810 / 818 | 918 / 978 / 994 (62 links) | 1933 / 2109 / 2152 | 2248 / 2417 / 2454 |
+
+Option A issues the 59 scene links in about 170 ms, inside the build's
+existing ~420 ms boot task, so it adds no new long task. The build then
+awaits the driver for about 1.15 s while the main thread is free (poster up,
+page scrollable). Frame 0 follows at about 300 ms.
+
+### KHR_parallel_shader_compile hidden (getExtension patched, 3 + 3 runs)
+
+| metric | baseline | option A |
+|---|---|---|
+| first scene frame | 2611 [2604-2686] | 2473 [2433-2820] |
+| frame-0 task | 1647 [1631-1679] | 1418 [1400-1559] |
+| links inside frame 0 | 68 | 9 (the other 62 were queued during the build) |
+| links after frame 0 | 0 | 0 |
+
+Without the extension three marks every program ready immediately, so
+`compileAsync` resolves after one 10 ms timeout. The links queued during the
+build overlap the rest of the build, and the warm render then blocks on the
+first status query. The result is still a little better than baseline,
+never worse. Behaviour is the same as before, with no timeout path.
+
+### Visual correctness
+
+Frame 0 was captured with `canvas.toDataURL()` inside the frame-0 task, 3
+interleaved runs per variant. All 15 pairwise comparisons (base-base,
+new-new, base-new) have **max channel difference 0**: option A's first
+frame is pixel-identical to the baseline's.
+
+## Pros and cons (measured)
+
+Pros
+- The frame-0 main-thread block drops from about 1.74 s to about 0.34 s (-80 %).
+  The longest main-thread task on the page goes from the scene's 1.7 s to
+  page boot (~0.45 s).
+- About 1.15 s of shader linking now runs on the driver's threads while the
+  poster shows and the page stays responsive.
+- The hitch on the frame after frame 0 is also smaller (177 -> 98 ms median).
+- Frame 0 is pixel-identical. The regression test pins zero program growth
+  after frame 0. On this machine no programs were linked after frame 0 in
+  10 s of scrolling, in either variant.
+- No extension: slightly better than baseline. Slow driver: the 2.5 s
+  timeout hands the remainder to the warm render, which is the old behaviour.
+
+Cons
+- **Time to first scene frame does not improve much** (-120 ms median with
+  KHR, within noise). The total link work is about the same; it moved off
+  the main thread, it did not shrink. The win is responsiveness and long-task
+  size, not an earlier frame.
+- The approach depends on three's key internals: the bound target decides
+  tone mapping and colour space, and `compile()` gathers lights with
+  `traverseVisible`. A three upgrade that changes either would quietly
+  double-compile. The browser spec guards this by allowing frame 0 to add at
+  most 12 programs on top of the warm-up (9 post passes today).
+- Frame 0 still costs about 330 ms: 9 post-pass programs, texture uploads
+  and the first full draw. Options to cut it further: precompile the post
+  passes' materials on a fullscreen quad through the same warm-up, or
+  combine with option B (slicing).
+- After a timeout, three's `compileAsync` keeps polling every 10 ms until
+  every program reports ready. This is harmless, but the poll would throw
+  once if the generation were disposed in the meantime.
+- The build waits for the parallel compile, so the first-frame watchdog
+  (5 s, armed before the build) now also covers the compile. The 2.5 s cap
+  keeps that bounded.
+
+## Verification
+
+`npx vue-tsc --noEmit` clean. `npx vitest run`: 84 files, 978 tests passed.
+`npm run build:preview`: exit 0, `"violations": []`.
+`npx playwright test tests/browser/shader-warmup.spec.ts`: 2 passed
+(desktop-chromium and mobile-chromium).
diff --git a/src/scene/post-processing.ts b/src/scene/post-processing.ts
index d53d09a..60bfa2c 100644
--- a/src/scene/post-processing.ts
+++ b/src/scene/post-processing.ts
@@ -15,6 +15,11 @@ export interface PostProcessor extends GpuRenderTargetReporter {
   setDynamics(acceleration: number, dawn?: number): void;
   setLens(state: LensState): void;
   render(scene: unknown, camera: unknown): void;
+  /**
+   * The off-screen target the scene pass draws into. Bind it while compiling
+   * scene programs: three keys tone mapping and output colour space on it.
+   */
+  sceneTarget(): unknown;
   dispose(): void;
 }
 
@@ -28,6 +33,8 @@ interface RenderTargetLike {
 interface ComposerLike {
   renderTarget1?: RenderTargetLike;
   renderTarget2?: RenderTargetLike;
+  /** RenderPass (needsSwap false) draws the scene into this target. */
+  readBuffer?: unknown;
   addPass(pass: unknown): void;
   setPixelRatio(pixelRatio: number): void;
   setSize(width: number, height: number): void;
@@ -146,6 +153,9 @@ export function createPostProcessor(
       renderPass.camera = camera;
       composer.render();
     },
+    sceneTarget() {
+      return composer.readBuffer ?? composer.renderTarget2 ?? null;
+    },
     dispose() {
       if (disposed) return;
       disposed = true;
diff --git a/src/scene/world/environment.ts b/src/scene/world/environment.ts
index 4744cc7..1ad09cd 100644
--- a/src/scene/world/environment.ts
+++ b/src/scene/world/environment.ts
@@ -54,6 +54,7 @@ import { readIgnition } from '@/journey/ignition-feed';
 import { createUnderpass, underpassUniforms } from '@/scene/world/underpass';
 import { DEFAULT_LEAD_VH, nextSectionId, shadeDepth } from '@/journey/underpass';
 import { signRouteFor, signTriggerMaximumVhFor } from '@/journey/sign-routes';
+import { warmUpShaders, type ShaderWarmupResult } from '@/scene/world/shader-warmup';
 
 declare const __PORTFOLIO_TEST_BUILD__: boolean;
 const TEST_BUILD =
@@ -71,6 +72,8 @@ export function createWorldFog(color: number) {
   return new THREE.Fog(color, HORIZON_FOG_START, HORIZON_FOG_END);
 }
 
+let warmupGenerations = 0;
+
 export async function createProceduralSceneGeneration(
   context: SceneBuildContext,
 ): Promise<SceneGeneration> {
@@ -353,6 +356,23 @@ export async function createProceduralSceneGeneration(
     }
   };
 
+  // SHADER WARM-UP: link the world's programs on the driver's compile threads
+  // while the poster is still up, so frame 0 finds them ready instead of
+  // compiling ~60 programs inside one task (src/scene/world/shader-warmup.ts).
+  // Size the canvas and the composer targets first: the camera and the
+  // targets are then exactly what frame 0 draws with.
+  resize();
+  const warmup: ShaderWarmupResult = await warmUpShaders(
+    renderer,
+    scene,
+    camera,
+    postProcessor?.sceneTarget() ?? null,
+  );
+  const warmupPrograms = (renderer.info.programs as unknown[] | null)?.length ?? 0;
+  // Test builds: renderer.info.programs must not grow after frame 0 (issue 2).
+  const warmupGeneration = (warmupGenerations += 1);
+  let programCount = -1;
+
   if (context.signal.aborted) {
     context.resources.dispose();
     throw new DOMException('Scene build aborted', 'AbortError');
@@ -939,11 +959,12 @@ export async function createProceduralSceneGeneration(
         // its program as a 70-280 ms hitch inside the opening shot (three slow
         // frames end the crane; one over 250 ms is a quality downgrade that
         // rebuilds the scene). Draw everything once now, behind the poster,
-        // through the real pipeline: renderer.compile() keys its programs for
-        // the screen (sRGB output, tone mapping) and never matches the post
-        // processor's linear targets, so every program compiled twice. The
-        // drawing buffer is presented once per task: the cold frame below
-        // is what the canvas shows.
+        // through the real pipeline. The build's shader warm-up has already
+        // linked the scene programs off-thread, so this draw is the safety
+        // net: it builds the post passes' programs (not in the scene), uploads
+        // textures, and finishes whatever a timed-out or extension-less
+        // warm-up left. The drawing buffer is presented once per task: the
+        // cold frame below is what the canvas shows.
         // Culling is off too: the car sits outside the crane's frustum.
         const hidden: { visible: boolean }[] = [];
         const culled: { frustumCulled: boolean }[] = [];
@@ -991,6 +1012,20 @@ export async function createProceduralSceneGeneration(
         }
       }
       renderedFrames += 1;
+      if (TEST_BUILD) {
+        const programs = (renderer.info.programs as unknown[] | null)?.length ?? 0;
+        if (programs !== programCount) {
+          programCount = programs;
+          window.dispatchEvent(new CustomEvent('portfolio-program-count', {
+            detail: {
+              generation: warmupGeneration,
+              frame: renderedFrames,
+              programs,
+              warmup: { ...warmup, programs: warmupPrograms },
+            },
+          }));
+        }
+      }
     },
     dispose() {
       if (disposed) return;
diff --git a/src/scene/world/shader-warmup.ts b/src/scene/world/shader-warmup.ts
new file mode 100644
index 0000000..e0b6221
--- /dev/null
+++ b/src/scene/world/shader-warmup.ts
@@ -0,0 +1,96 @@
+// SHADER WARM-UP: compile the world's programs off the main thread before the
+// first frame (issue 2, option A).
+//
+// renderer.compileAsync() asks the driver to link every material's program and
+// then polls KHR_parallel_shader_compile's COMPLETION_STATUS instead of blocking
+// on LINK_STATUS, so the scene build awaits the compile while the poster stays
+// up and the main thread stays free. Two details make the precompiled programs
+// the ones the real frames use:
+// - three r185 keys a program's tone mapping and output colour space on the
+//   BOUND render target (WebGLPrograms.getParameters): null means "the screen"
+//   (sRGB, tone mapped), any off-screen target means "linear, untouched". The
+//   scene is drawn into the composer's target, so that target must be bound.
+// - the light count is part of the key, and compile() gathers lights with
+//   traverseVisible(). The frame-0 warm render draws with every object visible,
+//   so the compile reveals the same hidden subtrees.
+// Without the extension three flags every program ready at once and resolves
+// after one 10 ms timeout: the links are queued, and the warm render waits on
+// them as before. A slow driver gets TIMEOUT_MS; the warm render then finishes
+// whatever is left synchronously, as it always did.
+
+export const SHADER_WARMUP_TIMEOUT_MS = 2_500;
+
+export type ShaderWarmupOutcome = 'ready' | 'timeout' | 'failed';
+
+export interface ShaderWarmupResult {
+  outcome: ShaderWarmupOutcome;
+  /** Wall time the build waited, ms. */
+  waited: number;
+  /** KHR_parallel_shader_compile was available (true off-thread compile). */
+  parallel: boolean;
+}
+
+interface WarmupObject {
+  visible: boolean;
+  traverse(callback: (object: WarmupObject) => void): void;
+}
+
+export interface WarmupRenderer {
+  getRenderTarget(): unknown;
+  setRenderTarget(target: unknown): void;
+  compileAsync(scene: unknown, camera: unknown): Promise<unknown>;
+  extensions: { has(name: string): boolean };
+}
+
+export interface WarmupScheduler {
+  now(): number;
+  setTimeout(callback: () => void, delay: number): unknown;
+  clearTimeout(handle: unknown): void;
+}
+
+const defaultScheduler: WarmupScheduler = {
+  now: () => performance.now(),
+  setTimeout: (callback, delay) => globalThis.setTimeout(callback, delay),
+  clearTimeout: (handle) => globalThis.clearTimeout(handle as ReturnType<typeof setTimeout>),
+};
+
+export async function warmUpShaders(
+  renderer: WarmupRenderer,
+  scene: WarmupObject,
+  camera: unknown,
+  /** The target the scene pass draws into; null when there is no composer. */
+  sceneTarget: unknown,
+  timeoutMs = SHADER_WARMUP_TIMEOUT_MS,
+  scheduler: WarmupScheduler = defaultScheduler,
+): Promise<ShaderWarmupResult> {
+  const started = scheduler.now();
+  const parallel = renderer.extensions.has('KHR_parallel_shader_compile');
+  let pending: Promise<unknown>;
+  // compile() itself is synchronous: target and visibility only need to hold
+  // for this call, not for the wait.
+  const previousTarget = renderer.getRenderTarget();
+  const hidden: WarmupObject[] = [];
+  try {
+    scene.traverse((object) => {
+      if (object.visible) return;
+      object.visible = true;
+      hidden.push(object);
+    });
+    renderer.setRenderTarget(sceneTarget);
+    pending = renderer.compileAsync(scene, camera);
+  } catch {
+    return { outcome: 'failed', waited: scheduler.now() - started, parallel };
+  } finally {
+    for (const object of hidden) object.visible = false;
+    renderer.setRenderTarget(previousTarget);
+  }
+  let timer: unknown = null;
+  const outcome = await Promise.race<ShaderWarmupOutcome>([
+    pending.then(() => 'ready' as const, () => 'failed' as const),
+    new Promise<ShaderWarmupOutcome>((resolve) => {
+      timer = scheduler.setTimeout(() => resolve('timeout'), timeoutMs);
+    }),
+  ]);
+  scheduler.clearTimeout(timer);
+  return { outcome, waited: scheduler.now() - started, parallel };
+}
diff --git a/tests/browser/shader-warmup.spec.ts b/tests/browser/shader-warmup.spec.ts
new file mode 100644
index 0000000..b4ef2e9
--- /dev/null
+++ b/tests/browser/shader-warmup.spec.ts
@@ -0,0 +1,55 @@
+import { expect, test } from '../helpers/serve-dist';
+
+// Issue 2 (cold-GPU first frame): the build links the world's programs with
+// renderer.compileAsync() against the composer's target, and frame 0 draws
+// everything once as a safety net. After frame 0 no program may be created:
+// each later one is a 70-280 ms hitch inside the opening shot. Test builds
+// dispatch portfolio-program-count whenever renderer.info.programs changes.
+
+interface ProgramCountEvent {
+  generation: number;
+  frame: number;
+  programs: number;
+  warmup: { outcome: 'ready' | 'timeout' | 'failed'; waited: number; parallel: boolean; programs: number };
+}
+
+test('compiles every scene program before the first frame and none after it', async ({ page, distUrl }) => {
+  await page.addInitScript(() => {
+    const events: unknown[] = [];
+    (window as unknown as { __programCounts: unknown[] }).__programCounts = events;
+    window.addEventListener('portfolio-program-count', (event) => {
+      events.push(structuredClone((event as CustomEvent).detail));
+    });
+  });
+  await page.goto(distUrl, { waitUntil: 'domcontentloaded' });
+  await expect(page.locator('canvas.journey-canvas')).toHaveAttribute('data-lit', '', { timeout: 20_000 });
+
+  // The ignition hold, then the pull-away and the first chapters.
+  await page.waitForTimeout(3_500);
+  await page.mouse.move(200, 300);
+  for (let step = 0; step < 12; step += 1) {
+    await page.mouse.wheel(0, 400);
+    await page.waitForTimeout(120);
+  }
+  await page.waitForTimeout(500);
+
+  const events = await page.evaluate(
+    () => (window as unknown as { __programCounts: ProgramCountEvent[] }).__programCounts,
+  );
+  expect(events.length).toBeGreaterThan(0);
+  for (const generation of new Set(events.map((event) => event.generation))) {
+    const own = events.filter((event) => event.generation === generation);
+    const first = own[0]!;
+    // The first report comes right after frame 0.
+    expect(first.frame).toBe(1);
+    // The warm-up compiled the scene before frame 0 (or gave up on a slow driver).
+    expect(['ready', 'timeout']).toContain(first.warmup.outcome);
+    expect(first.warmup.programs).toBeGreaterThan(0);
+    expect(first.programs).toBeGreaterThanOrEqual(first.warmup.programs);
+    // Frame 0 adds only the post passes (9 on desktop). A warm-up keyed for
+    // the wrong target or light count would add the whole scene again (~60).
+    expect(first.programs - first.warmup.programs).toBeLessThanOrEqual(12);
+    // No growth afterwards.
+    expect(own.slice(1)).toEqual([]);
+  }
+});
diff --git a/tests/unit/shader-warmup.test.ts b/tests/unit/shader-warmup.test.ts
new file mode 100644
index 0000000..0e914f5
--- /dev/null
+++ b/tests/unit/shader-warmup.test.ts
@@ -0,0 +1,115 @@
+import { describe, expect, it } from 'vitest';
+import { warmUpShaders, type WarmupRenderer, type WarmupScheduler } from '@/scene/world/shader-warmup';
+
+interface Node {
+  name: string;
+  visible: boolean;
+  children: Node[];
+  traverse(callback: (object: Node) => void): void;
+}
+
+const node = (name: string, visible: boolean, children: Node[] = []): Node => ({
+  name,
+  visible,
+  children,
+  traverse(callback) {
+    callback(this);
+    for (const child of this.children) child.traverse(callback);
+  },
+});
+
+const manualScheduler = () => {
+  let clock = 0;
+  const timers = new Map<number, () => void>();
+  let next = 1;
+  const scheduler: WarmupScheduler = {
+    now: () => clock,
+    setTimeout(callback, delay) {
+      const id = next++;
+      timers.set(id, () => {
+        clock += delay;
+        callback();
+      });
+      return id;
+    },
+    clearTimeout(handle) {
+      timers.delete(handle as number);
+    },
+  };
+  return { scheduler, fire: () => [...timers.values()].forEach((run) => run()), pending: () => timers.size };
+};
+
+const fakeRenderer = (compile: () => Promise<unknown>, parallel = true) => {
+  const seen: { target: unknown; visible: string[] }[] = [];
+  const state = { target: 'screen-previous' as unknown };
+  let scene: Node | null = null;
+  const renderer: WarmupRenderer = {
+    getRenderTarget: () => state.target,
+    setRenderTarget: (target) => {
+      state.target = target;
+    },
+    compileAsync: (root) => {
+      scene = root as Node;
+      const visible: string[] = [];
+      scene.traverse((object) => {
+        if (object.visible) visible.push(object.name);
+      });
+      seen.push({ target: state.target, visible });
+      return compile();
+    },
+    extensions: { has: (name) => parallel && name === 'KHR_parallel_shader_compile' },
+  };
+  return { renderer, seen, state };
+};
+
+describe('shader warm-up', () => {
+  it('compiles against the scene target with every subtree visible, then restores both', async () => {
+    const lamp = node('hidden-light', false);
+    const scene = node('root', true, [node('car', true), node('cone', false, [lamp])]);
+    const { renderer, seen, state } = fakeRenderer(() => Promise.resolve());
+    const { scheduler } = manualScheduler();
+
+    const result = await warmUpShaders(renderer, scene, {}, 'composer-target', 2_500, scheduler);
+
+    expect(result).toMatchObject({ outcome: 'ready', parallel: true });
+    expect(seen).toEqual([
+      { target: 'composer-target', visible: ['root', 'car', 'cone', 'hidden-light'] },
+    ]);
+    expect(state.target).toBe('screen-previous');
+    expect(scene.children[1]!.visible).toBe(false);
+    expect(lamp.visible).toBe(false);
+  });
+
+  it('gives up after the timeout so a slow driver never holds the build', async () => {
+    const { renderer } = fakeRenderer(() => new Promise(() => undefined));
+    const timers = manualScheduler();
+    const pending = warmUpShaders(renderer, node('root', true), {}, null, 2_500, timers.scheduler);
+    timers.fire();
+    await expect(pending).resolves.toMatchObject({ outcome: 'timeout', waited: 2_500 });
+  });
+
+  it('clears its timer once the compile resolves', async () => {
+    const { renderer } = fakeRenderer(() => Promise.resolve(), false);
+    const timers = manualScheduler();
+    const result = await warmUpShaders(renderer, node('root', true), {}, null, 2_500, timers.scheduler);
+    expect(result).toMatchObject({ outcome: 'ready', parallel: false });
+    expect(timers.pending()).toBe(0);
+  });
+
+  it('reports a throwing compile as failed and still restores the renderer and scene', async () => {
+    const hidden = node('hidden', false);
+    const { renderer, state } = fakeRenderer(() => {
+      throw new Error('link failed');
+    });
+    const result = await warmUpShaders(renderer, node('root', true, [hidden]), {}, 'target', 2_500, manualScheduler().scheduler);
+    expect(result.outcome).toBe('failed');
+    expect(state.target).toBe('screen-previous');
+    expect(hidden.visible).toBe(false);
+  });
+
+  it('reports a rejected compile as failed', async () => {
+    const { renderer } = fakeRenderer(() => Promise.reject(new Error('lost context')));
+    const result = await warmUpShaders(renderer, node('root', true), {}, null, 2_500, manualScheduler().scheduler);
+    expect(result.outcome).toBe('failed');
+  });
+});
diff --git a/tests/unit/world-models.test.ts b/tests/unit/world-models.test.ts
index c3f99a3..22e5b56 100644
--- a/tests/unit/world-models.test.ts
+++ b/tests/unit/world-models.test.ts
@@ -404,6 +404,8 @@ describe("procedural world models", () => {
     expect(bloomPass.strength).toBe(restingBloom.strength);
     expect(bloomPass.threshold).toBe(restingBloom.threshold);
     processor.render({}, {});
+    // The shader warm-up binds the scene pass's target (EffectComposer's readBuffer).
+    expect(processor.sceneTarget()).toBe(composer.renderTarget2);
     const allocations = resources.gpuBudgetSnapshot({
       tier: "high",
       defaultFramebuffer: {