fix(scene): land the opening crane on scroll or stutter and prewarm cut shaders

src/scene/world/environment.ts

@@ -187,6 +187,9 @@ export async function createProceduralSceneGeneration(
   // Opening shot: a crane down from the sun to the chase position. It plays
   // once, only when the visit starts at the top, and a scroll hurries it.
   let introProgress: number | null = null;
+  let introHurried = false;
+  let introFrames = 0;
+  let introSlowFrames = 0;
 
   const resize = (): void => {
     const nextWidth = Math.max(1, canvas.clientWidth || canvas.width || 1);
@@ -261,10 +264,8 @@ export async function createProceduralSceneGeneration(
         throw new Error('Injected post-first-frame exception');
       }
       resize();
-      const deltaSeconds =
+      const wallSeconds = lastNow === null ? 0 : Math.max(0, (now - lastNow) / 1000);
-        lastNow === null
+      const deltaSeconds = Math.min(0.1, wallSeconds);
-          ? 0
-          : Math.min(0.1, Math.max(0, (now - lastNow) / 1000));
       lastNow = now;
       dynamics.update(snapshot.acceleration, snapshot.direction, deltaSeconds);
       const response = dynamics.snapshot();
@@ -514,10 +515,25 @@ export async function createProceduralSceneGeneration(
       if (introProgress === null) {
         introProgress =
           snapshot.activeSectionId === 'start' && snapshot.scrollY < 40 ? 0 : 1;
+        // The crane opens on the sky, so the road, car and roadside would
+        // otherwise compile their shaders mid-drive when they first enter the
+        // frame. Compile every visible object now, as the chase framing would.
+        if (introProgress === 0) renderer.compile(scene, camera);
       }
       if (introProgress < 1) {
-        const hurry = snapshot.scrollY > 40 ? 5 : 1;
+        // The shot belongs to a visitor who waits at the top. Once they scroll
-        introProgress = Math.min(1, introProgress + (deltaSeconds * hurry) / INTRO_SECONDS);
+        // or navigate it lands within about 0.4 s of wall time, so the chase
+        // framing (and every sign placed against it) is back before arrival.
+        if (snapshot.scrollY > 40 || snapshot.activeSectionId !== 'start') introHurried = true;
+        // A crane that stutters is worse than none: on a device that cannot
+        // hold the shot smoothly, land it straight away.
+        // The first frames carry shader compilation and a lone hitch (a late
+        // texture upload) is forgiven; three slow frames end the shot.
+        introFrames += 1;
+        if (introFrames > 3 && wallSeconds > 0.1) introSlowFrames += 1;
+        if (introSlowFrames >= 3) introHurried = true;
+        const step = introHurried ? Math.min(0.5, wallSeconds) / 0.4 : deltaSeconds / INTRO_SECONDS;
+        introProgress = Math.min(1, introProgress + step);
       }
       const crane = easeInOutCubic(
         THREE.MathUtils.clamp((introProgress - 0.06) / 0.94, 0, 1),

src/scene/world/exit-gates.ts

@@ -76,6 +76,7 @@ export function createExitGates(resources: SceneResourceBag) {
 
   let progress = 1;
   let previous: SectionId | null = null;
+  let warmFrames = 2;
 
   const place = (index: number, x: number, y: number, z: number, sx: number, sy: number) => {
     dummy.position.set(x, y, z);
@@ -94,6 +95,14 @@ export function createExitGates(resources: SceneResourceBag) {
         if (progress >= 0.7) progress = 0;
       }
       previous = section;
+      if (warmFrames > 0) {
+        // Draw the gates unlit for the first frames so the shader program
+        // compiles at the on-ramp, not as a stall on the first exit.
+        warmFrames -= 1;
+        uniforms.uIntensity.value = 0;
+        group.visible = true;
+        return 0;
+      }
       if (progress >= 1) {
         group.visible = false;
         return 0;