fix(shader-transitions): address Copilot round-2 review

Three follow-up fixes from the Copilot review on commit 8cad2173:

1. Use strict `t.shader === undefined` instead of `!t.shader` (Copilot c4)
   in both the WebGL program compile loop and the page-side compositor.
   An empty-string `shader: ""` from a vanilla-JS caller (the IIFE bundle
   is hand-loaded via <script> tags in user HTML) should reach the shader
   registry and surface a loud "unknown shader" error, not silently
   degrade to a crossfade.

2. Graceful degradation when shader compile fails (Copilot c5). The
   previous `continue` dropped the transition from `cachedTransitions`,
   which also dropped its scene-visibility timeline entries and broke
   scene progression. Now: log a warning and downgrade to the CSS
   crossfade fallback (prog=null, fallback=true) so the opacity timeline
   still runs and the composition keeps playing.

3. Preserve index-to-scene-pair correlation when calling the page-side
   compositor (Copilot c6). The earlier filter `transitions.filter(t =>
   !!t.shader)` shifted indices, so a shader transition at original index
   2 (sitting between CSS crossfades) would be paired with scenes[1] and
   scenes[2] inside `installPageSideCompositor` instead of the correct
   scenes[2] and scenes[3]. The compositor now accepts the full array,
   makes `PageCompositeTransitionConfig.shader` optional, and skips
   CSS-only entries internally while keeping `transitions[i]` aligned
   with `scenes[i]`/`scenes[i+1]`.

Co-authored-by: Cursor <cursoragent@cursor.com>

packages/shader-transitions/src/engineModePageComposite.ts

@@ -42,7 +42,14 @@ import { isHtmlInCanvasCaptureSupported } from "./capture.js";
 
 interface PageCompositeTransitionConfig {
   time: number;
-  shader: ShaderName;
+  /**
+   * Shader id. Undefined entries are CSS crossfades — the page-side
+   * compositor skips them so the GSAP opacity timeline handles the blend,
+   * but the entry stays in the array to preserve `transitions[i]` ↔
+   * `scenes[i]`/`scenes[i+1]` index alignment for the surrounding shader
+   * entries.
+   */
+  shader?: ShaderName;
   duration?: number;
 }
 
@@ -114,6 +121,10 @@ export function installPageSideCompositor(options: PageCompositorInstallOptions)
 
   const programs = new Map<string, WebGLProgram>();
   for (const t of transitions) {
+    // CSS crossfade entries (shader undefined) carry no program. Use a
+    // strict undefined check so a misconfigured empty string still fails
+    // loudly through the createProgram path below.
+    if (t.shader === undefined) continue;
     if (programs.has(t.shader)) continue;
     try {
       programs.set(t.shader, createProgram(gl, getFragSource(t.shader)));
@@ -127,6 +138,10 @@ export function installPageSideCompositor(options: PageCompositorInstallOptions)
   for (let i = 0; i < transitions.length; i++) {
     const t = transitions[i];
     if (!t) continue;
+    // CSS-only transitions stay on the GSAP opacity timeline; the page-
+    // side compositor only handles shader entries. Index i is preserved
+    // so subsequent shader transitions still pair with the right scenes.
+    if (t.shader === undefined) continue;
     const fromSceneId = scenes[i];
     const toSceneId = scenes[i + 1];
     const prog = programs.get(t.shader);

packages/shader-transitions/src/hyper-shader.ts

@@ -903,7 +903,11 @@ export function init(config: HyperShaderConfig): GsapTimeline {
 
   const programs = new Map<string, WebGLProgram>();
   for (const t of transitions) {
-    if (!t.shader) continue; // CSS-only transitions have no WebGL program
+    // Strict undefined check — an explicit empty string from a vanilla-JS
+    // caller (the IIFE bundle is hand-loaded via <script> tags) should NOT
+    // be silently coerced into a CSS crossfade. The shader registry will
+    // throw a clear "unknown shader" error for it.
+    if (t.shader === undefined) continue;
     if (!programs.has(t.shader)) {
       try {
         programs.set(t.shader, createProgram(gl, getFragSource(t.shader)));
@@ -1302,11 +1306,19 @@ export function init(config: HyperShaderConfig): GsapTimeline {
     const toId = scenes[i + 1];
     if (!fromId || !toId) continue;
 
-    // CSS-only transition when shader is omitted — uses the fallback opacity
-    // crossfade path. No WebGL program or texture prewarming needed.
-    const isCssFallback = !t.shader;
-    const prog = isCssFallback ? null : (programs.get(t.shader!) ?? null);
-    if (!isCssFallback && !prog) continue; // shader requested but not compiled
+    // shader omitted → CSS crossfade. shader present but program failed to
+    // compile (logged above) → degrade gracefully to CSS crossfade so the
+    // opacity timeline still runs and scene progression isn't broken. Both
+    // paths land in the always-ready prog=null cache.
+    const requestedShader = t.shader !== undefined;
+    const compiledProg = requestedShader ? (programs.get(t.shader!) ?? null) : null;
+    const isCssFallback = !requestedShader || compiledProg === null;
+    if (requestedShader && compiledProg === null) {
+      console.warn(
+        `[HyperShader] Shader "${t.shader}" failed to compile — falling back to CSS crossfade.`,
+      );
+    }
+    const prog = isCssFallback ? null : compiledProg;
 
     const dur = t.duration ?? DEFAULT_DURATION;
     const ease = t.ease ?? DEFAULT_EASE;
@@ -1322,7 +1334,7 @@ export function init(config: HyperShaderConfig): GsapTimeline {
       frames: [],
       cacheKey: "",
       dirty: !isCssFallback,
-      ready: isCssFallback, // CSS fallback needs no prewarming
+      ready: isCssFallback,
       fallback: isCssFallback,
       persisted: isCssFallback,
       textureReady: false,
@@ -2274,16 +2286,13 @@ function initEngineMode(
     const rawH = Number(root?.getAttribute("data-height"));
     const compWidth = Number.isFinite(rawW) && rawW > 0 ? rawW : 1920;
     const compHeight = Number.isFinite(rawH) && rawH > 0 ? rawH : 1080;
-    // Page-side compositing only handles WebGL shader transitions. CSS
-    // crossfades are driven by GSAP opacity timelines elsewhere, so filter
-    // them out — passing them in would break the compositor's required
-    // `shader` field and produce a dead transition window with no rendering.
-    const shaderTransitions = transitions.filter(
-      (t): t is TransitionConfig & { shader: ShaderName } => !!t.shader,
-    );
+    // Pass the full transitions array so transition[i] still pairs with
+    // scenes[i]/scenes[i+1]. The compositor itself skips entries with
+    // `shader === undefined` while preserving the index↔scene mapping.
+    // (CSS crossfades remain driven by the GSAP opacity timeline.)
     installPageSideCompositor({
       scenes,
-      transitions: shaderTransitions,
+      transitions,
       bgColor,
       accentColors,
       width: compWidth,