Metabolism: browser fix — the arena stays a plain ArrayBuffer unless a region pool will share it; vite serves cross-origin-isolation headers so the browser can opt into SAB

metabolism/src/sim/world.js

@@ -209,7 +209,15 @@ export class World {
         const fcSide0 = Math.max(16, Math.round(field / 2));   // the layout needs the coarse shape
         const spec = layout ?? arenaLayout(field, this.m, B0, fcSide0);
         this.layout = spec;
-        this.arena = sab ?? new SharedArrayBuffer((spec.bytes + 3) & ~3);
+        // The arena is shared ONLY when a region pool will actually spawn:
+        // browsers expose SharedArrayBuffer solely under cross-origin
+        // isolation, and the serial world must never require it.
+        const wantPool = !bare && workers > 1 && !!workerCtor() &&
+            typeof SharedArrayBuffer !== 'undefined';
+        if (!wantPool) workers = 0;
+        this.arena = sab ?? ((wantPool)
+            ? new SharedArrayBuffer((spec.bytes + 3) & ~3)
+            : new ArrayBuffer((spec.bytes + 3) & ~3));
         {
             let off = 0;
             for (const it of spec.items) { it.offset = off; off += (it.bytes + 3) & ~3; }

metabolism/vite.config.js

@@ -142,4 +142,20 @@ function songBankPlugin() {
 
 export default defineConfig({
     plugins: [songBankPlugin()],
+    // Cross-origin isolation: browsers expose SharedArrayBuffer only when
+    // both headers are present. The region pool needs it once the sim runs
+    // inside a browser worker; the demo is fully same-origin, so requiring
+    // corp costs nothing here.
+    server: {
+        headers: {
+            'Cross-Origin-Opener-Policy': 'same-origin',
+            'Cross-Origin-Embedder-Policy': 'require-corp',
+        },
+    },
+    preview: {
+        headers: {
+            'Cross-Origin-Opener-Policy': 'same-origin',
+            'Cross-Origin-Embedder-Policy': 'require-corp',
+        },
+    },
 });