diff --git a/web/src/test/setup.ts b/web/src/test/setup.ts
index 7b0828b..d575873 100644
--- a/web/src/test/setup.ts
+++ b/web/src/test/setup.ts
@@ -1 +1,32 @@
 import '@testing-library/jest-dom';
+
+// Phase 1 (Foundation Primitives) global polyfills
+// =================================================
+// Headless UI's Combobox + Dialog use ResizeObserver internally to
+// track trigger-element movement (focus-management edge cases on
+// scroll / resize). jsdom does not implement ResizeObserver — without
+// a polyfill, Combobox.closeCombobox's async cleanup fires after the
+// vitest test exits and throws "ReferenceError: ResizeObserver is not
+// defined" as an Unhandled Error. The test assertions pass; the
+// unhandled exception causes vitest to exit 1.
+//
+// A minimal stub is sufficient. The component never reads the
+// observed dimensions in our test paths (those code paths fire only
+// after layout has settled in a real browser); it just needs the
+// constructor + observe/unobserve/disconnect to exist as no-ops.
+class ResizeObserverStub {
+  observe(): void {}
+  unobserve(): void {}
+  disconnect(): void {}
+}
+if (typeof globalThis.ResizeObserver === 'undefined') {
+  globalThis.ResizeObserver = ResizeObserverStub as unknown as typeof ResizeObserver;
+}
+
+// Headless UI also touches Element.prototype.scrollIntoView during
+// keyboard navigation of Combobox.Options. jsdom emits a noisy
+// warning but doesn't throw — still cheaper to stub it so the
+// CI log stays clean.
+if (typeof Element !== 'undefined' && !Element.prototype.scrollIntoView) {
+  Element.prototype.scrollIntoView = function () {};
+}