diff --git a/extensions/amp-iframe/1.0/component.js b/extensions/amp-iframe/1.0/component.js
index 6b9a051e7478..b1d83c6419f4 100644
--- a/extensions/amp-iframe/1.0/component.js
+++ b/extensions/amp-iframe/1.0/component.js
@@ -72,9 +72,7 @@ export function BentoIframe({
 
   useEffect(() => {
     const iframe = iframeRef.current;
-    // TODO(36239): Ensure that effects are properly isolated between test runs.
-    // Guarding for iframe truthiness should be enough.
-    if (!iframe?.ownerDocument.defaultView) {
+    if (!iframe) {
       return;
     }
     const win = getWin(iframe);
diff --git a/extensions/amp-iframe/1.0/test/test-amp-iframe.js b/extensions/amp-iframe/1.0/test/test-amp-iframe.js
index 5728b7ec0463..833c20a0d52c 100644
--- a/extensions/amp-iframe/1.0/test/test-amp-iframe.js
+++ b/extensions/amp-iframe/1.0/test/test-amp-iframe.js
@@ -3,6 +3,7 @@ import {htmlFor} from '#core/dom/static-template';
 import {toggleExperiment} from '#experiments';
 import {waitFor} from '#testing/test-helper';
 import {whenUpgradedToCustomElement} from '#core/dom/amp-element-helpers';
+import {flush} from '#testing/preact';
 
 describes.realWin(
   'amp-iframe-v1.0',
@@ -17,10 +18,8 @@ describes.realWin(
     async function waitRendered() {
       await whenUpgradedToCustomElement(element);
       await element.mount();
-      await waitFor(
-        () => element.shadowRoot.querySelector('iframe'),
-        'iframe rendered'
-      );
+      await flush();
+      await waitFor(() => element.shadowRoot.querySelector('iframe'));
     }
 
     beforeEach(() => {
diff --git a/testing/init-tests.js b/testing/init-tests.js
index 67eb039f5245..f176f27feaec 100644
--- a/testing/init-tests.js
+++ b/testing/init-tests.js
@@ -30,6 +30,7 @@ import {
   warnForConsoleError,
 } from './console-logging-setup';
 import * as describes from './describes';
+import {flush as flushPreactEffects} from './preact';
 import {TestConfig} from './test-config';
 import {installYieldIt} from './yield';
 
@@ -156,8 +157,10 @@ function resetTestingState() {
 
 /**
  * Cleans up global state added during tests.
+ * @return {Promise<void>}
  */
-function cleanupTestcase() {
+async function cleanupTestcase() {
+  await flushPreactEffects();
   setTestRunner(this);
   restoreConsoleSandbox();
   restoreConsoleError();
diff --git a/testing/preact.js b/testing/preact.js
new file mode 100644
index 000000000000..864930b83d14
--- /dev/null
+++ b/testing/preact.js
@@ -0,0 +1,64 @@
+import * as preact from /*OK*/ 'preact';
+
+/**
+ * This file introduces a helper for draining Preact's queue of renders and effects.
+ * We use this as part of the afterEach() cleanup in unit tests, to ensure no effects are run
+ * in subsequent tests.
+ *
+ * There is still a test isolation issue in that an effect can asynchronously schedule work
+ * which cannot be guarded from at this layer. For that we'd likely need to refresh the window
+ * in between each test run.
+ *
+ * We should be able to remove this file if this feature lands in Preact.
+ * @fileoverview
+ */
+
+const rafs = [];
+/**
+ * @param {(ts: (DOMHighResTimeStamp) => number)} cb
+ * @return {number}
+ */
+function flushableRaf(cb) {
+  rafs.push(cb);
+  return requestAnimationFrame(flushRaf);
+}
+
+function flushRaf(ts = performance.now()) {
+  for (const raf of rafs) {
+    raf(ts);
+  }
+  rafs.length = 0;
+}
+
+let pendingRender;
+
+/**
+ * @param {() => void} process
+ * @return {Promise<void>}
+ */
+async function flushableRender(process) {
+  pendingRender = () => {
+    pendingRender = null;
+    return process();
+  };
+  await Promise.resolve().then(pendingRender);
+}
+
+/**
+ * Flushes Preact renders and effects.
+ *
+ * Effects may queue up further rerenders, etc. etc,
+ * so this function will loop until everything to resolves.
+ *
+ * @return {Promise<void>}
+ */
+export async function flush() {
+  flushRaf();
+  while (pendingRender) {
+    await pendingRender();
+    flushRaf();
+  }
+}
+
+preact.options.requestAnimationFrame = flushableRaf;
+preact.options.debounceRendering = flushableRender;