diff --git a/packages/app/e2e/browser/developer-ui.spec.ts b/packages/app/e2e/browser/developer-ui.spec.ts
new file mode 100644
index 0000000000..7ea2fc2c95
--- /dev/null
+++ b/packages/app/e2e/browser/developer-ui.spec.ts
@@ -0,0 +1,58 @@
+import { metroTest as test, expect } from "../support/fixtures";
+import { installDeveloperToastDismissal } from "../support/developer-ui.mjs";
+import type { Page } from "@playwright/test";
+
+async function showTransientDeveloperToast(page: Page, shadow: boolean) {
+  await page.evaluate((useShadow) => {
+    const toast = document.createElement("div");
+    toast.id = "error-toast";
+    document.body.append(toast);
+    const root = useShadow ? toast.attachShadow({ mode: "open" }) : toast;
+    const button = document.createElement("button");
+    button.style.cssText = "width:32px;height:32px";
+    button.addEventListener("click", () => button.remove());
+    root.append(button);
+  }, shadow);
+}
+
+async function openTransientDeveloperToast(page: Page) {
+  await page.setContent("<button>Continue</button>");
+  await showTransientDeveloperToast(page, false);
+  await installDeveloperToastDismissal(page);
+}
+
+async function navigateWithDeveloperToastDismissal(page: Page) {
+  await installDeveloperToastDismissal(page);
+  await page.goto("data:text/html,<button>Continue</button>");
+  await showTransientDeveloperToast(page, true);
+}
+
+async function inspectLayoutWhileToastDisappears(page: Page) {
+  const action = page.getByRole("button", { name: "Continue", exact: true });
+  const boxes = await Promise.all(Array.from({ length: 4 }, () => action.boundingBox()));
+  for (const box of boxes) expect(box).not.toBeNull();
+  await expect(page.locator("#error-toast").getByRole("button")).toHaveCount(0);
+  await action.click();
+}
+
+test("a disappearing developer toast does not stall concurrent layout checks", async ({ page }) => {
+  test.setTimeout(10000);
+  await openTransientDeveloperToast(page);
+  await inspectLayoutWhileToastDisappears(page);
+});
+
+test("developer toast dismissal survives navigation and observes Expo shadow-root updates", async ({
+  page,
+}) => {
+  test.setTimeout(10000);
+  await navigateWithDeveloperToastDismissal(page);
+  await inspectLayoutWhileToastDisappears(page);
+  await page.evaluate(() => {
+    const root = document.getElementById("error-toast")!.shadowRoot!;
+    const replacement = document.createElement("button");
+    replacement.style.cssText = "width:32px;height:32px";
+    replacement.addEventListener("click", () => replacement.remove());
+    root.append(replacement);
+  });
+  await inspectLayoutWhileToastDisappears(page);
+});
diff --git a/packages/app/e2e/support/developer-ui.mjs b/packages/app/e2e/support/developer-ui.mjs
index 36d5752688..456f0d7088 100644
--- a/packages/app/e2e/support/developer-ui.mjs
+++ b/packages/app/e2e/support/developer-ui.mjs
@@ -1,10 +1,27 @@
 /** @param {import('@playwright/test').Page} page */
 export async function installDeveloperToastDismissal(page) {
-  // Developer chrome must not cover app actions or move the test's pointer/focus.
-  // Console errors remain in the runner's logs.
-  const dismiss = page.locator("#error-toast").getByRole("button", { name: "", exact: true });
-  await page.addLocatorHandler(dismiss, () => dismiss.dispatchEvent("click"), {
-    // A new logged error can replace the toast before the old one disappears.
-    noWaitAfter: true,
-  });
+  // Keep developer chrome out of app actions without pointer/focus changes.
+  // Locator handlers can deadlock when concurrent checks dismiss the same toast.
+  const install = () => {
+    let observedRoot;
+    const options = { childList: true, subtree: true };
+    const dismiss = () => {
+      const toast = document.getElementById("error-toast");
+      const root = toast?.shadowRoot ?? toast;
+      if (root && root !== observedRoot) {
+        observer.disconnect();
+        observer.observe(document, options);
+        observer.observe(root, options);
+        observedRoot = root;
+      }
+      // Expo's toast root contains dismiss buttons; its overlay has a different ID.
+      for (const button of root?.querySelectorAll("button") ?? [])
+        button.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    };
+    const observer = new MutationObserver(dismiss);
+    observer.observe(document, options);
+    dismiss();
+  };
+  await page.addInitScript(install);
+  await page.evaluate(install);
 }