app/(app)/create/components/CustomMethodCardWizard/CustomMethodCardWizard.container.tsx

@@ -6,6 +6,7 @@ import {
   useTranslation,
 } from "../../../../contexts/MessagesContext";
 import { useAsyncConfirm } from "../../../../hooks/useAsyncConfirm";
+import { useBeforeUnloadGuard } from "../../../../hooks/useBeforeUnloadGuard";
 import type { CustomMethodCardFieldBlock } from "../../../../../lib/create/customMethodCardFieldBlocks";
 import {
   CUSTOM_METHOD_CARD_WIZARD_MAX_DESCRIPTION_CHARS,
@@ -213,6 +214,8 @@ const CustomMethodCardWizardContainer = memo<CustomMethodCardWizardProps>(
       policyTitle,
     ]);
 
+    useBeforeUnloadGuard(isOpen && isWizardSessionDirty());
+
     const confirmAbandonWizardEdits = useCallback(async () => {
       if (!isWizardSessionDirty()) {
         return true;

app/(app)/create/components/FinalReviewChipEditModal.tsx

@@ -28,6 +28,7 @@ import {
 import CustomMethodCardModalBody from "./CustomMethodCardModalBody";
 import { buildCustomRuleModalKebabMenu } from "./customRuleModalKebabMenu";
 import { useDiscardCustomizeConfirm } from "../hooks/useDiscardCustomizeConfirm";
+import { useBeforeUnloadGuard } from "../../../hooks/useBeforeUnloadGuard";
 import {
   communicationPresetFor,
   conflictManagementPresetFor,
@@ -370,6 +371,12 @@ export function FinalReviewChipEditModal({
     draftFieldBlocks,
   ]);
 
+  useBeforeUnloadGuard(
+    isOpen &&
+      !addCustomWizardOpen &&
+      (!coreCustomizeSaveDisabled || !methodCustomizeSaveDisabled),
+  );
+
   const modalUsesWizardFieldBlocksBody = Boolean(
     target &&
       (usesWizardFieldBlocksModalBody({

app/(app)/create/components/FinalReviewCommunityContextEditModal.tsx

@@ -6,6 +6,7 @@
  */
 
 import { useEffect, useMemo, useRef, useState } from "react";
+import { useBeforeUnloadGuard } from "../../../hooks/useBeforeUnloadGuard";
 import Create from "../../../components/modals/Create";
 import TextInput from "../../../components/controls/TextInput";
 import ContentLockup from "../../../components/type/ContentLockup";
@@ -57,6 +58,8 @@ export function FinalReviewCommunityContextEditModal({
     [draft],
   );
 
+  useBeforeUnloadGuard(isOpen && isDirty);
+
   const characterHint = tField("characterCountTemplate")
     .replace("{current}", String(draft.length))
     .replace("{max}", String(COMMUNITY_CONTEXT_FIELD_MAX_LENGTH));

app/(app)/create/screens/card/CommunicationMethodsScreen.tsx

@@ -19,6 +19,7 @@ import { useState, useCallback, useMemo, useRef } from "react";
 import { useMessages } from "../../../../contexts/MessagesContext";
 import { useCreateFlow } from "../../context/CreateFlowContext";
 import { useCreateFlowMdUp } from "../../hooks/useCreateFlowMdUp";
+import { useBeforeUnloadGuard } from "../../../../hooks/useBeforeUnloadGuard";
 import { useDiscardCustomizeConfirm } from "../../hooks/useDiscardCustomizeConfirm";
 import { useMethodCardDeckOrdering } from "../../hooks/useMethodCardDeckOrdering";
 import { CreateFlowHeaderLockup } from "../../components/CreateFlowHeaderLockup";
@@ -57,6 +58,7 @@ import { buildMethodCardWizardInitialValues } from "../../../../../lib/create/me
 import type { MethodCardWizardInitialValues } from "../../../../../lib/create/methodCardWizardPrefill";
 import {
   captureMethodCardCustomizeSnapshot,
+  isMethodCardCustomizeUnloadBlocked,
   type MethodCardCustomizeSnapshot,
   type MethodCardHeaderDraft,
 } from "../../../../../lib/create/methodCardCustomizeSession";
@@ -88,6 +90,16 @@ export function CommunicationMethodsScreen() {
     CustomMethodCardFieldBlock[] | null
   >(null);
 
+  useBeforeUnloadGuard(
+    isMethodCardCustomizeUnloadBlocked(
+      createModalOpen,
+      customizeSnapshotRef.current,
+      pendingDraft,
+      draftFieldBlocks,
+      customizeSnapshotRef.current?.headerDraft ?? null,
+    ),
+  );
+
   const selectedIds = state.selectedCommunicationMethodIds ?? [];
 
   const mergedMethods = useMemo(

app/(app)/create/screens/card/ConflictManagementScreen.tsx

@@ -16,6 +16,7 @@ import { useState, useCallback, useMemo, useRef } from "react";
 import { useMessages } from "../../../../contexts/MessagesContext";
 import { useCreateFlow } from "../../context/CreateFlowContext";
 import { useCreateFlowMdUp } from "../../hooks/useCreateFlowMdUp";
+import { useBeforeUnloadGuard } from "../../../../hooks/useBeforeUnloadGuard";
 import { useDiscardCustomizeConfirm } from "../../hooks/useDiscardCustomizeConfirm";
 import { useMethodCardDeckOrdering } from "../../hooks/useMethodCardDeckOrdering";
 import { CreateFlowHeaderLockup } from "../../components/CreateFlowHeaderLockup";
@@ -54,6 +55,7 @@ import { buildMethodCardWizardInitialValues } from "../../../../../lib/create/me
 import type { MethodCardWizardInitialValues } from "../../../../../lib/create/methodCardWizardPrefill";
 import {
   captureMethodCardCustomizeSnapshot,
+  isMethodCardCustomizeUnloadBlocked,
   type MethodCardCustomizeSnapshot,
   type MethodCardHeaderDraft,
 } from "../../../../../lib/create/methodCardCustomizeSession";
@@ -85,6 +87,16 @@ export function ConflictManagementScreen() {
     CustomMethodCardFieldBlock[] | null
   >(null);
 
+  useBeforeUnloadGuard(
+    isMethodCardCustomizeUnloadBlocked(
+      createModalOpen,
+      customizeSnapshotRef.current,
+      pendingDraft,
+      draftFieldBlocks,
+      customizeSnapshotRef.current?.headerDraft ?? null,
+    ),
+  );
+
   const selectedIds = state.selectedConflictManagementIds ?? [];
 
   const mergedMethods = useMemo(

app/(app)/create/screens/card/MembershipMethodsScreen.tsx

@@ -17,6 +17,7 @@ import { useState, useCallback, useMemo, useRef } from "react";
 import { useMessages } from "../../../../contexts/MessagesContext";
 import { useCreateFlow } from "../../context/CreateFlowContext";
 import { useCreateFlowMdUp } from "../../hooks/useCreateFlowMdUp";
+import { useBeforeUnloadGuard } from "../../../../hooks/useBeforeUnloadGuard";
 import { useDiscardCustomizeConfirm } from "../../hooks/useDiscardCustomizeConfirm";
 import { useMethodCardDeckOrdering } from "../../hooks/useMethodCardDeckOrdering";
 import { CreateFlowHeaderLockup } from "../../components/CreateFlowHeaderLockup";
@@ -55,6 +56,7 @@ import { buildMethodCardWizardInitialValues } from "../../../../../lib/create/me
 import type { MethodCardWizardInitialValues } from "../../../../../lib/create/methodCardWizardPrefill";
 import {
   captureMethodCardCustomizeSnapshot,
+  isMethodCardCustomizeUnloadBlocked,
   type MethodCardCustomizeSnapshot,
   type MethodCardHeaderDraft,
 } from "../../../../../lib/create/methodCardCustomizeSession";
@@ -86,6 +88,16 @@ export function MembershipMethodsScreen() {
     CustomMethodCardFieldBlock[] | null
   >(null);
 
+  useBeforeUnloadGuard(
+    isMethodCardCustomizeUnloadBlocked(
+      createModalOpen,
+      customizeSnapshotRef.current,
+      pendingDraft,
+      draftFieldBlocks,
+      customizeSnapshotRef.current?.headerDraft ?? null,
+    ),
+  );
+
   const selectedIds = state.selectedMembershipMethodIds ?? [];
 
   const mergedMethods = useMemo(

app/(app)/create/screens/right-rail/DecisionApproachesScreen.tsx

@@ -26,6 +26,7 @@ import type { InfoMessageBoxItem } from "../../../../components/controls/InfoMes
 import { useMessages } from "../../../../contexts/MessagesContext";
 import { useCreateFlow } from "../../context/CreateFlowContext";
 import { useCreateFlowMdUp } from "../../hooks/useCreateFlowMdUp";
+import { useBeforeUnloadGuard } from "../../../../hooks/useBeforeUnloadGuard";
 import { useDiscardCustomizeConfirm } from "../../hooks/useDiscardCustomizeConfirm";
 import { useMethodCardDeckOrdering } from "../../hooks/useMethodCardDeckOrdering";
 import { CreateFlowTwoColumnSelectShell } from "../../components/CreateFlowTwoColumnSelectShell";
@@ -60,6 +61,7 @@ import { buildMethodCardWizardInitialValues } from "../../../../../lib/create/me
 import type { MethodCardWizardInitialValues } from "../../../../../lib/create/methodCardWizardPrefill";
 import {
   captureMethodCardCustomizeSnapshot,
+  isMethodCardCustomizeUnloadBlocked,
   type MethodCardCustomizeSnapshot,
   type MethodCardHeaderDraft,
 } from "../../../../../lib/create/methodCardCustomizeSession";
@@ -91,6 +93,16 @@ export function DecisionApproachesScreen() {
     CustomMethodCardFieldBlock[] | null
   >(null);
 
+  useBeforeUnloadGuard(
+    isMethodCardCustomizeUnloadBlocked(
+      createModalOpen,
+      customizeSnapshotRef.current,
+      pendingDraft,
+      draftFieldBlocks,
+      customizeSnapshotRef.current?.headerDraft ?? null,
+    ),
+  );
+
   const selectedIds = state.selectedDecisionApproachIds ?? [];
   const messageBoxCheckedIds = decisionApproachKeyResourceCheckboxIds({
     detailsById: state.decisionApproachDetailsById,

app/(app)/create/screens/select/CoreValuesSelectScreen.tsx

@@ -9,6 +9,7 @@ import { useMessages } from "../../../../contexts/MessagesContext";
 import { buildCoreValueChipOptionsFromDraft } from "../../../../../lib/create/coreValueChipOptionsFromDraft";
 import { useCreateFlow } from "../../context/CreateFlowContext";
 import { useAsyncConfirm } from "../../../../hooks/useAsyncConfirm";
+import { useBeforeUnloadGuard } from "../../../../hooks/useBeforeUnloadGuard";
 import type {
   CommunityStructureChipSnapshotRow,
   CoreValueDetailEntry,
@@ -257,6 +258,14 @@ export function CoreValuesSelectScreen() {
     });
   }, [cv.detailModal, draft, modalSession, requestConfirm]);
 
+  useBeforeUnloadGuard(
+    activeModalChipId != null &&
+      !addCustomWizardOpen &&
+      initialDraftRef.current != null &&
+      (draft.meaning !== initialDraftRef.current.meaning ||
+        draft.signals !== initialDraftRef.current.signals),
+  );
+
   const handleDuplicateCoreChip = useCallback(() => {
     if (!activeModalChipId || !modalSession) return;
     markCreateFlowInteraction();

app/hooks/index.ts

@@ -18,6 +18,7 @@ export { useMediaQuery,
 } from "./useMediaQuery";
 export { useAsyncConfirm } from "./useAsyncConfirm";
 export type { AsyncConfirmOptions } from "./useAsyncConfirm";
+export { useBeforeUnloadGuard } from "./useBeforeUnloadGuard";
 export type {
   SchemaOrganization,
   SchemaWebSite,

app/hooks/useBeforeUnloadGuard.ts

@@ -0,0 +1,33 @@
+"use client";
+
+import { useEffect } from "react";
+
+/**
+ * Attach the browser’s native leave-site prompt while `enabled` is true.
+ *
+ * Modern browsers ignore custom copy; this only blocks accidental tab or
+ * window close. No-ops during SSR and when `enabled` is false so clean
+ * sessions never register a listener.
+ *
+ * @param enabled Whether unsaved work would be lost on unload.
+ *
+ * @example
+ * useBeforeUnloadGuard(isWizardSessionDirty());
+ */
+export function useBeforeUnloadGuard(enabled: boolean): void {
+  useEffect(() => {
+    if (!enabled || typeof window === "undefined") {
+      return;
+    }
+
+    const onBeforeUnload = (event: BeforeUnloadEvent) => {
+      event.preventDefault();
+      event.returnValue = "";
+    };
+
+    window.addEventListener("beforeunload", onBeforeUnload);
+    return () => {
+      window.removeEventListener("beforeunload", onBeforeUnload);
+    };
+  }, [enabled]);
+}

lib/create/methodCardCustomizeSession.ts

@@ -25,6 +25,28 @@ export function captureMethodCardCustomizeSnapshot<TDraft>(
   };
 }
 
+/**
+ * True when a method-card create/customize modal is open with edits that
+ * are not yet persisted — the condition for a tab/window `beforeunload` guard.
+ */
+export function isMethodCardCustomizeUnloadBlocked<TDraft>(
+  modalOpen: boolean,
+  snapshot: MethodCardCustomizeSnapshot<TDraft> | null,
+  pendingDraft: TDraft | null,
+  draftFieldBlocks: CustomMethodCardFieldBlock[] | null,
+  headerDraft: MethodCardHeaderDraft | null,
+): boolean {
+  if (!modalOpen || snapshot === null) {
+    return false;
+  }
+  return isMethodCardCustomizeSessionDirty(
+    snapshot,
+    pendingDraft,
+    draftFieldBlocks,
+    headerDraft,
+  );
+}
+
 export function isMethodCardCustomizeSessionDirty<TDraft>(
   snapshot: MethodCardCustomizeSnapshot<TDraft>,
   pendingDraft: TDraft | null,

tests/components/CommunicationMethodsScreenPersistence.test.tsx

@@ -6,6 +6,7 @@ import {
   cleanup,
   within,
   waitFor,
+  dispatchBeforeUnload,
 } from "../utils/test-utils";
 import { fireEvent } from "@testing-library/react";
 import "@testing-library/jest-dom/vitest";
@@ -576,4 +577,62 @@ describe("CommunicationMethodsScreen — Add Platform persistence", () => {
     expect(labels[1]).toMatch(/Code of Conduct/);
     expect(labels[2]).toMatch(/Core Principle/);
   });
+
+  it("does not block tab close when the create modal is unchanged", async () => {
+    render(
+      <ScreenWithStateProbe
+        onState={() => {
+          /* noop */
+        }}
+      />,
+    );
+
+    fireEvent.click(
+      screen.getAllByRole("button", { name: /Signal: Encrypted messaging/ })[0],
+    );
+    await screen.findByRole("dialog");
+    expect(dispatchBeforeUnload()).toBe(false);
+  });
+
+  it("blocks tab close while the create modal has unsaved field edits", async () => {
+    render(
+      <ScreenWithStateProbe
+        onState={() => {
+          /* noop */
+        }}
+      />,
+    );
+
+    fireEvent.click(
+      screen.getAllByRole("button", { name: /Signal: Encrypted messaging/ })[0],
+    );
+    const dialog = await screen.findByRole("dialog");
+    const textboxes = within(dialog).getAllByRole(
+      "textbox",
+    ) as HTMLTextAreaElement[];
+    fireEvent.change(textboxes[0], { target: { value: "Unsaved principle" } });
+    expect(dispatchBeforeUnload()).toBe(true);
+  });
+
+  it("blocks tab close while the custom-policy wizard has unsaved edits", async () => {
+    render(
+      <ScreenWithStateProbe
+        onState={() => {
+          /* noop */
+        }}
+      />,
+    );
+
+    fireEvent.click(
+      screen.getAllByRole("button", { name: /Signal: Encrypted messaging/ })[0],
+    );
+    const dialog = await screen.findByRole("dialog");
+    fireEvent.click(within(dialog).getByRole("button", { name: "More options" }));
+    fireEvent.click(screen.getByRole("menuitem", { name: "Customize" }));
+
+    const nameInput = await screen.findByPlaceholderText("Policy name");
+    expect(dispatchBeforeUnload()).toBe(false);
+    fireEvent.change(nameInput, { target: { value: "Renamed in wizard" } });
+    expect(dispatchBeforeUnload()).toBe(true);
+  });
 });

tests/components/CoreValuesSelectScreen.test.tsx

@@ -1,7 +1,7 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { screen, fireEvent, waitFor, within } from "@testing-library/react";
 import "@testing-library/jest-dom/vitest";
-import { renderWithProviders } from "../utils/test-utils";
+import { renderWithProviders, dispatchBeforeUnload } from "../utils/test-utils";
 import { CoreValuesSelectScreen } from "../../app/(app)/create/screens/select/CoreValuesSelectScreen";
 
 describe("CoreValuesSelectScreen", () => {
@@ -65,6 +65,20 @@ describe("CoreValuesSelectScreen", () => {
     });
   });
 
+  it("does not block tab close when a pending value is unchanged", async () => {
+    renderWithProviders(<CoreValuesSelectScreen />);
+    fireEvent.click(screen.getByText("Accessibility"));
+    await screen.findByRole("dialog");
+    expect(dispatchBeforeUnload()).toBe(false);
+  });
+
+  it("blocks tab close after editing a pending value", async () => {
+    renderWithProviders(<CoreValuesSelectScreen />);
+    fireEvent.click(screen.getByText("Accessibility"));
+    await editMeaningInOpenDialog("Changed meaning");
+    expect(dispatchBeforeUnload()).toBe(true);
+  });
+
   it("keeps the pending value modal open when Keep editing is chosen", async () => {
     renderWithProviders(<CoreValuesSelectScreen />);
     fireEvent.click(screen.getByText("Accessibility"));

tests/components/CustomMethodCardWizardUnload.test.tsx

@@ -0,0 +1,44 @@
+import { describe, it, expect } from "vitest";
+import {
+  renderWithProviders as render,
+  screen,
+  fireEvent,
+  dispatchBeforeUnload,
+} from "../utils/test-utils";
+import "@testing-library/jest-dom/vitest";
+import CustomMethodCardWizard from "../../app/(app)/create/components/CustomMethodCardWizard";
+
+describe("CustomMethodCardWizard — tab close guard", () => {
+  it("does not block unload when the wizard is open but unchanged", async () => {
+    render(
+      <CustomMethodCardWizard
+        isOpen
+        onClose={() => {
+          /* noop */
+        }}
+        onFinalize={() => {
+          /* noop */
+        }}
+      />,
+    );
+    await screen.findByPlaceholderText("Policy name");
+    expect(dispatchBeforeUnload()).toBe(false);
+  });
+
+  it("blocks unload after the user types a policy name", async () => {
+    render(
+      <CustomMethodCardWizard
+        isOpen
+        onClose={() => {
+          /* noop */
+        }}
+        onFinalize={() => {
+          /* noop */
+        }}
+      />,
+    );
+    const name = await screen.findByPlaceholderText("Policy name");
+    fireEvent.change(name, { target: { value: "Garden hours" } });
+    expect(dispatchBeforeUnload()).toBe(true);
+  });
+});

tests/components/FinalReviewPage.test.tsx

@@ -5,6 +5,7 @@ import {
   renderWithProviders as render,
   screen,
   waitFor,
+  dispatchBeforeUnload,
 } from "../utils/test-utils";
 import "@testing-library/jest-dom/vitest";
 import { FinalReviewScreen } from "../../app/(app)/create/screens/review/FinalReviewScreen";
@@ -623,6 +624,28 @@ describe("FinalReviewScreen — chip edit modal save semantics", () => {
     expect(latest.communicationMethodDetailsById).toBeUndefined();
   });
 
+  it("blocks tab close while chip edits are unsaved", async () => {
+    render(
+      <FinalReviewWithStateProbe
+        onState={() => {
+          /* noop */
+        }}
+        initial={baseSelections}
+      />,
+    );
+
+    fireEvent.click(await screen.findByRole("button", { name: "Signal" }));
+    const dialog = await screen.findByRole("dialog");
+    expect(dispatchBeforeUnload()).toBe(false);
+    const principleField = within(dialog).getByRole("textbox", {
+      name: /core principle/i,
+    });
+    fireEvent.change(principleField, {
+      target: { value: "Unsaved on tab close" },
+    });
+    expect(dispatchBeforeUnload()).toBe(true);
+  });
+
   it("shows consolidated placeholder for user-authored communication chips", async () => {
     const customId = "550e8400-e29b-41d4-a716-446655440000";
     render(

tests/lib/methodCardCustomizeSession.test.ts

@@ -4,6 +4,7 @@ import {
   captureMethodCardCustomizeSnapshot,
   confirmDiscardMethodCardCustomizeSession,
   isMethodCardCustomizeSessionDirty,
+  isMethodCardCustomizeUnloadBlocked,
 } from "../../lib/create/methodCardCustomizeSession";
 
 const HEADER_0 = { title: "", description: "" };
@@ -88,4 +89,24 @@ describe("methodCardCustomizeSession", () => {
     ).toBe(false);
     expect(confirmFn).toHaveBeenCalled();
   });
+
+  it("unload block is false when the modal is closed or snapshot is missing", () => {
+    const snap = captureMethodCardCustomizeSnapshot({ x: 1 }, null, HEADER_0);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(false, snap, { x: 2 }, null, HEADER_0),
+    ).toBe(false);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(true, null, { x: 2 }, null, HEADER_0),
+    ).toBe(false);
+  });
+
+  it("unload block is true only when the open modal is dirty", () => {
+    const snap = captureMethodCardCustomizeSnapshot({ x: 1 }, null, HEADER_0);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(true, snap, { x: 1 }, null, HEADER_0),
+    ).toBe(false);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(true, snap, { x: 2 }, null, HEADER_0),
+    ).toBe(true);
+  });
 });

tests/unit/hooks/useBeforeUnloadGuard.test.ts

@@ -0,0 +1,26 @@
+import { renderHook } from "@testing-library/react";
+import { describe, it, expect } from "vitest";
+import { useBeforeUnloadGuard } from "../../../app/hooks/useBeforeUnloadGuard";
+import { dispatchBeforeUnload } from "../../utils/test-utils";
+
+describe("useBeforeUnloadGuard", () => {
+  it("does not intercept unload when disabled", () => {
+    renderHook(() => useBeforeUnloadGuard(false));
+    expect(dispatchBeforeUnload()).toBe(false);
+  });
+
+  it("prevents unload when enabled", () => {
+    renderHook(() => useBeforeUnloadGuard(true));
+    expect(dispatchBeforeUnload()).toBe(true);
+  });
+
+  it("drops the listener when enabled flips to false", () => {
+    const { rerender } = renderHook(
+      ({ enabled }) => useBeforeUnloadGuard(enabled),
+      { initialProps: { enabled: true } },
+    );
+    expect(dispatchBeforeUnload()).toBe(true);
+    rerender({ enabled: false });
+    expect(dispatchBeforeUnload()).toBe(false);
+  });
+});

tests/unit/methodCardCustomizeUnloadBlocked.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+import {
+  captureMethodCardCustomizeSnapshot,
+  isMethodCardCustomizeUnloadBlocked,
+} from "../../lib/create/methodCardCustomizeSession";
+
+const HEADER_0 = { title: "", description: "" };
+
+describe("isMethodCardCustomizeUnloadBlocked", () => {
+  it("is false when the modal is closed or snapshot is missing", () => {
+    const snap = captureMethodCardCustomizeSnapshot({ x: 1 }, null, HEADER_0);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(false, snap, { x: 2 }, null, HEADER_0),
+    ).toBe(false);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(true, null, { x: 2 }, null, HEADER_0),
+    ).toBe(false);
+  });
+
+  it("is true only when the open modal is dirty", () => {
+    const snap = captureMethodCardCustomizeSnapshot({ x: 1 }, null, HEADER_0);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(true, snap, { x: 1 }, null, HEADER_0),
+    ).toBe(false);
+    expect(
+      isMethodCardCustomizeUnloadBlocked(true, snap, { x: 2 }, null, HEADER_0),
+    ).toBe(true);
+  });
+});

tests/utils/test-utils.tsx

@@ -25,5 +25,12 @@ export function renderWithProviders(
   return render(ui, { wrapper: Wrapper, ...options });
 }
 
+/** True when a `beforeunload` listener called `preventDefault` (tab-close guard). */
+export function dispatchBeforeUnload(): boolean {
+  const event = new Event("beforeunload", { cancelable: true });
+  window.dispatchEvent(event);
+  return event.defaultPrevented;
+}
+
 // Re-export everything from @testing-library/react for convenience
 export * from "@testing-library/react";