diff --git a/web/src/App.test.tsx b/web/src/App.test.tsx
index 61259f6d4..810e62080 100644
--- a/web/src/App.test.tsx
+++ b/web/src/App.test.tsx
@@ -1,4 +1,4 @@
-import { cleanup, fireEvent, render, screen, waitFor, within } from '@testing-library/react';
+import { act, cleanup, fireEvent, render, screen, waitFor, within } from '@testing-library/react';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { App } from './App';
 import { Button, Checkbox, Input, ProgressBar, Switch, Tabs, Toast, Tooltip } from './components';
@@ -1274,19 +1274,17 @@ describe('ChicoryTV SPA scaffold', () => {
     expect(fetchCount('/api/libraries/scan-status')).toBe(1);
 
     const sourceFetchesBeforePoll = fetchCount('/api/media-sources');
-    intervalHandlers.forEach((handler) => handler());
+    await runPollTick(intervalHandlers);
 
-    await waitFor(() => {
-      expect(fetchCount('/api/libraries/scan-status')).toBeGreaterThan(1);
-    });
+    expect(fetchCount('/api/libraries/scan-status')).toBeGreaterThan(1);
     expect(fetchCount('/api/media-sources')).toBe(sourceFetchesBeforePoll);
   });
 
   it('stops Libraries scan polling and refreshes sources once when scans complete', async () => {
     const intervalHandlers: Array<() => void> = [];
     let clearCount = 0;
-    vi.spyOn(window, 'setInterval').mockImplementation((handler: TimerHandler) => {
-      if (typeof handler === 'function') {
+    vi.spyOn(window, 'setInterval').mockImplementation((handler: TimerHandler, timeout?: number) => {
+      if (typeof handler === 'function' && (timeout ?? 0) >= 10000) {
         intervalHandlers.push(handler as () => void);
       }
 
@@ -1311,11 +1309,9 @@ describe('ChicoryTV SPA scaffold', () => {
     expect(await screen.findByText('75%')).toBeInTheDocument();
 
     const sourceFetchesBeforeCompletion = fetchCount('/api/media-sources');
-    intervalHandlers.forEach((handler) => handler());
+    await runPollTick(intervalHandlers);
 
-    await waitFor(() => {
-      expect(screen.queryByText('75%')).not.toBeInTheDocument();
-    });
+    expect(screen.queryByText('75%')).not.toBeInTheDocument();
     expect(fetchCount('/api/media-sources')).toBe(sourceFetchesBeforeCompletion + 1);
     expect(clearCount).toBeGreaterThan(0);
   });
@@ -1390,14 +1386,12 @@ describe('ChicoryTV SPA scaffold', () => {
 
     const sourceFetchesBeforeCompletion = fetchCount('/api/media-sources');
 
-    intervalHandlers.forEach((handler) => handler());
-    expect(await screen.findByText('30%')).toBeInTheDocument();
+    await runPollTick(intervalHandlers);
+    expect(screen.getByText('30%')).toBeInTheDocument();
     expect(screen.getByRole('button', { name: 'Scan Movies' })).toBeDisabled();
 
-    intervalHandlers.forEach((handler) => handler());
-    await waitFor(() => {
-      expect(screen.queryByText('30%')).not.toBeInTheDocument();
-    });
+    await runPollTick(intervalHandlers);
+    expect(screen.queryByText('30%')).not.toBeInTheDocument();
     expect(screen.getByRole('button', { name: 'Scan Movies' })).not.toBeDisabled();
     expect(fetchCount('/api/media-sources')).toBe(sourceFetchesBeforeCompletion + 1);
     expect(clearCount).toBeGreaterThan(0);
@@ -1440,17 +1434,13 @@ describe('ChicoryTV SPA scaffold', () => {
     expect(intervalHandlers.length).toBeGreaterThan(0);
 
     // The id never shows up in scan-status. It survives a couple of ticks...
-    intervalHandlers.forEach((handler) => handler());
-    await waitFor(() => {
-      expect(fetchCount('/api/libraries/scan-status')).toBe(3);
-    });
+    await runPollTick(intervalHandlers);
+    expect(fetchCount('/api/libraries/scan-status')).toBe(3);
     expect(screen.getByRole('button', { name: 'Scan Movies' })).toBeDisabled();
 
     // ...but expires once the grace window runs out, freeing the button and the poll.
-    intervalHandlers.forEach((handler) => handler());
-    await waitFor(() => {
-      expect(screen.getByRole('button', { name: 'Scan Movies' })).not.toBeDisabled();
-    });
+    await runPollTick(intervalHandlers);
+    expect(screen.getByRole('button', { name: 'Scan Movies' })).not.toBeDisabled();
     expect(clearCount).toBeGreaterThan(0);
   });
 
@@ -1496,18 +1486,14 @@ describe('ChicoryTV SPA scaffold', () => {
     expect(intervalHandlers.length).toBeGreaterThan(0);
 
     // scan-status keeps erroring on every poll tick...
-    intervalHandlers.forEach((handler) => handler());
-    await waitFor(() => {
-      expect(fetchCount('/api/libraries/scan-status')).toBe(3);
-    });
+    await runPollTick(intervalHandlers);
+    expect(fetchCount('/api/libraries/scan-status')).toBe(3);
     expect(screen.getByRole('button', { name: 'Scan Movies' })).toBeDisabled();
 
     // ...but the same grace budget burns down on failures too, so persistent failure
     // eventually frees the button and stops the interval instead of polling forever.
-    intervalHandlers.forEach((handler) => handler());
-    await waitFor(() => {
-      expect(screen.getByRole('button', { name: 'Scan Movies' })).not.toBeDisabled();
-    });
+    await runPollTick(intervalHandlers);
+    expect(screen.getByRole('button', { name: 'Scan Movies' })).not.toBeDisabled();
     expect(clearCount).toBeGreaterThan(0);
   });
 
@@ -3833,6 +3819,24 @@ function mockDashboardApi({
   });
 }
 
+// Deterministically fires one captured poll tick for the mocked-setInterval polling tests.
+//
+// The Libraries poll interval is registered in a passive useEffect that only runs after
+// an out-of-act state commit (the async fetch that flips hasActiveScans), so `findBy*`
+// queries - which resolve on the DOM mutation, via MutationObserver - can win the race
+// against React's effect flush and observe an empty handler list (the #157 flake).
+// The leading empty act() flushes those pending effects, the length assertion turns a
+// still-missing registration into a clear failure instead of a silent no-op forEach,
+// and the act() around the handlers applies the fetch microtasks + state updates before
+// the caller asserts, so no assertion depends on wall-clock scheduling.
+async function runPollTick(intervalHandlers: Array<() => void>): Promise<void> {
+  await act(async () => {});
+  expect(intervalHandlers.length).toBeGreaterThan(0);
+  await act(async () => {
+    intervalHandlers.forEach((handler) => handler());
+  });
+}
+
 function fetchCount(path: string): number {
   return vi.mocked(window.fetch).mock.calls.filter(([input]) => input.toString() === path).length;
 }