Dark Mode Visual Testing Patterns

1npm install --save-dev @playwright/test
2npx playwright install --with-deps chromium
3
4# Generate initial baseline screenshots:
5npx playwright test tests/visual/ --update-snapshots
6
7# Run visual comparison tests:
8npx playwright test tests/visual/
9
10# On failure, screenshots are in test-results/
11# The diff image shows exactly what changed

1// playwright.config.ts
2export default defineConfig({
3  expect: {
4    toHaveScreenshot: {
5      maxDiffPixelRatio: 0.02,  // 2% pixel difference allowed
6      threshold: 0.2,           // per-pixel color tolerance
7      animations: 'disabled',   // freeze CSS animations
8    },
9  },
10  snapshotDir: './tests/visual/__screenshots__',
11});

1await expect(page).toHaveScreenshot('page.png', {
2  // Mask elements that change between test runs
3  mask: [
4    page.locator('[data-testid="timestamp"]'),
5    page.locator('[data-testid="user-avatar"]'),
6    page.locator('.advertisement'),
7  ],
8  animations: 'disabled',
9});
10
11// Freeze dynamic content before screenshot:
12await page.evaluate(() => {
13  document.querySelectorAll('[data-testid="live-count"]')
14    .forEach(el => el.textContent = '1,234');
15  const style = document.createElement('style');
16  style.textContent = '*, *::before, *::after { transition-duration: 0s !important; }';
17  document.head.appendChild(style);
18});

1- name: Run visual tests
2  run: npx playwright test tests/visual/
3  continue-on-error: true
4
5- name: Upload visual test artifacts
6  uses: actions/upload-artifact@v4
7  if: always()
8  with:
9    name: visual-results
10    path: |
11      playwright-report/
12      test-results/
13    retention-days: 30

1test('dark mode visual consistency', async ({ page }) => {
2  // Light mode baseline
3  await page.goto('/courses');
4  await expect(page).toHaveScreenshot('courses-light.png');
5
6  // Dark mode — emulate prefers-color-scheme: dark
7  await page.emulateMedia({ colorScheme: 'dark' });
8  await page.reload();
9  await expect(page).toHaveScreenshot('courses-dark.png');
10
11  // Verify dark mode is actually applied
12  const bodyBg = await page.evaluate(() =>
13    getComputedStyle(document.body).backgroundColor
14  );
15  // Dark backgrounds have low luminance
16  const [r, g, b] = bodyBg.match(/\d+/g)?.map(Number) || [];
17  const luminance = 0.299 * r + 0.587 * g + 0.114 * b;
18  expect(luminance).toBeLessThan(128); // dark background
19});

1const BREAKPOINTS = [
2  { name: 'mobile', width: 375, height: 812 },
3  { name: 'tablet', width: 768, height: 1024 },
4  { name: 'desktop', width: 1280, height: 800 },
5];
6
7for (const { name, width, height } of BREAKPOINTS) {
8  test(`homepage at ${name}`, async ({ page }) => {
9    await page.setViewportSize({ width, height });
10    await page.goto('/');
11    await expect(page).toHaveScreenshot(`homepage-${name}.png`, {
12      fullPage: true,
13      animations: 'disabled',
14    });
15  });
16}
17
18// Also check for horizontal overflow:
19test('no horizontal scroll on mobile', async ({ page }) => {
20  await page.setViewportSize({ width: 375, height: 812 });
21  await page.goto('/');
22  const hasScroll = await page.evaluate(() =>
23    document.documentElement.scrollWidth > document.documentElement.clientWidth
24  );
25  expect(hasScroll).toBe(false);
26});