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If Loki screenshot tests are flaky, first make each story’s ready state explicit and stop uncontrolled motion. Then run baseline creation and CI in the same pinned browser and operating-system environment, choose a diff engine using real examples, and review every baseline change. Retries and looser tolerances cannot make an unpredictable page deterministic.
1. Make story readiness explicit
A story that fetches data, computes state on mount, or re-renders after an event can be captured before its final UI appears. Loki’s Handling Flaky Tests guide describes an async callback pattern for signaling that work is complete. Use that signal in stories with asynchronous work instead of guessing at a sleep duration.
Loki handles most network traffic and image loading, but other asynchronous work may need an explicit completion signal. Identify what the component is waiting for, then signal completion only after the state represented by the screenshot is ready. A fixed delay can be too short on a slow run and needlessly long on a fast one.
2. Remove motion and changing content
Loki disables common CSS transitions and animations, as well as requestAnimationFrame, by default. That does not freeze every animated source: the documentation notes limits such as looping requestAnimationFrame, GIF, animated SVG, Lottie, and React Native Animated content.
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For animation outside Loki’s automatic controls, provide a stable test-only state or disable the animation in the story. Loki documents both a context/decorator approach and Loki-targeted CSS. Choose a representative still state; if an animation cannot meaningfully be represented by one frame, selectively skip that story rather than accepting unexplained image changes.
Also fix other changing inputs in the fixture—such as data or state that differs between runs—so the story has a stable visual result. The goal is not to hide legitimate UI changes, but to make the same input produce the same capture.
3. Pin the capture environment
Keep baseline creation and test runs on the same operating-system image, browser build, headless mode, viewport, fonts, and relevant browser settings. Playwright’s visual comparisons guidance warns that rendering can vary with host OS, browser version, settings, hardware, power source, and headless mode. A difference caused by changing machines is still a difference to the test, even when application code is unchanged.
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Loki supports Docker image selection from its CLI, named targets, and viewport dimensions in configuration. Its documentation includes a versioned Docker image example; check that example against the installed Loki and browser versions instead of copying an old image tag blindly. Use the same pinned capture setup both to create references and to compare CI screenshots.
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Loki fails a test by default when the story makes a network request that fails. Investigate the request and fixture first. Set fetchFailIgnore only for a deliberately understood URL pattern; using it to suppress an unexplained failure can turn a broken or incomplete story into a misleading screenshot.
Check chromeSelector when a capture contains unexpected content. Loki describes this option as cropping to the selector’s dimensions, not taking a DOM-only screenshot. Absolutely positioned elements above that area may therefore still appear in the captured image.
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5. Choose a diff engine with representative screenshots
Loki documents three diff engines. Their sensitivity, tolerance behavior, speed, and dependencies differ; test them against known intentional changes and known noise before changing engines or thresholds.
| Engine | Documented trade-off | When to evaluate it |
|---|---|---|
pixelmatch |
More sensitive to changes on large images and less susceptible to anti-alias flakiness. | Use representative large captures and check that expected UI changes remain detectable. |
gm (GraphicsMagick) |
Generally faster, but requires GraphicsMagick; tolerance is based on the overall image. | Consider it when runtime matters and the external dependency is acceptable. |
looks-same |
JavaScript-only and slower; measures tolerance across neighboring pixels, which may help with differing pixel densities. | Evaluate it if pixel-density differences are relevant and its runtime trade-off is acceptable. |
These trade-offs are described in Loki’s configuration documentation, which displays an August 27, 2024 update date. There is no universally safe tolerance in that guidance: choose one from your own diffs and confirm that representative real UI changes still fail the comparison.
6. Use retries and tolerance narrowly
Loki’s CLI documentation lists chromeRetries with a default of zero, chromeLoadTimeout with a default of 60,000 milliseconds, and chromeTolerance with a default of zero. Tolerance behavior depends on the selected diff engine. Confirm these flags and defaults against the Loki version installed in your project.
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- Use a retry only for an occasional capture failure whose cause is understood; repeated differing pixels usually point to unstable state or rendering conditions.
- Keep tolerance small and document the specific expected noise it excludes.
- Do not raise tolerance to make a test pass until you know whether the difference is rendering noise or a real regression.
7. Treat reference images as reviewed code
Loki separates testing, updating, and approving references, and stores reference images, current screenshots, and diffs in configurable directories. Its CI guide recommends requiring references and running against a static Storybook build. That avoids silently treating a missing reference as an ordinary passing comparison.
- Build the static Storybook used for the test run.
- Require references in CI and run Loki against that build.
- When a change is intentional, inspect the expected image, actual image, and diff before updating or approving the reference.
- Commit the reviewed reference change with the code change it represents.
Playwright likewise documents explicit snapshot updates and reviewing and committing screenshot changes in its snapshot guidance. A baseline update should record an accepted visual change, not erase a failure whose cause is unknown.
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If your goal is to capture a page rather than test a Storybook story, ScreenshotNeo offers a screenshot API and MCP server. A single request can return an image or PDF; it is not a replacement for making Loki stories deterministic or reviewing visual-regression baselines.
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