Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsBackstopJS tests the viewport sizes you configure; it does not discover your CSS breakpoints for you. Add project-specific widths around the layout transitions that matter, capture approved reference images with backstop reference, then run backstop test to compare new screenshots against them.
Configure widths that exercise your breakpoints
BackstopJS applies the configured viewports to your scenarios. Use widths based on your application’s CSS and known layout behavior—not only generic phone, tablet, and desktop sizes. For a breakpoint at 768 pixels, for example, test the widths immediately below and above it, as well as any other widths where the layout is especially sensitive. Those are practical test choices, not sizes BackstopJS infers automatically.
Add labeled width-and-height objects to the root viewports array in backstop.json:
{
"viewports": [
{ "label": "mobile-below-nav-change", "width": 767, "height": 900 },
{ "label": "desktop-at-nav-change", "width": 768, "height": 900 },
{ "label": "wide-desktop", "width": 1280, "height": 900 }
],
"scenarios": [
{
"label": "Home page",
"url": "http://localhost:3000/"
}
]
}This example assumes a transition at 768 pixels; replace the example widths and local URL with values for your project. BackstopJS requires at least one viewport. A scenario needs a label and URL, and the configured viewport list is applied to relevant scenarios. Add separate scenarios for routes, content, or application states that need distinct coverage.
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Capture and compare screenshots
- Prepare the intended page state. Make sure the application is running at the scenario URL and that the page shows the state you want to preserve.
- Create references: run
backstop reference. BackstopJS captures the configured scenarios and viewport sizes as the baseline images. - Run the regression check: run
backstop test. BackstopJS creates test bitmaps, compares them with the current references, and presents a report for review. - Inspect every relevant difference. If a change is intentional and correct, run
backstop approveto promote the latest changed captures to the reference collection. Do not approve a difference simply to make a failed test pass.
Later tests compare against the approved references. When layout changes intentionally, review the affected scenarios and sizes before updating that baseline.
Choose what each screenshot captures
BackstopJS supports three capture scopes. Select the smallest one that still exposes the problem; use more than one when page-wide and component-level views answer different questions.
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| Capture scope | Useful for | Trade-off |
|---|---|---|
document |
Seeing changes throughout the full page, including below the first screen. | More page content must render consistently to make comparisons useful. |
viewport |
Focusing on what is visible within the current screen area. | Content outside that area is not included in the capture. |
| CSS selector | Isolating a component whose layout changes at a breakpoint. | Useful for diagnosis, but it will not show unrelated page-wide effects. |
Full-page captures can reveal overflow or spacing defects lower down the page; viewport captures focus on the visible region; selector captures help narrow a failure to a component.
Make asynchronous pages reproducible
A screenshot taken before the relevant content appears may be blank or incomplete. BackstopJS documents three readiness controls:
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readySelectorwaits for a chosen selector.readyEventwaits for an application console event.delayadds a fixed pause before capture.
Prefer a readiness signal tied to the content under test where possible. A fixed delay can be fragile when load time varies. For dynamic content, the project documentation recommends using static data stubs to make output deterministic. Hiding or removing unstable elements is also documented, but do not hide a region whose size or responsive behavior is what you are testing.
Set comparison rules without masking defects
Two settings answer different questions:
misMatchThresholdcontrols the tolerated pixel difference. The documented default is0.1, described as the percentage of different pixels tolerated before a scenario fails.requireSameDimensionscontrols whether a change in screenshot dimensions causes failure. Its documented default istrue.
Review actual diffs before relaxing either setting. A higher mismatch tolerance can hide small layout defects, while disabling same-dimensions checks can allow a size change that should fail. Choose based on the variation your page legitimately produces, rather than treating either setting as a universal value.
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Debug a failing or inconsistent run
- Only one viewport fails: rerun the matching scenario and inspect that viewport’s report. Check the nearby CSS transition before replacing a reference.
- The screenshot is blank or incomplete: verify that the URL and expected page state are available, then inspect the readiness condition. The capture may be happening before content appears.
- Dynamic content changes between runs: use stable test data or stubs where possible. Hide or remove unstable regions only if they are not part of the responsive behavior under test.
- Text or rendering differs across operating systems: the project recommends Docker rendering to reduce environment-related variation. It can improve repeatability, but does not guarantee identical output for every application or dependency.
- You need to isolate a rerun: use
--filterwith a matching scenario label to rerun only selected scenarios. Give scenarios and viewports meaningful labels; report capture names include scenario and viewport information. - A diff looks intentional: verify the rendered result at the affected widths first, then use
backstop approveto update the reference if the change is correct.
Or skip the browser setup
If you need screenshots without setting up browser capture locally, ScreenshotNeo returns an image or PDF from one GET request. For a responsive check, request the same URL at the widths you need and compare the resulting captures with your approved images. See the ScreenshotNeo API documentation.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpReplace YOUR_API_KEY and the example URL with your credentials and target. ScreenshotNeo accepts cookie or consent banners and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each step can be turned off. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and the response identifies the page verdict and billing status in X-Page-Verdict and X-Billed headers. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for AI agents. The Free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 screenshots.
Sign up for ScreenshotNeo’s free plan to try 1,000 screenshots a month with no card.
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Sources and version note
BackstopJS configuration details can vary by installed version. Consult the project documentation for the version you use: BackstopJS project documentation and README and BackstopJS on npm.
Frequently Asked Questions
Does BackstopJS find my CSS media-query breakpoints automatically?
No. You choose the viewport widths to test and define them in configuration.
Can I test a single responsive component instead of the whole page?
Yes. Configure a CSS selector capture for the element you want to isolate.
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How do I compare mobile and desktop screenshots?
Include both viewport sizes in the configuration, create references, and run the test so each configured size is compared with its baseline.
Quick Recap
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