Why can visual tests be flaky?

Visual tests can be flaky because they are too sensitive to any changes, even ones that are insignificant to the user. They compare pixels, and this makes them fragile.

The main reasons for instability:

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#1. Changes that don't affect the substance but change pixels

Even a small thing, and the screenshot is already different:

• a different font after a browser update
• a different rendering subsystem on Windows/Mac/Linux
• anti-aliasing and text smoothing
• a minimal 1-pixel shift

The test will fail, even though the interface is essentially "the same."

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#2. Unstable data and dynamic content

If something on the UI changes with every render, the test will keep catching "changes":

• random IDs
• the current date/time
• random images
• animations
• loading states

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#3. Animations and asynchronous states

The interface can be screenshotted in the middle of an animation, so the resulting frame is unpredictable. As a result, tests sometimes pass and sometimes fail.

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#4. Dependency on the environment

Differences can come from:

• browser version
• OS version
• screen scale
• DPI
• theme (light/dark)

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#5. Unstable styles

If CSS depends on computed values or third-party libraries, the visual result can drift.

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#Summary

Visual tests are unstable because they rely on an exact pixel match, and pixels easily change for reasons unrelated to real bugs. That is why such tests require discipline, a stabilized environment and configured filters to provide value instead of false alarms.