Cypress scrollHeight Value Discrepancy in Headless Firefox Mode

Other Questions Headless Browser
1

I’m encountering an issue with my Cypress script, which includes the following code:

let elementHeight = $element.prop('scrollHeight');
cy.get('@otherElement').should('contain.text', elementHeight);

Although my tests complete without any errors, I’ve noticed that the elementHeight value occasionally varies by 1 pixel, and this happens exclusively when I execute the following command:

cypress run --headless --browser firefox

Is there a solution to address this inconsistency?

2

Hello Alice,

Addressing the scrollHeight discrepancy during Cypress tests in headless Firefox mode often boils down to how these environments render elements differently. Here’s how you can tackle this:

  1. Implement a Range Check: Instead of checking for an exact pixel value, allow for a small range. This handles minor rendering variations effectively:

    cy.get('@otherElement').invoke('text').then((text) => {
  const height = parseInt(text, 10);
  expect(height).to.be.within(elementHeight - 1, elementHeight + 1);
});

  2. Ensure Consistent Setup: Verify that your test environment is uniform. This includes using the same browser versions and confirming that CSS properties do not influence the outcome unexpectedly, such as box-sizing.

  3. Adjust Firefox Settings: Configure Firefox in headless mode with specific flags (like --window-size) to control rendering behaviors and reduce inconsistencies.

These methods aim to optimize your test conditions, offering more stability in results. If you still experience issues, consider reaching out for more detailed logging strategies to pinpoint the root cause. Best of luck!

Regards,
David Grant

3

Hey Alice,

This scrollHeight variance in headless Firefox can indeed be a pesky issue. It's common due to different rendering behavior, but here's a simple fix:

  1. Allow Small Variances: Instead of exact matches, use a small range check:

    cy.get('@otherElement').invoke('text').then((text) => {
  const height = parseInt(text, 10);
  expect(height).to.be.within(elementHeight - 1, elementHeight + 1);
});

  2. Check CSS & Environment: Ensure CSS or browser settings aren't influencing this. Consistency helps.

This should keep your tests stable. Good luck!

4

The issue with scrollHeight varies due to how rendering might differ between headless and headful modes, particularly in Firefox. While existing suggestions focus on accommodating this variance with range checks and environment consistency, I would like to offer an additional perspective:

  1. Emulate Full Display Environment: Consider setting up a virtual desktop environment that mimics headful conditions. Tools like Xvfb can run a virtual framebuffer that replicates a headful browser display, potentially reducing rendering differences.

    CYPRESS_INTERNAL_ENV=production xfvb-run --auto-servernum cypress run

  2. Review Rendering Configuration: Customizing Firefox’s rendering settings might prove helpful. For instance, adjusting the Firefox configuration to disable certain graphics or rendering optimizations could reduce discrepancies.

    firefox --headless --disable-gpu --no-sandbox

  3. Dimension Locking: Lock the dimensions of both the testing viewport and the elements being tested to ensure they always render the same way, disregarding any natural pixel shifting due to environmental changes.

These strategies can help you maintain the fidelity of your tests across different execution environments, especially for visual elements where precision matters. By emulating similar headful conditions even in headless mode, you can ensure that your test results remain consistent and reliable.

5

To add to Harry's points, another viable approach is to ensure that your test setup and execution avoid any factors that might introduce unnecessary variances. Here are some additional strategies:

  1. Use Browser-specific Flags: When executing in headless mode, you might want to use specific flags or configurations tailored for the Firefox browser to control how rendering is handled. This includes emulating certain display characteristics that can reduce discrepancies.

    {
  "headless": "true",
  "visualMetrics": "true",
  "viewport": "1280x720"
}

  2. Comparative Testing: Run your test suite across multiple browser configurations (headful vs. headless) and document any discrepancies. This can help isolate whether the issue is persistent across environments or unique to Firefox headless mode.

  3. Optimize Element Visibility: Ensure that the elements you are working with are fully loaded and properly visible before measuring. Consider using Cypress commands like cy.wait() or cy.scrollIntoView() for ensuring complete rendering.

By adopting these strategies, you can improve the robustness of your tests when running in headless modes. Addressing these variabilities will help in achieving more reliable and consistent outcomes.