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Visual Regression Test Docker Instructions

The Visual Regression Tests utilize headless browsers provided by the Playwright project. Browsers tend to be very platform specific in terms of exactly how pixels end up being rendered out. The differences should be small, but they tend to be big enough such that a simple image comparison algorithm cannot reliably tell what is a real regression and what is just a platform difference.

In order to prevent this issue, the Visual Regression Tests are built to run inside a Docker container which guarantees a consistent platform environment for the given headless browser to run in.

Whenever new commits to a PR are pushed, a GitHub Action runs the Visual Regression Tests in a Linux-based Docker container to determine if the PR should be allowed to merge or not.

When you need to capture new snapshots or update existing ones, you must run the Visual Regression Test Runner locally in --ci mode. This launches the tests in a Docker container giving you exactly the same snapshot results as the tests that run in the cloud on GitHub Actions.

Below are the instructions for both installing Docker and building the Visual Regression Test Image that is used when you run the tests in --ci.

Installing Docker

Mac

If you have a license for Docker Desktop you can install that and all should be well. However, if you don't follow the alternative steps below:

  1. Using Homebrew install the Docker Client:
brew install docker
  1. Install Colima
brew install colima
  1. Start Colima
colima start
  1. Test Docker
docker ps

It should run without errors.

Building the Test Image

After installing Docker, you must build the Visual Regression Test Image before running the test runner in --ci mode.

pnpm build:docker

If all goes well it should create an image called visual-regression locally.

❯ docker images
REPOSITORY                     TAG             IMAGE ID       CREATED         SIZE
visual-regression              latest          40476ed4acae   3 minutes ago   2.09GB