test: Playwright visual-regression harness (v1.1.1 baselines)

[mmcky]

Adds a Playwright visual-regression harness — modelled on quantecon-book-theme's setup but adapted for this theme's runtime nature (rendered through a live myst start, not static HTML).

What's here

• Self-contained fixture (tests/visual/fixture/): intro.md, features.md (math/code/admonitions/tables/figures), and notebook.ipynb with baked stream / execute_result / error outputs — the surface the @myst-theme v1.0.0 output-node AST change touches.
• serve.sh runs myst start with the theme chosen at runtime via THEME_TEMPLATE (a local build dir or a GitHub archive zip).
• playwright.config.ts (desktop + mobile-chrome), theme.spec.ts (full-page snapshots), README.
• npm scripts test:visual / test:visual:update; @playwright/test devDep.

Baselines = the last release (v1.1.1)

The committed __snapshots__/ were captured against the deployed v1.1.1 theme (…/quantecon-theme/archive/refs/heads/main.zip) — validated rendering live, including the notebook stream/error outputs. So we can diff the 2.0.0 upgrade against the last release:

make build-theme
THEME_TEMPLATE="$PWD/.deploy/quantecon-theme" npm run test:visual   # diff vs v1.1.1

Once the 2.0.0 visual changes are reviewed/accepted, refresh baselines with test:visual:update.

Notes

• Requires Node 24 (.nvmrc), myst CLI, and npx playwright install chromium.
• Generated fixture/myst.yml, fixture/_build/, playwright-report/ are gitignored.
• Follow-up: wire this into CI (a visual.yml matrix that builds the 2.0.0 bundle and runs the snapshots).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a Playwright-based visual regression harness for this theme’s runtime rendering (via myst start) using a self-contained MyST fixture project and committed snapshot baselines (intended to represent v1.1.1) to support reviewing/accepting visual changes from the upcoming upgrade work.

Changes:

• Introduces a visual fixture project (tests/visual/fixture/) including markdown surfaces and a notebook with baked outputs.
• Adds Playwright configuration + spec to run full-page screenshot comparisons across desktop and mobile.
• Wires up local serving (serve.sh), documentation, npm scripts, and Playwright devDependencies; ignores generated artifacts.

Reviewed changes

Copilot reviewed 9 out of 17 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/visual/theme.spec.ts	Adds full-page screenshot tests for intro/features/notebook routes.
tests/visual/serve.sh	Starts a myst start server for the fixture with a theme selected via THEME_TEMPLATE.
tests/visual/README.md	Documents how to run and update visual baselines locally.
tests/visual/fixture/notebook.ipynb	Adds notebook surface to exercise output rendering (stream/execute_result/error).
tests/visual/fixture/myst.yml.in	Provides MyST config template with __THEME__ placeholder for runtime injection.
tests/visual/fixture/intro.md	Adds landing-page content surface for baseline snapshots.
tests/visual/fixture/features.md	Adds rich content surface (math/code/admonitions/tables/definitions) for baseline snapshots.
playwright.config.ts	Adds Playwright runner config, devices, snapshot pathing, and webServer integration.
package.json	Adds test:visual scripts and @playwright/test devDependency.
package-lock.json	Locks Playwright-related dependencies and updates lock metadata accordingly.
.gitignore	Ignores generated Playwright and MyST build artifacts while keeping snapshots committed.

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