visual-regression

name	visual-regression
description	Visual regression testing — Storybook-based VRT, Playwright screenshot comparison, determinism/flake control, baseline review workflow. Use when setting up or maintaining visual tests, screenshot diffs, Chromatic, or investigating "the screenshot test broke" failures. 日本語の依頼例:「スクショテスト導入」「ビジュアルリグレッション」「見た目の差分テスト」「スクリーンショットが毎回変わる」「Chromatic」。

Visual Regression Testing

VRT catches what DOM assertions can't: layout breaks, token regressions, theme bugs, overflow. It complements — never replaces — behavioral tests. If a DOM assertion can express the check (text content, element presence), use that; VRT is for how things look.

Stories are the VRT surface

• Component-level VRT runs over Storybook stories (Chromatic, or a Playwright/Vitest screenshot run against the built Storybook). Every visual state already has a story per the storybook skill — that catalog IS the snapshot suite. Don't build a parallel page-screenshot harness for component states.
• Runner precedence: if a vendor service (Chromatic) is already configured, add the story and stop — it's covered. Otherwise default to Playwright toHaveScreenshot against the built Storybook, baselines committed alongside the spec and (re)generated only in the CI container.
• Page-level VRT: expect(page).toHaveScreenshot() in Playwright for a handful of critical pages/themes only, as its own Playwright project running on fixture data — never bolt toHaveScreenshot onto a real-backend functional E2E spec (those follow testing-playwright). Full-page screenshots of every route is a flake farm, not coverage.

Determinism — every flake source must be pinned

A VRT suite that cries wolf gets ignored; eliminate nondeterminism at the source:

• Time: mock the clock (page.clock / fake timers); seed any randomized data. Relative dates ("3 minutes ago") in fixtures, never real Date.now().
• Network: all data via MSW/fixtures (same handlers as the test suites). No real API in VRT.
• Fonts & images: wait for document.fonts.ready AND for in-view images to finish decoding before capture — a half-loaded image is a diff. Self-hosted fonts only (a CDN hiccup is a diff).
• Animation: disable it — toHaveScreenshot({ animations: 'disabled' }) is the primary mechanism. Emulating prefers-reduced-motion alone is insufficient: per motion, state-conveying animations reduce to a quick fade, which can still be captured mid-fade. Never screenshot mid-animation.
• Environment: baselines are generated in the SAME environment that compares them — the CI container (one OS, one browser, pinned versions, pinned TZ=UTC + locale, since Intl output differs per environment). Local-macOS baselines vs Linux CI = permanent font-rendering diffs. Use --update-snapshots in the CI container (or the vendor cloud) to (re)baseline (the bare flag updates only changed snapshots; pass all to force everything).
• Scrollbars & caret: content-dependent scrollbars and a blinking text caret cause intermittent diffs — size stories to their content and keep the caret hidden (Playwright's toHaveScreenshot does this by default).
• Leftovers: fixed viewport sizes; mask genuinely dynamic regions (avatars, maps) rather than widening thresholds.

Thresholds & scope

• Keep maxDiffPixelRatio strict (≤ 0.01); a loose threshold silently approves real regressions. If a region forces looseness, mask that region instead.
• Screenshot the component/element under test, not the whole page, when the subject is a component — smaller surface, fewer false positives.
• Cover both themes and the key breakpoints for layout-critical pieces; don't multiply every story × every viewport × every browser reflexively (cost grows multiplicatively, signal doesn't).

Baseline review — diffs are code review artifacts

• A visual diff is either a regression (fix the code) or an intended change (approve the new baseline in the PR). Never auto-accept baselines, never regenerate-and-commit without looking at the diff — that converts the suite into a rubber stamp.
• Baseline updates ship in the same PR as the change that caused them, so the reviewer sees code and pixels together.
• Recurring flake in one snapshot = fix the determinism cause or delete the snapshot; regenerating or re-running until the diff happens to pass is banned — same retried-pass-is-a-flake-report policy as testing-playwright.