---
name: visual-regression
description:
  Prove a refactor did not change how the site looks. Use before and after any
  page migration, token consolidation, or breakpoint change on ai-for-dummies.
---

# Visual regression

"Maintain the same styles" is a testable claim. Test it.

## Capture

The repo already has a Playwright pattern (`scripts/inspect.py`). Extend it
rather than inventing one.

```python
from playwright.sync_api import sync_playwright
ROUTES = ['/', '/full-guide/', '/summary/', '/models/', '/agents/',
          '/skills/', '/rules/', '/skills-review/',
          '/hands-on/starter/', '/hands-on/rules/']
WIDTHS = [560, 800, 1100, 1600]

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    for route in ROUTES:
        for w in WIDTHS:
            page = browser.new_page(viewport={'width': w, 'height': 900})
            page.goto(f'{BASE}{route}', wait_until='networkidle')
            page.screenshot(path=f'{OUT}/{route.strip("/").replace("/","_") or "index"}-{w}.png',
                            full_page=True)
            page.close()
    browser.close()
```

Run once against `pnpm run preview`. To compare against the vanilla site, serve
a pre-cutover worktree on :4173 first — those files are no longer on `main`.
Keep both sets.

## Compare

```bash
for f in before/*.png; do
  compare -metric AE "$f" "after/$(basename $f)" null: 2>&1   # ImageMagick
  echo "  <- $(basename $f)"
done
```

Pixel-exact is not the bar — antialiasing differs. Judge by eye where the metric
is non-trivial, and attach the pair to the task report.

## The three widths that catch the most

- **560px** — where the 16 ad-hoc breakpoints collapse to `--bp-sm`. Highest
  risk in the whole migration.
- **800px** — the most common existing breakpoint; layout flips here.
- **1600px** — `min-width` rules that only fire on large screens are the ones
  nobody notices are broken.

Also screenshot at the **old** breakpoint values you removed (520, 530, 600,
620, 720, 850, 880, 900), not just the new ones. Regressions hide exactly there.

## What a real difference looks like

Expect and accept: sub-pixel text shifts, antialiasing.

Investigate: anything that moves by more than ~2px, any colour change (that is a
token bug), any element that appears or disappears (that is content loss — stop
and check the snapshot diff).

## Reduced motion

Capture one pass with `prefers_reduced_motion='reduce'`. Animations must land in
their correct end state, not vanish.
