UI Screenshots

Capture screenshots of web apps and graphical UIs during development to document visual changes.

When to Use This Skill

Use this skill when you need to:

• Capture the current state of a running web app
• Document a UI before and after a code change
• Screenshot interactive states (tooltips, hovers, selected elements)
• Capture specific sections of a page without re-screenshotting

Prerequisites

pip install playwright Pillow -q
playwright install chromium

Core Workflow

1. Take a raw full-page screenshot

from playwright.async_api import async_playwright

async def capture(url="http://localhost:3000", out="screenshot-raw.png", width=1400, height=5000):
    async with async_playwright() as p:
        browser = await p.chromium.launch()
        page = await browser.new_page(viewport={"width": width, "height": height})
        await page.goto(url, wait_until="networkidle")
        await page.wait_for_timeout(4000)  # let charts/animations render
        await page.screenshot(path=out, full_page=True)
        await browser.close()

• Use a tall viewport (height=5000) so the page renders everything without scrolling
• wait_until="networkidle" + wait_for_timeout(4000) ensures async charts load
• full_page=True captures the entire scrollable content

2. View the raw image, then crop with PIL

Do NOT try to get perfect crops via Playwright's clip parameter. It's unreliable with full-page captures.

from PIL import Image

img = Image.open("screenshot-raw.png")
cropped = img.crop((left, top, right, bottom))  # adjust based on what you see
cropped.save("screenshot-final.png")

1. Take the raw screenshot
2. View it to see actual pixel positions
3. Crop with PIL based on what you see
4. View the result — if not right, re-crop (instant, no re-screenshot needed)

3. Iterate on crop, not on capture

• Re-screenshotting is slow (browser launch + page load + render wait)
• Re-cropping is instant (just PIL)
• Get one good raw capture, then slice it as many ways as needed

4. Interactive states

element = page.locator("selector").first
await element.hover()
await page.wait_for_timeout(1000)  # let tooltip appear
await page.screenshot(path="screenshot-hover.png", full_page=True)

For "selected" state without hover effect, move the mouse away after clicking:

await element.click()
await page.mouse.move(300, 300)  # move away so hover doesn't show
await page.wait_for_timeout(500)
await page.screenshot(path="screenshot-selected.png", full_page=True)

5. Section-specific captures

Crop different sections from a single full-page screenshot:

img.crop((0, 200, 920, 900)).save("screenshot-header.png")
img.crop((0, 900, 920, 1600)).save("screenshot-main.png")

Guidelines

1. Always capture before state BEFORE making any changes — if you forget, you have to revert code to get a before shot
2. Before/after pairs must use the same viewport width and crop — otherwise the comparison is useless
3. To get a "before" after you already changed code: use git checkout HEAD~1 -- <files> to revert, screenshot, then git checkout HEAD -- <files> to restore
4. For interactive states: capture before AND after for each state — don't assume the "normal" before covers all cases
5. Use device_scale_factor=1 in Playwright to force 1x pixels so screenshots match what users see at 100% zoom
6. Charts need extra wait time — Plotly, D3, etc. render asynchronously; 4s minimum after networkidle
7. Narrow viewport reveals rendering bugs — some border/alignment issues only appear at specific widths

Non-Web App Screenshots

For desktop apps (VS, WPF, WinForms, console apps, terminals) where Playwright can't reach.

mss + ctypes (recommended for desktop windows)

Find a window by title via Win32 API, capture its region with mss. Tested at ~33ms per capture.

import ctypes
from ctypes import c_int, Structure, byref, windll
import mss
from PIL import Image

user32 = windll.user32

def find_window(title_contains):
    """Find visible windows matching a title substring."""
    results = []
    WNDENUMPROC = ctypes.WINFUNCTYPE(ctypes.c_bool, ctypes.c_void_p, ctypes.c_void_p)
    def cb(hwnd, _):
        if user32.IsWindowVisible(hwnd):
            buf = ctypes.create_unicode_buffer(256)
            user32.GetWindowTextW(hwnd, buf, 256)

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