UI Screenshots

UI screenshots

Capture web, Electron, and desktop UI screenshots for development evidence by taking raw screenshots, preserving before/after comparability, cropping with PIL, and selecting the correct capture tool for each UI surface.

When to invoke

• "Capture the current state of this web app."
• "Take before and after screenshots for this UI change."
• "Screenshot this hover or selected state."
• "Crop a specific page section from a full-page capture."
• "Capture a desktop or Electron app window."

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

Invocation details

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)
            if title_contains.lower() in buf.value.lower():
                results.append((hwnd, buf.value))
        return True
    user32.EnumWindows(WNDENUMPROC(cb), 0)
    return results

def capture_window(title_contains, output_path):
    """Capture a window by title substring."""
    windows = find_window(title_contains)
    if not windows:
        raise ValueError(f"No window matching '{title_contains}'")
    hwnd = windows[0][0]

    class RECT(Structure):
        _fields_ = [('left', c_int), ('top', c_int), ('right', c_int), ('bottom', c_int)]
    rect = RECT()
    user32.GetWindowRect(hwnd, byref(rect))
    w, h = rect.right - rect.left, rect.bottom - rect.top

    with mss.mss() as sct:
        shot = sct.grab({'left': rect.left, 'top': rect.top, 'width': w, 'height': h})
        img = Image.frombytes('RGB', shot.size, shot.rgb)
        img.save(output_path)
        return img

# Usage:
capture_window('Visual Studio Code', 'vscode-capture.png')

Prerequisites: pip install mss pillow Limitation: Window must be visible (not behind other windows or minimized).

Electron apps (VS Code, etc.)

Node.js Playwright only — Python Playwright has no electron API. Captures via CDP (Chrome DevTools Protocol), not from the screen — works even while minimized.

const { _electron: electron } = require('playwright');
const app = await electron.launch({
    executablePath: 'C:\\Program Files\\Microsoft VS Code\\Code.exe',
    args: ['--new-window', '--disable-extensions', '--user-data-dir=' + tmpDir]
});
const window = await app.firstWindow();
await window.waitForLoadState('domcontentloaded');

// Minimize immediately — captures still work via CDP
await app.evaluate(({ BrowserWindow }) => {
    BrowserWindow.getAllWindows()[0].minimize();
});

await window.screenshot({ path: 'capture.png' }); // works while minimized!
await app.close();

Critical: --user-data-dir=<temp> is required or VS Code hands off to the existing instance and the launched process exits immediately.

Decision tree

Limitations

• Web capture requires a locally running app or accessible URL
• Desktop capture (mss) requires the window to be visible and unobstructed
• Electron capture requires Node.js Playwright (not Python)
• Some SPAs with heavy client-side rendering may need custom wait logic beyond networkidle

Output template

## UI screenshot result

**Status:** captured | blocked
**Target:** `{{url_or_window_title}}`
**Viewport:** `{{width}}x{{height}}`, device scale factor `1`

| Image | Path | Notes |
| --- | --- | --- |
| Raw | `{{screenshot-raw.png}}` | full-page or full-window capture |
| Final | `{{screenshot-final.png}}` | crop box `({{left}}, {{top}}, {{right}}, {{bottom}})` |

### Validation
- Before/after parity: same viewport and crop | not applicable
- Async wait: `networkidle` plus render wait | desktop capture not applicable

Quality gate

• A before screenshot is captured before code changes when visual comparison is requested.
• Before and after screenshots use the same viewport width, height, device scale factor, and crop.
• Web screenshots use full_page=True for raw capture and PIL for final crops.
• Interactive states wait for hover, tooltip, selected, or animation effects before capture.
• The chosen capture method matches the decision tree: Playwright for web, Node Playwright for Electron, mss plus ctypes for visible desktop windows.

References

• http://localhost:3000