# Webapp Testing

> Toolkit for interacting with and testing local web applications using Playwright. Supports verifying frontend functionality, debugging UI behavior, capturing browser screenshots, and viewing browser logs.

- Skill: `lanej/webapp-testing` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add lanej/webapp-testing`
- Raw SKILL.md: https://api.skillmd.com/api/skills/lanej/webapp-testing/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- License: Complete terms in LICENSE.txt
- Author: lanej (https://skillmd.com/u/lanej)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/lanej/webapp-testing

---


# Web Application Testing

To test local web applications, write native Python Playwright scripts.

**Helper Scripts Available**:
- `scripts/with_server.py` - Manages server lifecycle (supports multiple servers)

**Always run scripts with `--help` first** to see usage. DO NOT read the source until you try running the script first and find that a customized solution is abslutely necessary. These scripts can be very large and thus pollute your context window. They exist to be called directly as black-box scripts rather than ingested into your context window.

## Decision Tree: Choosing Your Approach

```
User task → Is it static HTML?
    ├─ Yes → Read HTML file directly to identify selectors
    │         ├─ Success → Write Playwright script using selectors
    │         └─ Fails/Incomplete → Treat as dynamic (below)
    │
    └─ No (dynamic webapp) → Is the server already running?
        ├─ No → Run: python scripts/with_server.py --help
        │        Then use the helper + write simplified Playwright script
        │
        └─ Yes → Reconnaissance-then-action:
            1. Navigate and wait for networkidle
            2. Take screenshot or inspect DOM
            3. Identify selectors from rendered state
            4. Execute actions with discovered selectors
```

## Example: Using with_server.py

To start a server, run `--help` first, then use the helper:

**Single server:**
```bash
python scripts/with_server.py --server "npm run dev" --port 5173 -- python your_automation.py
```

**Multiple servers (e.g., backend + frontend):**
```bash
python scripts/with_server.py \
  --server "cd backend && python server.py" --port 3000 \
  --server "cd frontend && npm run dev" --port 5173 \
  -- python your_automation.py
```

To create an automation script, include only Playwright logic (servers are managed automatically):
```python
from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True) # Always launch chromium in headless mode
    page = browser.new_page()
    page.goto('http://localhost:5173') # Server already running and ready
    page.wait_for_load_state('networkidle') # CRITICAL: Wait for JS to execute
    # ... your automation logic
    browser.close()
```

## Reconnaissance-Then-Action Pattern

1. **Inspect rendered DOM**:
   ```python
   page.screenshot(path='/tmp/inspect.png', full_page=True)
   content = page.content()
   page.locator('button').all()
   ```

2. **Identify selectors** from inspection results

3. **Execute actions** using discovered selectors

## Common Pitfall

❌ **Don't** inspect the DOM before waiting for `networkidle` on dynamic apps
✅ **Do** wait for `page.wait_for_load_state('networkidle')` before inspection

## Debugging CSS Layout Bugs (sticky, scroll containers, height chains)

A screenshot or a plausible-sounding root cause is not sufficient confirmation for a layout bug (sticky positioning, scroll containers, flex/grid height chains). These bugs frequently have root causes stacked across the DOM ancestor chain — a locally correct fix (e.g. `position: sticky` on the target element) can still fail because a parent or the shared app shell constrains it (e.g. `align-items: start` collapsing the grid item's height, or a shell using `min-height: 100vh` instead of `height: 100vh` so no ancestor ever actually becomes a scrolling container).

Protocol:
1. **Pin down the exact failing measurement before attempting a fix** — not "does it look right," but a specific `getBoundingClientRect()` / computed-style / `scrollTop` value at a specific scroll position that demonstrates the bug. Example:
   ```python
   page.evaluate("window.scrollTo(0, 1200)")
   rect = page.locator(".rightColumn").evaluate("el => el.getBoundingClientRect()")
   scroll_top = page.locator(".content").evaluate("el => el.scrollTop")
   ```
2. **Re-run that identical measurement after every fix attempt.** Only declare the bug fixed when that specific measurement now passes — not when a new theory sounds more precise than the last one, and not from a screenshot alone.
3. **Expect to walk up the ancestor chain.** If a fix is syntactically correct but the measurement still fails, the real constraint is usually one level higher (grid/flex alignment → the element's own overflow container → a shared shell's height model). Don't stop at the first plausible explanation; keep measuring until the failure condition itself is gone.

## Best Practices

- **Use bundled scripts as black boxes** - To accomplish a task, consider whether one of the scripts available in `scripts/` can help. These scripts handle common, complex workflows reliably without cluttering the context window. Use `--help` to see usage, then invoke directly. 
- Use `sync_playwright()` for synchronous scripts
- Always close the browser when done
- Use descriptive selectors: `text=`, `role=`, CSS selectors, or IDs
- Add appropriate waits: `page.wait_for_selector()` or `page.wait_for_timeout()`

## Rendering HTML to PNG

Use `uv run --with playwright` — no venv setup required. Install the browser once with `uv run --with playwright python3 -m playwright install chromium`.

```python
from playwright.sync_api import sync_playwright
import os

html_path = os.path.abspath('path/to/file.html')
out_path = os.path.abspath('path/to/output.png')

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    page = browser.new_page(device_scale_factor=4)   # 4–5x for sharp output
    page.set_viewport_size({'width': 1400, 'height': 900})
    page.goto(f'file://{html_path}')
    page.wait_for_load_state('networkidle')
    # Screenshot a specific element to avoid body padding:
    page.locator('.my-container').screenshot(path=out_path)
    # Or full page:
    # page.screenshot(path=out_path, full_page=True)
    browser.close()
```

**Key decisions:**
- `device_scale_factor=4` or `5` — required for sharp text; 1x (default) looks blurry
- **Element screenshot** (`locator.screenshot()`) crops to content, avoids body padding
- **`full_page=True`** captures everything but includes body margins — use only if you want the full document

## Reference Files

- **examples/** - Examples showing common patterns:
  - `element_discovery.py` - Discovering buttons, links, and inputs on a page
  - `static_html_automation.py` - Using file:// URLs for local HTML
  - `console_logging.py` - Capturing console logs during automation