Control UI

Use local browser automation to verify UI behavior with evidence. First reuse the repo's own Playwright, browser, or Electron harness if it exists; otherwise assemble a temporary local harness around the app's dev server or Chromium debug port.

What It Is Used For

• Reproducing UI bugs that depend on real browser focus, keyboard input, scrolling, resizing, or rendering.
• Verifying visual or accessibility changes with screenshots and snapshots.
• Checking local web, IDE, or Electron behavior before shipping.
• Capturing console logs, network logs, CPU profiles, traces, or heap snapshots.
• Creating before/after evidence for verify-this.

Setup Pattern

1. Start the app locally using the repo's documented dev command.
2. Discover existing local harnesses: Playwright tests, Cypress specs, Storybook, browser scripts, Electron launch scripts, or snapshot tools.
3. For a web app, connect to the local URL with the existing browser tooling.
4. For Electron/Chromium, enable a remote debugging port when supported.
5. Select the correct page by stable app markers, not by tab order alone.
6. Prefer accessibility roles, labels, and stable data-* selectors over coordinates.

Generic Web Harness

Use the repo's installed browser tooling when possible. If the repo already has Playwright, a minimal one-off probe looks like:

import { chromium } from "playwright";

const browser = await chromium.launch();
const page = await browser.newPage({ viewport: { width: 1280, height: 800 } });
await page.goto("http://127.0.0.1:<port>");
await page.getByRole("button", { name: /submit/i }).click();
await page.screenshot({ path: "/tmp/ui-harness-after.png", fullPage: true });
await browser.close();

Do not add Playwright as a project dependency just for this probe unless the user asks. Prefer existing dev dependencies or external browser tools already available in the environment.

Generic CDP Harness

For Electron or a Chromium app launched with --remote-debugging-port=<port>, connect over CDP:

import { chromium } from "playwright";

const browser = await chromium.connectOverCDP("http://127.0.0.1:<debug-port>");
const pages = browser.contexts().flatMap((context) => context.pages());
let page;
for (const candidate of pages) {
  if (await candidate.locator("<app-root-selector>").count()) {
    page = candidate;
    break;
  }
}

if (!page) {
  console.log(await Promise.all(pages.map(async (p) => ({
    title: await p.title(),
    url: p.url(),
  }))));
  throw new Error("No matching app page found");
}

await page.screenshot({ path: "/tmp/ui-harness-cdp.png", fullPage: true });
await browser.close();

Replace <app-root-selector> with a stable marker from the current repo, such as a root app node, landmark, or product-specific data-* attribute.

Interaction Loop

1. Capture a page snapshot or screenshot before acting.
2. Choose a target from the latest page structure.
3. Perform exactly one structural action: click, type, keypress, drag, scroll, navigate, or resize.
4. Capture a fresh snapshot/screenshot.
5. Verify the expected state change.
6. Save artifacts for before/after comparisons when the user asked for proof.

CDP Capabilities

Use raw CDP only when higher-level browser APIs are insufficient:

• Performance: CPU profiles, traces, paint flashing, FPS meter, layout shift inspection.
• Memory: heap snapshots and forced GC for leak investigations.
• Network: request blocking, throttling, cache disablement, request/response logs.
• Rendering: viewport changes, color scheme emulation, reduced motion, accessibility checks.
• Debugging: console streaming, exception capture, DOM snapshots.

Page Selection

When multiple app windows/tabs share a debug port:

• Prefer a positive marker for the surface under test, such as an app root selector.
• Use a negative marker to avoid the wrong surface when necessary.
• If no page matches, list available page titles and URLs instead of guessing.

Guardrails

• Do not rely on stale element references after navigation or structural changes.
• Avoid coordinate clicks unless a fresh screenshot was captured immediately before the click.
• Keep test data local and disposable.
• Do not store screenshots or heap snapshots from privacy-sensitive workspaces unless the user explicitly agrees.
• Do not hard-code selectors, ports, or script paths from another repository. Discover the current repo's local app markers.
• Clean up dev servers, debug sessions, and temp profiles when done.