Verifying Tauri UIs Headlessly (CDP + IPC mock)

How to drive and screenshot a Tauri 2 + vite frontend end-to-end when the dev machine's desktop is in active use (popping real windows fights the user for focus and they'll close them mid-test). Worked out on AssettoCar (2026-07); recipe lives in AssettoCar/.claude/skills/verify/SKILL.md.

Core idea

Run only the vite dev server, load it in headless Chrome, and inject a window.__TAURI_INTERNALS__ mock before page load via CDP Page.addScriptToEvaluateOnNewDocument. Feed the mock real payloads dumped from the Rust backend (an #[ignore]d test that serializes probe results to JSON) so the UI renders real data, not hand-typed fixtures.

The mock needs three members:

• invoke(cmd, args) — switch on command name; route plugin:dialog|open to a canned path (bypasses native dialogs entirely), record mutation commands' args on window for assertions (e.g. what generate was sent).
• transformCallback(cb) — assign window['_' + id] = cb, return id (Tauri's event system delivers via these globals).
• metadata: { currentWebview: {label:'main'}, currentWindow: {label:'main'} } — required by getCurrentWebview() / onDragDropEvent.

Simulate backend events (progress/log) by calling registered plugin:event|listen handlers from the mock.

Driving

CDP over WebSocket (npm i ws --no-save, Node 20 lacks global WebSocket): Runtime.evaluate (with awaitPromise) for state assertions, Input.dispatchMouseEvent for trusted clicks (needed for autoplay/user activation), Page.captureScreenshot for evidence — works fully offscreen, no focus stolen.

Gotchas

• React StrictMode double-mounts effects; if the mock's plugin:event|unlisten is a no-op, every event renders twice. Mock artifact, not an app bug — real unlisten removes the first listener.
• A harness/background-task-owned npm run tauri dev dies when the task ends; Start-Process (detached) if the app must survive tool calls.
• Automating the real app's native file dialogs (UIA/SendKeys) is flaky and lands on the user's desktop — avoid; stub at the plugin:dialog|open boundary instead.
• WebView2 accepts WEBVIEW2_ADDITIONAL_BROWSER_ARGUMENTS=--remote-debugging-port=9223 for CDP against the real app when the desktop IS free; same driver works, but native dialogs still pop real windows.
• cargo run --example linking a tauri lib crate can fail with STATUS_ENTRYPOINT_NOT_FOUND on Windows; put dev utilities in #[ignore]d integration tests instead.