paseo/docs/browser-capture-harness.md

Browser Capture Harness

The desktop capture harness is the real-Electron verification path for browser screenshots. It validates the compositor behavior that unit tests cannot see:

• the resident automation <webview> starts in the production parking state;
• the parked guest remains paintable and has a copyable viewport frame;
• the resident webview guest is sized to 1280x800 logical pixels;
• multiple resident webviews are parked as an overlapping stack without per-capture stacking changes;
• a newly attached resident webview whose first useful frame is delayed can be captured by retrying until the frame appears;
• both viewport capturePage and full-page CDP screenshots return real pixels from the permanent production parking state;
• guest background throttling can be disabled once at attach without per-capture renderer coordination;
• the real-Electron host-composer sentinel proves guest Enter cannot submit a focused host composer;
• the automation group loads the compiled production keyboard boundary and guest preload, then proves that initial page window handlers get first refusal, unhandled shortcuts synchronously suppress editable browser defaults before crossing the host boundary, shortcuts marked unavailable in editable targets retain the browser field's native behavior, handlers registered after preload still get first refusal, focused iframes share the same boundary, digit wildcard shortcuts cross, and background automation stays in the guest.

Run it with the repo Electron:

npm run capture-harness --workspace=@getpaseo/desktop

Build the desktop main process before the automation group so its production guest preload is available:

npm run build:main --workspace=@getpaseo/desktop
PASEO_CAPTURE_HARNESS_GROUP=automation npm run capture-harness --workspace=@getpaseo/desktop

Run the shared browser profile fixture with:

PASEO_CAPTURE_HARNESS_GROUP=browser-profile npm run capture-harness --workspace=@getpaseo/desktop

The browser profile group runs two Electron processes in sequence. It verifies that each renderer-side did-attach identity maps to the correct main-process guest, that two live tabs share cookies and local storage through one persistent session, and that the data is still present after the first Electron process exits and the second starts.

The automation group uses a real guest webview to verify the page-side ref contract: ARIA-like snapshot text includes headings, static text, and controls; refs survive pushState when the element still matches; same-URL rerenders stale old refs; and a file-input ref can be resolved to a CDP backend node id for upload. It also verifies page-context evaluation, including passing a resolved ref element as the function argument. Keyboard containment runs last because the host-composer sentinel intentionally leaves native focus in the host. It reuses an existing fixture button: adding a test-only control changes the inline fixture geometry exercised by the earlier actionability checks.

On macOS the harness process must set app.setActivationPolicy("accessory") and hide the Dock icon before creating any window. showInactive() only prevents window focus; a normal Electron app launch can still activate the app and steal focus. Harness windows are then created hidden, positioned in a screen corner, skipped from the taskbar where Electron supports it, and revealed with showInactive() from ready-to-show. Do not replace this with show(), focus(), or app.focus(): the compositor only needs visible inactive windows, and harness runs must not steal focus from the person using the machine.

The harness writes PNG evidence and results.json to:

packages/desktop/capture-harness/out/

A passing run prints PASS lines for the production P1 attach-off parking state, including fresh, settled, 75-second soak, multi-tab, viewport, and full-page checks. The PNG sizes may be device-pixel scaled; on a Retina display the 1280x800 logical viewport is usually saved as 2560x1600.

Mechanism

Electron captures copy from the guest web contents' compositor surface. A resident webview parked with display:none, offscreen coordinates, or opacity:0 can lose its copyable surface. The production parking state keeps the host fixed at left:0, top:0, width:1px, height:1px, overflow:hidden, opacity:1, and pointer-events:none. The webviews inside stay full-size at 1280x800, display:inline-flex, and absolutely overlap at left:0, top:0.

There is no renderer prep/restore handshake. Main disables guest background throttling once when the webview attaches, then screenshot capture uses the shared serialized queue, invalidates before each attempt, and retries known first-frame failures within the 5-second capture budget. Viewport screenshots use capturePage({ stayHidden:false }); full-page screenshots use the existing CDP path with layout metrics and screenshot clip.