cua-driver/Windows: background Chrome click can miss or blur the foreground sentinel

Problem

On a hosted Windows Server 2025 interactive desktop, a real Chrome target does not produce a deterministic safe result for background coordinate clicks on current main (3cadb5f82e7d2ed071a2082764276ec872a52135).

Observed outcomes across repeated runs include:

• a background scene click reaching the target once;
• point and scene clicks missing the target;
• the foreground sentinel recording a blur during the action.

The last outcome is the most serious: a background request must either deliver without disturbing the foreground desktop or return a structured refusal before acting.

Our canonical Windows matrix proves the stricter target-state, focus, z-order, cursor, and leaked-input oracles on repo-built Electron, Tauri, WPF, WinUI 3, and WebView2 fixtures. It does not currently prove the same route against standalone Chrome, so the surface-specific regression can escape that matrix.

Reproduction shape

1. Launch Chrome with a deterministic page that journals button state.
2. Fully occlude Chrome with a foreground sentinel that records focus, keyboard, pointer, and wheel events.
3. Call click with Chrome's pid, window_id, window-local x/y, and delivery_mode:"background".
4. Assert target state and all foreground side-effect oracles.
5. Repeat with fresh processes and both point- and scene-addressed routes.

Acceptance criteria

• The same action deterministically delivers under full occlusion or returns one exact structured refusal.
• A background result never blurs or lowers the foreground sentinel.
• Run at least three fresh-process seeds on windows-latest and an interactive Windows VM.
• Add an optional real-Chrome compatibility row that uses the same typed report and independent oracles as the canonical Rust catalog.
• Keep the repo-built shared harness as the canonical contract; use the Chrome row to detect materially different renderer/window-class behavior.
• Retain per-cell video, target journal, focus journal, and driver trajectory for failures.

Related

• #2176 tracks page/JS escalation when native background delivery is unavailable on web surfaces.
• #2193 introduced the current strict background E2E oracles and canonical Windows matrix.

Start with the canonical Rust Windows matrix and shared harness introduced by #2193, then trace the background click route for a Chrome process using pid, window_id, and window-local coordinates. Add the optional real-Chrome compatibility row with the existing typed report and independent target, focus, z-order, cursor, and leaked-input oracles. Done means three fresh-process seeds run on windows-latest and an interactive Windows VM, with failure artifacts retained.