Project host pointer coordinates into nested webview viewports #522

Parent epic: #268. Top-level release epic: #227.

Proven gap

Native host pointer input is hit-tested in top-level coordinates. When the target is inside a nested iframe, WebScene currently publishes those host coordinates as clientX/clientY and pageX/pageY in the child realm. Unchanged Code OSS uses the child coordinates for webview context-menu anchoring and target ancestry handoff, so menus and pointer-driven integrations can be displaced by every enclosing frame offset.

Focused implementation

• Project script-visible client coordinates into the nearest owning iframe content viewport after native hit testing and realm selection.
• Include iframe border and padding in the host-to-child translation.
• Add the child window scroll offset for page coordinates.
• Preserve host-space screen/movement coordinates and target-relative offset coordinates.
• Keep top-level event behavior and typed host authority unchanged.
• Cover the complete secondary-button context-menu event sequence and a browser/native primary-activation contract.

Acceptance and deferred gates

• Focused implementation committed from exact WebScene main 276d5b77737927a4bd67bfeb523b8a515a7b115f.
• Two-level same-origin native regression source authored.
• Browser/native contract and profile authored.
• git diff --check passed.
• Execute Chromium/native contracts.
• Run nested scroll, transform, zoom, lifecycle, and unchanged Code OSS webview acceptance.
• Record performance and memory evidence.

Execution remains deferred under the current implementation-first direction.