Use opt-in Android window backdrops

Haze will expose HazeInput.Backdrop(fallback: HazeInput.Sources) as an experimental input. On a supported hardware-accelerated Android window, built-in effects may filter pixels already drawn behind the effect through a dedicated RenderNode. The fallback is mandatory and remains the authoritative result on older Android releases, other platforms, unsupported renderers, and after native setup or drawing fails.

Backdrop input is not another source-selection policy. It samples the combined earlier pixels in the current window surface; it cannot select individual Haze sources, include later drawing, or cross a dialog, popup, or window boundary. Callers that require those semantics continue to use HazeInput.Sources directly.

Core owns attachment-scoped backend selection, geometry, clipping, transforms, draw ordering, and fallback capture demand. A missing capability, unavailable canvas, unusable root effect, or native failure selects source fallback until the modifier node detaches. A healthy native node neither allocates nor records its fallback source layers. This keeps fallback reliable without charging the native path for dormant capture work.

The Android platform renderer is gated on the full 37.2 SDK version and hardware acceleration, not major API 37 alone. Its backend and built-in renderer capability remain internal; callers cannot select a backend or supply a custom backdrop root effect through the supported public API.

Consequences

Existing HazeInput.Sources and HazeInput.Content behavior is unchanged. Blur and Glass retain ownership of their complete platform effect graphs and may opt into the internal capability only when their styling can be represented without weakening their public contract. A native-to-source transition may take one frame, but known failures are not retried every frame.

ADR-0003 remains authoritative for source-backed Android Glass. Backdrop is an explicit input exception: it may reuse the fused effect graph, but it must not replace or simplify the retained source-backed renderer.