cbf-compositor

cbf-compositor is the scene-based browser surface compositor for CBF applications.

• ownership tree: backend object relationships such as transient browsing contexts being owned by a parent browsing context
• composition tree: which window displays which browser surface, in what order, and at what bounds

Within a window, WindowCompositionSpec.items defines front-to-back stacking order. The first item is topmost.

Each scene item declares a hit-test policy:

• Passthrough: never receives pointer hit-tests
• Bounds: receives pointer hit-tests across the full item bounds
• RegionSnapshot: receives pointer hit-tests only inside the latest pushed region snapshot

The compositor only manages browser-managed surfaces:

• BrowsingContext
• TransientBrowsingContext

Host-native UI remains outside of the scene graph.

The current implementation provides:

• scene/state management for BrowsingContext and TransientBrowsingContext
• Chrome event adapters for surface handles and transient popup lifecycle hints
• macOS host-window attachment through raw-window-handle
• scene rendering via a single CompositorViewMac that manages multiple CALayerHost surfaces
• input routing for key, edit action, mouse wheel, IME, and transient popup commands
• host-side context menu, choice menu, and native drag integration for the active surface target

The macOS implementation attaches exactly one compositor-owned NSView to the host content view. That responder view becomes first responder and routes keyboard, IME, mouse, wheel, menu, and drag interactions to the appropriate scene item based on composition order and hit-testing. Region snapshots use item-local CSS-pixel rectangles with a top-left origin.