Authoritative closed vocabulary and first-milestone mapping: native-capture-contracts.md (Coverage).
The Android path reports engineSurface for PixelCopy of the active
FlutterSurfaceView. The Apple plugin reports engineSurface when it renders
the live Flutter layer. Neither path is the final window composition.
Platform views and separate video or map surfaces can be missing. Secure or
DRM content can also be missing. Those cases are unsupported modes or missing
layers. They are never an unmasked substitute.
windowComposite is forbidden until that path exists. viewHierarchy is an
optional Apple compatibility mode. It uses drawHierarchy of the Flutter
view. Incomplete descendants set incomplete=true.