LayerRasterizerHost class

Hosts the layers a LayerRasterizer is capturing.

Mount this above anything that captures — typically once around the whole app — and pass the same rasterizer to LayerRasterizer.capture:

MaterialApp(
  builder: (context, child) => LayerRasterizerHost(
    rasterizer: rasterizer,
    child: child!,
  ),
);

While idle the host adds a Stack around child and nothing else. During a capture it renders the requested layers behind child: they have to be painted for their repaint boundaries to hold an image, but child covers them so they never reach the user. This mirrors how the editor's own screenshot capture stays invisible — which also means child has to be opaque and cover the host, otherwise the layers show through for the two frames a capture takes.