Why not a virtual/shadow FS? The application would be given a virtual home dir, and would try to access the "~/Downloads" folder, for instance, and then this wo

Why not a virtual/shadow FS? The application would be given a virtual home dir, and would try to access the "~/Downloads" folder, for instance, and then this would take the form of a macOS "Firefox wants to access your downloads folder". Then, OS can easily set up a mount on the container for that folder, with restricted perms. Same with drag and drop. Data could be put into a folder in /tmp, a mount could be set up whilst it uploads your photo or whatever to Facebook, and then the mount and the temporary dir could be deleted. Having a "shadow fs" displayed to the container so it can function as if it is not in a container, but then when it wants to access the actual fs, it is done via either prebaked whitelist or user interaction/confirmation, would be great because apps would not have to be modified and security would be incredible. You could even have it write to shadow FS and that could be synced later.