Behavior inside a Flatpak sandbox?

[elsiehupp]

I don't know how to test this, and it isn't immediately clear to me from the code, but does this choose the correct directories within a Flatpak sandbox? According to the Flatpak documentation, the environmental variables within a Flatpak sandbox should return as follows:

Base directory	Usage	Default location
XDG_CONFIG_HOME	User-specific configuration files	~/.var/app/<app-id>/config
XDG_DATA_HOME	User-specific data	~/.var/app/<app-id>/data
XDG_CACHE_HOME	Non-essential user-specific data	~/.var/app/<app-id>/cache

I don't know about XDG variables beyond these within the Flatpak sandbox.

Basically, it would be nice for the documentation (e.g. README.md) to explicitly state the behavior of appdirs within a Flatpak sandbox. (Similarly, the documentation could explicitly state the behavior within a Snap sandbox and within a macOS container, as well.)

I'd be happy to help if you can point me in the right direction.

Thanks!