Provide a gnome-flashback-portals.conf for xdg-desktop-portal >= 1.17.1
Since xdg-desktop-portal 1.17.1, the recommendation is for each desktop environment implementation XDG_CURRENT_DESKTOP=foo
to provide a /usr/share/xdg-desktop-portal/foo-portals.conf
specifying its preferred portal implementations. This ensures that GNOME-Shell-specific implementations of interfaces like screencasting are used if and only if we're running GNOME Shell.
Since GNOME Flashback uses metacity rather than GNOME Shell, its mix of desired portals is probably somewhat different from the full-fat GNOME session, so probably it should have its own gnome-flashback-portals.conf
that doesn't try to provide GNOME-Shell- or Mutter-specific interfaces?
See also gnome-session#114 (closed), gnome-session!95 (merged) which are the equivalent of this for full-fat GNOME.