Comment # 25 on bug 1226112 from Michael Pujos

xdg-desktop-portal requires an implementation to also be installed for it to
work. 


Here's the list (packages):


xdg-desktop-portal-cosmic
xdg-desktop-portal-gnome
xdg-desktop-portal-gtk
xdg-desktop-portal-hyprland      
xdg-desktop-portal-kde6          
xdg-desktop-portal-lxqt          
xdg-desktop-portal-pantheon      
xdg-desktop-portal-wlr           


If a user does not have at least one of these installed, it cannot work.
It's also possible the less common implementations may have bugs.
At least the gtk and kde6 implementation work fine, I tested them both in
Firefox/Thunderbird for file dialogs.

To force the use of a specific implementation, simply link to the
implementation portals.conf file in ~/.config/xdg-desktop-portal (create that
folder if necesary). For example to force the use of the kde portal:


ln -s /usr/share/xdg-desktop-portal/kde-portals.conf portals.conf


(then relogin for the change to take effect. A 'systemd --user restart
xdg-desktop-portal' may also be sufficient). 

Also, for proper operation when using a Window Manager, I have these line in my
Xorg startup script:

dbus-update-activation-environment --systemd DBUS_SESSION_BUS_ADDRESS DISPLAY
XAUTHORITY QT_AUTO_SCREEN_SCALE_FACTOR QT_SCREEN_SCALE_FACTORS GDK_SCALE
GDK_DPI_SCALE GTK_THEME

This will make these environment variables seen by the xdg-desktop-portal*
processes. Some of them are necessary for proper scaling/theming if they are
set  by the user in his startup scripts.