Indeed, and thanks for pointing that out! I am not a fan of the simplified Gtk file dialog, even though I learned the trick of pressing Ctrl-L to edit the location. Let's see if that works with other Gtk apps. Edit: `GTK_USE_PORTAL=1 inkscape/gimp` does not work, seems it requires sandboxing? The xdg-desktop-portal-kde package says "This allows sandboxed applications to request services from outside the sandbox using KDE GUIs".
The environment variable only works with apps using GtkFileChooserNative, a GTK3-only API that landed in GTK 3.20. (In Firefox, my patch will fall back if you have an older version of GTK3.) GtkFileChooserNative was designed for use by sandboxed Flatpaks, but the environment variable can force it on outside of a sandbox.