How do I configure Flatpak applications to interact with each other?
Here is a specific example.
The .deb version of Darktable is able to interact with the .deb versions of GIMP and Hugin Panorama Stitcher. However, the Flatpak version of Darktable cannot interact with the .deb. version of GIMP nor the Flatpak version of Hugin. In fact, there is a whole raft of functionality (provided by lua scripts that call other programs) that no longer works because it is a Flatpak.
I have installed Flatseal so that I can have some control over Flatpak applications. I have managed to give Darktable access to my home folder, which is great, but that's all I have achieved.
What do I need to do to allow Flatpak applications to work with each other?