https://github.com/flathub/org.kde.krita/pull/15 1. Install the test build from the PR: flatpak install --user https://dl.flathub.org/build-repo/4076/org.kde.krita.flatpakref 2. Try to open a file. Notice how the app freezes. This works under GNOME with xdg-desktop-portal-gtk.
I'm not sure this is our fault, because: 1) It works with older version of Krita (tried 4.1.8 from Flathub now) 2) It doesn't work for me even with xdg-desktop-portal-gtk
I tested it under Ubuntu 19.04 (GNOME) with - flatpak 1.2.4 - xdg-desktop-portal 1.2.0 - xdg-desktop-portal-gtk 1.2.0 and there it definitely works.
This does work under Gnome because Krita doesn't use native file dialogs there, which means no portal is involved. I tested it with xdg-desktop-portal-gtk running under KDE and there the result was same, which means the issue is in Krita or in Qt's portal support. I will try to investigate this further.
After updating Krita to 4.2.3, this bug seems to be gone. Maybe something in the runtime was fixed?!