I'm not familiar enough with flatpak to say if this bug is in the package itself, in one of the xdg-desktop-portal* package or in many flatpaks. Sorry about that, feel free to close the bug, or to reassign. I use xmonad under X11, with an azerty keyboard layout and I can't use dead keys in a lot of flatpaks (for example org.mozilla.Firefox, im.riot.Riot, but cc.arduino.arduinoide works fine). For example, typing "^" then "e" should result in "ê", but it gives an "e". I'd be happy to help if you could give me advice on how to debug this! Thanks!