[KevinH]

So something is preventing subprocesses from communicating and running with flatpak. Does flatpak use some sort of sandboxing to prevent it from launching sub processes? I am not a flatpak user, as building Sigil from scratch on most linux systems is well documented and quite easy.

The code that does the work in Sigil itself is quite simple:

if (QFile::exists(filePath) && QFile::exists(application)) {
        QStringList arguments = QStringList(QDir::toNativeSeparators(filePath));
        return QProcess::startDetached(QDir::toNativeSeparators(application), arguments, QFileInfo(filePath).absolutePath());
    }

So as long as the image file exists and the shell script exists and is executable, it should pass the full file path to the shell script.

Try changing the script to echo the input file and see if that helps you figure out how to modify the shell script to make it work.

As for removing unwanted things look in the sigil.ini for the editors_image settings and delete the lines you do not want but I recommend backing that file up first to be safe.