[KevinH]

I am obviously not seeing this on my mac. In Sigil/src/Misc/OpenExternally.cpp
you will see that Sigil creates a detached process and runs the following command:

   if (QFile::exists(filePath) && QDir(application).exists()) {
        QStringList arguments = QStringList() << "-a" << application << filePath;
        return QProcess::startDetached("/usr/bin/open", arguments);
    }

So in Terminal.app please try running the following:

open -a YOUR_APPLICATION_NAME FULLPATHTOAFILETOBEOPENED

Does that work?

Or is this just a silly case of having a space in the application name and us needing to add quotes around the application name itself?