John,
if you set the central widget to an empty QWidget(), this is true. I just can't code now, I have a terrible headache. I don't plan to take API changes over from QtOctave. As for QScintilla, I have a feeling that it's not that simple as
setWidget(new QScintillaWidget());
If anyone thinks I am wrong here, he may fork the project on github, modify it and send me a pull request. It's not that I am not open to QScintilla, it's just that I don't have the time see whether and how it works out.