Switch to fullscreen in a desktop app which is running windowed

I have a desktop application that runs with a transparent masked border, so since it's not a fullscreen application i have <fullScreen>false</fullScreen> in the app descriptor. BUT I'd like to be able to switch to fullscreen mode in some instances. I try to do so with:

But this doesn't seem to work. The app is still not fullscreen after issuing that line. I suppose it's because of the descriptor, but if I set  <fullScreen>true</fullScreen> the app will start fullscreen, which I don't necessarily want (usually it's a windowed application with transparent chrome).

So is this possible?

Thanks!