im not very sure myself, but i will try to help you, in examples.
I have the gqview to view manga with real Fullscreen, but i got two details to do it, that seems related to your work.
1) when i go to fullscreen it put the image in fullscreen but i know that i haven't closed the contentlistener, but i'm not seeing it(interesting)
2) if i press the devicemanager or another button like news, books...
i go to the part it's ok, but if i recall the gqview and goes another time for the fullscreen i only got the first image (i got now two gqview running)
i think that your problem it's the "priority of the gtk window" that you created