Exiting fullscreen broken on Linux / X11 in multimediawidgets example

Going to fullscreen with `QWidget::showFullscreen()` then back to normal with `QWidget::showNormal()` doesn't work, the unfullscreened window stays detached from the main application.

This can easily be reproduced with the qt5-multimedia player example, I have uploaded a screen capture at https://www.youtube.com/watch?v=Dr6a9gcXVro showing the behaviour on my end, but it can probably be reproduced without involving qt5-multimedia at all.

Please let me know if I can provide more information!