X11 child window reparenting doesn't seem to work if window is already visible

As seen in the parent bug's test case, if you enable the container->setVisible(true); line, the widget ends up not being parented into the quick window

Which is strange because we do end up in QXcbWindow::setParent(this=0x0000aaaaaac348c0, parent=0x0000aaaaaada2ee0) and we don't call setParent again after that.