...unless private QQuickWindow API is used, like in QQuickWidget::show() and hide().

qquickitemgrabresult.cpp contains this code:

{{
if (!item->window()->isVisible())

}}

that makes it impossible to use grabToImage() on a hidden QWindow. This window, however, may be a hidden window that QQuickWidget uses internally to render results into a framebuffer object. The example "rendercontrol" also uses such a window. This makes it impossible to grab items, that are visible, but contained in an internal, hidden, QWindow, effectively disabling a part of Qt functionality.