can not go back to the previous window

I have a window(inherit from QWidget, call it W1). in the window I create another window(inherit from QWidget, call it W2).
In W1, I call W2.showFullScreen(), W2 shows; In W2, I call hide(), the W2 hide, W1 show.
this works well.

another case:
I have a window(inherit from QWidget, call it V1). in the window I create another window(inherit from QGraphicsView, call it V2, in V2 it has scene).
In V1, I call V2.showFullScreen(), V2 shows; In V2, I call hide(), the V2 hide, V1 does not show.
why?

resolved, I use QGraphicsSceneMouseEvent::Widget to get the widget, but do not why, it is wrong.