Class QGraphicsView and QGraphicsScene.
1, scene's size is bigger than view.
2, view display scroll bar.
3, when no operation more than 3 seconds, hide view's scroll bar.
4, After scroll bar is hided, the view can not fill the whole screen to enlarge visible area.
And, seems that current view's scroll bar overlay on the top of scene which part should be visible also.
View's scroll bar area should not overlay with scene's area.