A graphics view's last center point may drift if only a scroll bar is moving

If only a scroll bar, for example the horizontal scroll bar, is moved, QGraphicsView::scrollContentsBy(dx, dy) will be called with a zero dy. Then updateLastCenterPoint() will be called by scrollContentsBy(). Both the x and y coordinates of lastCenterPoint will be updated. But actually only the x coordinate should be updated while the y coordinate must be unchanged. The existing code will make lastCenterPoint drift away.

I uploaded a test case. The horizontal scroll bar is moved every 3 seconds. The horizontal line should not move but it's drifting away slowly.

It'll be appreciated if this bug is fixed.