QMainWindow doesn't update new splitter handle after dropping a dockwidget when running in classic style.

To reproduce, run the dockwidgets example in Classic mode (windows xp, right-click desktop and swich theme to Classic). Undock a dockwidget, and redock it. Notice that the vertical splitter (the right-most one that separates the two dock widgets vertically, the handle is horizontal) is not redrawn when you redock the dock widget.