QWidget::stackUnder causes unnecessary repaint events.

Let's say we have four widgets with opaque background and the same geometry in this stacking order:

widget3 <- top
widget2
widget4
widget1

widget1->stackUnder(widget3) => repaint on widget3

widget3 is unchanged and still on top, so the repaint is waste of time . See tst_QWidget::stackUnder QEXPECT_FAIL.