WA_StaticContents broken on Windows in Qt5

Qt::WA_StaticContents is used to restrict the region of a QPaintEvent to the actual dirty region. This worked as advertised in Qt4 in OSX and Windows (modulo a bug in the Core Graphics Engine, which I also reported). In Qt 5.1.1, the attribute is completely ignored on Windows.

Simply drop this line into the paintEvent in the 'scribble' example which ships with Qt, and see that the dirty region is the entire window.

for( const QRect& rect : event->region().rects() )
qDebug() << rect;

Output:
QRect(0,0 500x479)
QRect(0,0 501x479)
QRect(0,0 504x480)
QRect(0,0 507x482)
QRect(0,0 513x483)
QRect(0,0 521x485)
QRect(0,0 529x486)
QRect(0,0 535x486)
QRect(0,0 543x489)
QRect(0,0 550x489)
QRect(0,0 557x489)

IMHO this is a fairly serious performance regression, particularly for large data-driven widgets such as grids, lists, and trees.