Active palette role doesnt work on OS X

The example below displays a button which changes its color to red when the window is active.
This works on X11/Linux.
On Mac OS X the button is always red, regardless if the window is active or not.

—

#include <QApplication>
#include <QPushButton>

int main(int argc, char **argv)
{
QApplication a(argc, argv);

QPushButton *b=new QPushButton();
b->setStyleSheet("QPushButton:active

");
b->setText("Test");
b->show();
a.exec();

return 0;
}