QPixmap::grabWidget() produces a QPixmap of a wrong size if the widget hasn't been resized previously

The widget itself is painted in a correct size, but has a very thick border over the rest of the QPixmap.
Two possible workarounds are:
1) w.show(); and then call the grabWidget()

or

2) w.resize(..., ...); and then grabWidget()

Reproducible with the following code:

#include <QtGui>
#include <QDebug>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QTreeWidget w;
QPixmap pixmap = QPixmap::grabWidget(&w);
pixmap.save("widget.png");
return 0;
}