QHBoxLayout miscalculates size needed for QPushButton-derived subclasses

#include <QtGui>

class MyButton : public QPushButton {
public:
  explicit MyButton(Qt::GlobalColor color, QWidget *parent = NULL)
    : QPushButton(parent), color_(color) {
    setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
  }
  QSize sizeHint() const {
    return QSize(50, 25);
  }
protected:
  void paintEvent(QPaintEvent *) {
    QPainter painter(this);
    painter.setOpacity(0.5);
    painter.fillRect(0, 0, width(), height(), color_);
  }
private:
  Qt::GlobalColor color_;
};

int main(int argc, char **argv) {
  QApplication app(argc, argv);
  QWidget widget;
  QHBoxLayout *layout = new QHBoxLayout;
  layout->setSpacing(0);
  MyButton *w1 = new MyButton(Qt::red);
  MyButton *w2 = new MyButton(Qt::green);
  MyButton *w3 = new MyButton(Qt::blue);
  layout->addWidget(w1);
  layout->addWidget(w2);
  layout->addWidget(w3);
  widget.setLayout(layout);
  widget.show();
  return app.exec();
}