QTreeView checkbox items do not draw in sunken state

If a checkbox is placed in a QTreeView, the checkbox correctly displays the checked and unchecked states.

However it does not display the pressed (sunken) state, and so the user only sees the correct state when the mouse is released rather than on mouse-down.

This can be seen using the "styles" example app, by modifying widgetgallery.cpp's WidgetGallery::createBottomLeftTabWidget to contain:

QWidget *tab1 = new QWidget;
tableWidget = new QTableWidget(10, 10);

// Insert a checkbox
QTableWidgetItem *theItem = new QTableWidgetItem("Test");
theItem->setCheckState(Qt::Checked);
theItem->setFlags(Qt::ItemIsEnabled | Qt::ItemIsUserCheckable);
tableWidget->setItem(0, 0, theItem);

The screenshot shows the visual difference between a QCheckBox widget and a checkable item on Mac OS X.