poor vertical alignment of QCheckBox, QComboBox, using QGridLayout on Mac

-create a qcheckbox, a qcombobox, and another qcheckbox
-layout the three items horizontally using a QGridLayout
-with Qt 4.6.3 on Mac, the baseline of the text in the combo box is 1 pixel lower than that in the checkboxes.

Below is some basic repro code:

QWidget* widget = new QWidget();
QGridLayout* grid = new QGridLayout(widget);

grid->addWidget( new QCheckBox("Higher"), 0,0);

QComboBox* combo = new QComboBox();
combo->addItem("Lower");
grid->addWidget( combo, 0,1);

grid->addWidget( new QCheckBox("Also Higher"), 0,2);
grid->setColumnStretch(3,1);
grid->setMargin(0);