Strange behaviour of QCombobox::setItemData function - qt

Question

I was working with a simple class derived(publicly) from QComboBox and noticed a strange behaviour of setItemData function:
a simple example:
QFont font;
font.setItalic(true);
setItemData(0, font, Qt::FontRole);
I was sure that this had to change the font of the first element in combobox to italic. But it does NOTHING.
I was more surprised when this code works porperly in this combination:
setEditable(true); //setting my derived class editable changes the situation
QFont font;
font.setItalic(true);
setItemData(0, font, Qt::FontRole);
Moreover, if I run my application(which uses my custom combobox class) giving style option from terminal f.e. -style plastique then everything works properly, without setting combobox editable.
I couldn't find anything said about this in documentation.
Does anybody have idea what is the reason of all this?
Here is the source code:
.h file:
class ComboBoxWrapper : public QComboBox
{
Q_OBJECT;
public:
ComboBoxWrapper(QWidget* parent = 0);
const QString& getDefaultValue() const;
void setDefaultValue(const QString& defaultValue);
private:
QString defaultValue_;
};
And here is .cpp:
ComboBoxWrapper::ComboBoxWrapper(QWidget* parent)
: QComboBox(parent)
{
setAutoFillBackground(true);
}
const QString& ComboBoxWrapper::getDefaultValue() const
{
return defaultValue_;
}
void ComboBoxWrapper::setDefaultValue(const QString& defaultValue)
{
int index = findText(defaultValue);
if(-1 == index)
{ /// current value is invalid.
defaultValue_.clear();
Q_ASSERT(false);
}
defaultValue_ = defaultValue;
QFont font = QApplication::font();
font.setItalic(true);
setEditable(true); //setItemData doesn't work without this trick
setItemData(index, font, Qt::FontRole);
setEditable(false);
}