In the attached code the check in the checkbox does not always draw correctly.
I found a workaround that fixes it.

class CustomStyle : public QProxyStyle
{
    void drawControl(ControlElement element, const QStyleOption* option, QPainter* painter, const QWidget* widget) const
    {
        if (element == QStyle::CE_CheckBox && dynamic_cast<QWebView*>(option->styleObject))
            option->styleObject->setProperty("_q_no_animation", true);
        QProxyStyle::drawControl(element, option, painter, widget);
    }
};

since the QStyleOption::styleObject is the QWebView, when the animation is generated it has a bad starting point. QWindowsVistaStyle::drawPrimitive uses QStyleOption::styleObject->property("_q_stylestate") for the starting point. The stylestate of the QWebView has nothing to do with the checkbox state.