When a style is calling QWidget::setWindowFlag() inside polish/unpolish to set e.g. Qt::NoDropShadowWindowHint it might create an endless loop when there is also a stylesheet involved.

main.cpp:

QComboBox cbx;
cbx.show();
a.setStyleSheet("blub");

Then add something like this in style polish/unpolsih (see e.g. https://codereview.qt-project.org/c/qt/qtbase/+/613492)

polish(QWidget* widget) {
    widget->setWindowFlag(Qt::NoDropShadowWindowHint, true);
}
unpolish(QWidget* widget) {
    widget->setWindowFlag(Qt::NoDropShadowWindowHint, false);
}

This gives a stackoverflow like this: