QComboBox: Disable items marked as disabled in native popups

Fixes: QTBUG-56064
Pick-to: 6.5 6.2 5.15
Change-Id: I00a17a19b4c14c874e16ec863e794218f0570346
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

Author: gxalpha

src/widgets/widgets/qcombobox.cpp

@@ -2501,10 +2501,12 @@ bool QComboBoxPrivate::showNativePopup()
         QVariant textVariant = model->data(rowIndex, Qt::EditRole);
         item->setText(textVariant.toString());
         QVariant iconVariant = model->data(rowIndex, Qt::DecorationRole);
+        const Qt::ItemFlags itemFlags = model->flags(rowIndex);
         if (iconVariant.canConvert<QIcon>())
             item->setIcon(iconVariant.value<QIcon>());
         item->setCheckable(true);
         item->setChecked(i == currentIndex);
+        item->setEnabled(itemFlags & Qt::ItemIsEnabled);
         if (!currentItem || i == currentIndex)
             currentItem = item;