QComboBox uses QPersistentModelIndex to store the current index of the
underlying model. When the model is cleared, that index is automatically
invalidated, so calling QComboBoxPrivate::setCurrentIndex(QModelIndex())
does not result in signals being emitted, because we do not detect the
index change.
This patch uses indexBeforeChange to detect such situation and emit all
necessary signals.
Fixes: QTBUG-103007
Pick-to: 6.4 6.3 6.2
Change-Id: I29326830a30a17900839e8d1737a08bd940081ea
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
662184ac68