In the documentation for QAbstractItemModel::layoutChanged() we see the following instruction on use:

Subclasses should update any persistent model indexes before emitting layoutChanged(). In other words, when the structure changes:

Call beginLayoutChanged()
Remember the QModelIndex that will change
Update your internal data
Call changePersistentIndex()
Call endLayoutChanged()

Neither beginLayoutChanged() nor endLayoutChanged() are defined in any Qt classes documented in the Qt Assistant