This was a regression caused by the code attempting to insert new items
at drop position, if available. `setSortMode` was being called in a slot
connected to the dir model's rowsInserted, but the Positioner has to be
initialized earlier as a proxy needs to handle
`sourceRowsAboutToBeInserted` as well.
Thanks to an investigation and patch by Oleg Solovyov in D17689 for
helping to get to the bottom of this.
This is aimed at 5.12+.