If "Autodownload maps" is enabled, in practice, one also needs to
enable "Autodownload Only Gets Missing Maps" (each time viking is
started, because this is not the default and this option is not
saved). Otherwise moving the map often makes viking hang for a few
seconds: it seems that it tries to re-download parts of the maps,
even if this has already been done several seconds ago, and this
makes viking hardly usable.
The issue with enabling "Autodownload Only Gets Missing Maps" is that
parts of the maps could have been downloaded years ago, so that they
can have become obsolete.
The user should be able to control cache expiry time.